Clauses are included that detail the C language itself and the contents of the C language
execution library. Annexes summarize aspects of both of them, and enumerate factors
that influence the portability of C programs.
Although this International Standard is intended to guide knowledgeable C language
programmers as well as implementors of C language translation systems, the document
itself is not designed to serve as a tutorial.
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Contents
Foreword ............................ 1
Introduction ........................... 4
1. Scope ............................. 5
2. Normative references ....................... 6
3. Terms and definitions ....................... 6
4. Conformance .......................... 9
5. Environment ........................... 11
5.1 Conceptual models ...................... 11
5.1.1 Translation environment ................ 11
5.1.2 Execution environments ................ 13
5.2 Environmental considerations ................. 19
5.2.1 Character sets .................... 19
5.2.2 Character display semantics ............... 21
5.2.3 Signals and interrupts ................. 22
5.2.4 Environmental limits ................. 22
6. Language ............................ 31
6.1 Notation .......................... 31
6.2 Concepts .......................... 31
6.2.1 Scopes of identifiers .................. 31
6.2.2 Linkages of identifiers ................. 32
6.2.3 Name spaces of identifiers ............... 33
6.2.4 Storage durations of objects ............... 34
6.2.5 Types ....................... 35
6.2.6 Representations of types ................ 39
6.2.7 Compatible type and composite type ........... 41
6.3 Conversions ........................ 43
6.3.1 Arithmetic operands .................. 43
6.3.2 Other operands .................... 47
6.4 Lexical elements ....................... 50
6.4.1 Ke ywords ...................... 51
6.4.2 Identifiers ...................... 52
6.4.3 Universal character names ............... 54
6.4.4 Constants ...................... 54
6.4.5 String literals .................... 63
6.4.6 Punctuators ..................... 64
6.4.7 Header names .................... 65
6.4.8 Preprocessing numbers ................. 66
6.4.9 Comments ..................... 66
6.5 Expressions ......................... 68
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6.5.1 Primary expressions .................. 70
6.5.2 Postfix operators ................... 70
6.5.3 Unary operators ................... 78
6.5.4 Cast operators .................... 82
6.5.5 Multiplicative operators ................ 83
6.5.6 Additive operators .................. 83
6.5.7 Bitwise shift operators ................. 85
6.5.8 Relational operators .................. 86
6.5.9 Equality operators .................. 87
6.5.10 Bitwise AND operator ................. 88
6.5.11 Bitwise exclusive OR operator .............. 89
6.5.12 Bitwise inclusive OR operator .............. 89
6.5.13 Logical AND operator ................. 89
6.5.14 Logical OR operator .................. 90
6.5.15 Conditional operator .................. 90
6.5.16 Assignment operators ................. 92
6.5.17 Comma operator ................... 94
6.6 Constant expressions ..................... 95
6.7 Declarations ........................ 97
6.7.1 Storage-class specifiers ................ 98
6.7.2 Type specifiers .................... 99
6.7.3 Type qualifiers .................... 108
6.7.4 Function specifiers .................. 112
6.7.5 Declarators ..................... 113
6.7.6 Type names ..................... 120
6.7.7 Type definitions ................... 121
6.7.8 Initialization ..................... 123
6.8 Statements ......................... 130
6.8.1 Labeled statements .................. 130
6.8.2 Compound statement, or block .............. 131
6.8.3 Expression and null statements ............. 131
6.8.4 Selection statements .................. 132
6.8.5 Iteration statements .................. 134
6.8.6 Jump statements ................... 135
6.9 External definitions ..................... 140
6.9.1 Function definitions .................. 141
6.9.2 External object definitions ............... 143
6.10 Preprocessing directives .................... 145
6.10.1 Conditional inclusion ................. 147
6.10.2 Source file inclusion .................. 149
6.10.3 Macro replacement .................. 150
6.10.4 Line control ..................... 157
6.10.5 Error directive .................... 158
6.10.6 Pragma directive ................... 158
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6.10.7 Null directive .................... 159
6.10.8 Predefined macro names ................ 159
6.10.9 Pragma operator ................... 160
6.11 Future language directions ................... 162
6.11.1 Floating Types .................... 162
6.11.2 Character escape sequences ............... 162
6.11.3 Storage-class specifiers ................ 162
6.11.4 Function declarators .................. 162
6.11.5 Function definitions .................. 162
6.11.6 Pragma directives ................... 162
7. Library ............................. 163
7.1 Introduction ........................ 163
7.1.1 Definitions of terms .................. 163
7.1.2 Standard headers ................... 164
7.1.3 Reserved identifiers .................. 165
7.1.4 Use of library functions ................ 166
7.2 Diagnostics <assert. h> .................. 168
7.2.1 Program diagnostics .................. 168
7.3 Complex arithmetic <complex. h> ............... 169
7.3.1 Introduction ..................... 169
7.3.2 Conventions ..................... 170
7.3.3 Branch cuts ..................... 170
7.3.4 The CX_ LIMITED_ RANGE pragma ........... 170
7.3.5 Trigonometric functions ................ 171
7.3.6 Hyperbolic functions ................. 174
7.3.7 Exponential and logarithmic functions .......... 176
7.3.8 Power and absolute-value functions ............ 177
7.3.9 Manipulation functions ................ 179
7.4 Character handling <ctype. h> ................ 182
7.4.1 Character testing functions ............... 182
7.4.2 Character case mapping functions ............ 186
7.5 Errors <errno. h> ..................... 187
7.6 Floating-point environment <fenv. h> ............. 188
7.6.1 The FENV_ ACCESS pragma .............. 190
7.6.2 Exceptions ..................... 191
7.6.3 Rounding ...................... 193
7.6.4 Environment ..................... 195
7.7 Characteristics of floating types <float. h> ........... 197
7.8 Format conversion of integer types <inttypes. h> ........ 198
7.8.1 Macros for format specifiers .............. 198
7.8.2 Conversion functions for greatest-width integer types ..... 199
7.9 Alternative spellings <iso646. h> ............... 201
7.10 Sizes of integer types <limits. h> ............... 202
7.11 Localization <locale. h> .................. 203
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7.11.1 Locale control .................... 204
7.11.2 Numeric formatting convention inquiry .......... 205
7.12 Mathematics <math. h> ................... 211
7.12.1 Treatment of error conditions .............. 213
7.12.2 The FP_ CONTRACT pragma .............. 214
7.12.3 Classification macros ................. 214
7.12.4 Trigonometric functions ................ 217
7.12.5 Hyperbolic functions ................. 220
7.12.6 Exponential and logarithmic functions .......... 223
7.12.7 Power and absolute-value functions ............ 229
7.12.8 Error and gamma functions ............... 231
7.12.9 Nearest integer functions ................ 233
7.12.10 Remainder functions ................. 237
7.12.11 Manipulation functions ................ 239
7.12.12 Maximum, minimum, and positive difference functions .... 241
7.12.13 Floating multiply-add ................. 242
7.12.14 Comparison macros .................. 243
7.13 Nonlocal jumps <setjmp. h> ................. 247
7.13.1 Save calling environment ................ 247
7.13.2 Restore calling environment ............... 248
7.14 Signal handling <signal. h> ................. 250
7.14.1 Specify signal handling ................ 251
7.14.2 Send signal ..................... 252
7.15 Variable arguments <stdarg. h> ............... 253
7.15.1 Variable argument list access macros ........... 253
7.16 Boolean type and values <stdbool. h> ............. 257
7.17 Common definitions <stddef. h> ............... 258
7.18 Integer types <stdint. h> .................. 259
7.18.1 Integer types ..................... 259
7.18.2 Limits of specified-width integer types .......... 261
7.18.3 Limits of other integer types .............. 263
7.18.4 Macros for integer constants .............. 264
7.19 Input/ output <stdio. h> ................... 266
7.19.1 Introduction ..................... 266
7.19.2 Streams ....................... 268
7.19.3 Files ........................ 269
7.19.4 Operations on files .................. 272
7.19.5 File access functions .................. 274
7.19.6 Formatted input/ output functions ............. 278
7.19.7 Character input/ output functions ............. 300
7.19.8 Direct input/ output functions .............. 306
7.19.9 File positioning functions ................ 307
7.19.10 Error-handling functions ................ 309
7.20 General utilities <stdlib. h> ................. 312
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7.20.1 String conversion functions ............... 313
7.20.2 Pseudo-random sequence generation functions ....... 318
7.20.3 Memory management functions ............. 319
7.20.4 Communication with the environment ........... 321
7.20.5 Searching and sorting utilities .............. 323
7.20.6 Integer arithmetic functions ............... 325
7.20.7 Multibyte character functions .............. 326
7.20.8 Multibyte string functions ............... 328
7.21 String handling <string. h> ................. 331
7.21.1 String function conventions ............... 331
7.21.2 Copying functions .................. 331
7.21.3 Concatenation functions ................ 333
7.21.4 Comparison functions ................. 334
7.21.5 Search functions ................... 337
7.21.6 Miscellaneous functions ................ 341
7.22 Type-generic math <tgmath. h> ................ 343
7.22.1 Type-generic macros ................. 343
7.23 Date and time <time. h> ................... 346
7.23.1 Components of time .................. 346
7.23.2 Time manipulation functions .............. 347
7.23.3 Time conversion functions ............... 349
7.24 Extended multibyte and wide-character utilities <wchar. h> ..... 356
7.24.1 Introduction ..................... 356
7.24.2 Formatted wide-character input/ output functions ...... 357
7.24.3 Wide-character input/ output functions ........... 375
7.24.4 General wide-string utilities ............... 381
7.24.5 Wide-character time conversion functions ......... 396
7.24.6 Extended multibyte and wide-character conversion
utilities ....................... 397
7.25 Wide-character classification and mapping utilities <wctype. h> ... 404
7.25.1 Introduction ..................... 404
7.25.2 Wide-character classification utilities ........... 405
7.25.3 Wide-character mapping utilities ............. 411
7.26 Future library directions .................... 413
7.26.1 Complex arithmetic <complex. h> ........... 413
7.26.2 Character handling <ctype. h> ............. 413
7.26.3 Errors <errno. h> .................. 413
7.26.4 Format conversion of integer types <inttypes. h> .... 413
7.26.5 Localization <locale. h> ............... 413
7.26.6 Signal handling <signal. h> ............. 413
7.26.7 Boolean type and values <stdbool. h> ......... 414
7.26.8 Integer types <stdint. h> .............. 414
7.26.9 Input/ output <stdio. h> ............... 414
7.26.10 General utilities <stdlib. h> ............. 414
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7.26.11 String handling <string. h> ............. 414
7.26.12 Extended multibyte and wide-character utilities
<wchar. h> .................... 414
7.26.13 Wide-character classification and mapping utilities
<wctype. h> .................... 415
Annex A (informative) Language syntax summary ............. 416
A. 1 Lexical grammar ...................... 416
A. 2 Phrase structure grammar ................... 422
A. 3 Preprocessing directives .................... 429
Annex B (informative) Library summary ................ 431
B. 1 Diagnostics <assert. h> .................. 431
B. 2 Complex <complex. h> ................... 431
B. 3 Character handling <ctype. h> ................ 433
B. 4 Errors <errno. h> ..................... 433
B. 5 Floating-point environment <fenv. h> ............. 433
B. 6 Characteristics of floating types <float. h> ........... 434
B. 7 Format conversion of integer types <inttypes. h> ........ 434
B. 8 Alternative spellings <iso646. h> ............... 435
B. 9 Sizes of integer types <limits. h> ............... 435
B. 10 Localization <locale. h> .................. 436
B. 11 Mathematics <math. h> ................... 436
B. 12 Nonlocal jumps <setjmp. h> ................. 441
B. 13 Signal handling <signal. h> ................. 441
B. 14 Variable arguments <stdarg. h> ............... 441
B. 15 Boolean type and values <stdbool. h> ............. 441
B. 16 Common definitions <stddef. h> ............... 441
B. 17 Integer types <stdint. h> .................. 442
B. 18 Input/ output <stdio. h> ................... 443
B. 19 General utilities <stdlib. h> ................. 445
B. 20 String handling <string. h> ................. 446
B. 21 Type-generic math <tgmath. h> ................ 447
B. 22 Date and time <time. h> ................... 448
B. 23 Extended multibyte and wide-character utilities <wchar. h> ..... 448
B. 24 Wide-character classification and mapping utilities <wctype. h> ... 451
Annex C (informative) Sequence points ................. 452
Annex D (normative) Universal character names for identifiers ........ 453
Annex E (informative) Implementation limits ............... 455
Annex F (normative) IEC 60559 floating-point arithmetic .......... 457
F. 1 Introduction ........................ 457
F. 2 Types ........................... 457
F. 3 Operators and functions .................... 458
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F. 4 Floating to integer conversion ................. 460
F. 5 Binary-decimal conversion ................... 460
F. 6 Contracted expressions .................... 461
F. 7 Environment ........................ 461
F. 8 Optimization ........................ 464
F. 9 Mathematics <math. h> ................... 467
Annex G (informative) IEC 60559-compatible complex arithmetic ...... 482
G. 1 Introduction ........................ 482
G. 2 Types ........................... 482
G. 3 Conversions ........................ 482
G. 4 Binary operators ....................... 483
G. 5 Complex arithmetic <complex. h> ............... 488
G. 6 Type-generic math <tgmath. h> ................ 496
Annex H (informative) Language independent arithmetic .......... 497
H. 1 Introduction ........................ 497
H. 2 Types ........................... 497
H. 3 Notification ......................... 501
Annex I (informative) Common warnings ................ 503
Annex J (informative) Portability issues ................. 505
J. 1 Unspecified behavior ..................... 505
J. 2 Undefined behavior ..................... 507
J. 3 Implementation-defined behavior ................ 521
J. 4 Locale-specific behavior ................... 528
J. 5 Common extensions ..................... 529
Bibliography .......................... 532
Index ............................. 535
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Programming languages C
Foreword
1 ISO (the International Organization for Standardization) and IEC (the International
Electrotechnical Commission) form the specialized system for worldwide
standardization. National bodies that are member of ISO or IEC participate in the
development of International Standards through technical committees established by the
respective org anization to deal with particular fields of technical activity. ISO and IEC
technical committees collaborate in fields of mutual interest. Other international
organizations, governmental and non-governmental, in liaison with ISO and IEC, also
take part in the work.
2 International Standards are drafted in accordance with the rules given in the ISO/ IEC
Directives, Part 3. *
3 In the field of information technology, ISO and IEC have established a joint technical
committee, ISO/ IEC JTC 1. Draft International Standards adopted by the joint technical
committee are circulated to national bodies for voting. Publication as an International
Standard requires approval by at least 75% of the national bodies casting a vote.
4 International Standard ISO/ IEC 9899 was prepared by Joint Technical Committee
ISO/ IEC JTC 1, '' Information Technology'', subcommittee 22, '' Programming
languages, their environments and system software interfaces''. The Working Group
responsible for this standard (WG14) maintains a site on the World Wide Web at
http:// www. dkuug. dk/ JTC1/ SC22/ WG14/ containing additional information
relevant to this standard such as a Rationale for many of the decisions made during its
preparation and a log of Defect Reports and Responses.
5 This edition replaces the previous edition, ISO/ IEC 9899: 1990, as amended and corrected
by ISO/ IEC 9899/ COR1: 1994, ISO/ IEC 9899/ COR2: 1995, and ISO/ IEC
9899/ AMD1: 1995. Major changes from the previous edition include:
restricted character set support in <iso646. h> (originally specified in AMD1)
wide-character library support in <wchar. h> and <wctype. h> (originally
specified in AMD1)
restricted pointers
variable-length arrays
Foreword
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flexible array members
complex (and imaginary) support in <complex. h>
type-generic math macros in <tgmath. h>
the long long int type and library functions
increased translation limits
remove implicit int
the vscanf family of functions
reliable integer division
universal character names
extended identifiers
binary floating-point literals and printf/ scanf conversion specifiers
compound literals
designated initializers
// comments
extended integer types in <inttypes. h> and <stdint. h>
remove implicit function declaration
preprocessor arithmetic done in intmax_ t/ uintmax_ t
mixed declarations and code
integer constant type rules
integer promotion rules
vararg macros
additional math library functions in <math. h>
floating-point environment access in <fenv. h>
IEC 60559 (also known as IEC 559 or IEEE arithmetic) support
trailing comma allowed in enum declaration
%lf conversion specifier allowed in printf
inline functions
the snprintf family of functions
Foreword
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boolean type in <stdbool. h>
idempotent type qualifiers
empty macro arguments
new struct type compatibility rules (tag compatibility)
_Prama preprocessing operator
standard pragmas
__ func__ predefined identifier
VA_ COPY macro
additional strftime conversion specifiers
LIA compatibility annex
deprecate ungetc at the beginning of a binary file
remove deprecation of aliased array parameters
6 Annexes D and F form a normative part of this standard; annexes A, B, C, E, G, H, I, J,
the bibliography, and the index are for information only. In accordance with the ISO/ IEC
Directives, Part 3, this foreword, the introduction, notes, footnotes, and examples are for
information only.
Foreword
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Introduction
1 With the introduction of new devices and extended character sets, new features may be
added to this International Standard. Subclauses in the language and library clauses warn
implementors and programmers of usages which, though valid in themselves, may
conflict with future additions.
2 Certain features are obsolescent, which means that they may be considered for
withdrawal in future revisions of this International Standard. They are retained because
of their widespread use, but their use in new implementations (for implementation
features) or new programs (for language [6.11] or library features [7.26]) is discouraged.
3 This International Standard is divided into four major subdivisions:
the introduction and preliminary elements (clauses 14);
the characteristics of environments that translate and execute C programs (clause 5);
the language syntax, constraints, and semantics (clause 6);
the library facilities (clause 7).
4 Examples are provided to illustrate possible forms of the constructions described.
Footnotes are provided to emphasize consequences of the rules described in that
subclause or elsewhere in this International Standard. References are used to refer to
other related subclauses. Recommendations are provided to give advice or guidance to
implementors. Annexes provide additional information and summarize the information
contained in this International Standard. A bibliography lists documents that were
referred to during the preparation of the standard.
5 The language clause (clause 6) is derived from '' The C Reference Manual''.
6 The library clause (clause 7) is based on the 1984 /usr/ group Standard.
Introduction
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1. Scope
1 This International Standard specifies the form and establishes the interpretation of
programs written in the C programming language. 1) It specifies
the representation of C programs;
the syntax and constraints of the C language;
the semantic rules for interpreting C programs;
the representation of input data to be processed by C programs;
the representation of output data produced by C programs;
the restrictions and limits imposed by a conforming implementation of C.
2 This International Standard does not specify
the mechanism by which C programs are transformed for use by a data-processing
system;
the mechanism by which C programs are invoked for use by a data-processing
system;
the mechanism by which input data are transformed for use by a C program;
the mechanism by which output data are transformed after being produced by a C
program;
the size or complexity of a program and its data that will exceed the capacity of any
specific data-processing system or the capacity of a particular processor;
all minimal requirements of a data-processing system that is capable of supporting a
conforming implementation.
1) This International Standard is designed to promote the portability of C programs among a variety of
data-processing systems. It is intended for use by implementors and programmers.
1 General 1
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2. Normative references
1 The following normative documents contain provisions which, through reference in this
text, constitute provisions of this International Standard. For dated references,
subsequent amendments to, or revisions of, any of these publications do not apply.
However, parties to agreements based on this International Standard are encouraged to
investigate the possibility of applying the most recent editions of the normative
documents indicated below. For undated references, the latest edition of the normative
document referred to applies. Members of ISO and IEC maintain registers of currently
valid International Standards.
2 ISO/ IEC 646: 1991, Information technology ISO 7-bit coded character set for
information interchange.
3 ISO/ IEC 23821: 1993, Information technology Vocabulary Part 1: Fundamental
terms.
4 ISO 4217: 1995, Codes for the representation of currencies and funds.
5 ISO 8601: 1988, Data elements and interchange formats Information interchange
Representation of dates and times.
6 ISO/ IEC 10646: 1993, Information technology Universal Multiple-Octet Coded
Character Set (UCS).
7 IEC 60559: 1989, Binary floating-point arithmetic for microprocessor systems, second
edition (previously designated IEC 559: 1989).
3. Terms and definitions
1 For the purposes of this International Standard, the following definitions apply. Other
terms are defined where they appear in italic type or on the left side of a syntax rule.
Terms explicitly defined in this International Standard are not to be presumed to refer
implicitly to similar terms defined elsewhere. Terms not defined in this International
Standard are to be interpreted according to ISO/ IEC 23821.
3.1 1 alignment
requirement that objects of a particular type be located on storage boundaries with
addresses that are particular multiples of a byte address
3.2 1 argument
actual argument
actual parameter (deprecated)
expression in the comma-separated list bounded by the parentheses in a function call
expression, or a sequence of preprocessing tokens in the comma-separated list bounded
2 General 3.2
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by the parentheses in a function-like macro invocation
3.3 1 bit
unit of data storage in the execution environment large enough to hold an object that may
have one of two values
2 NOTE It need not be possible to express the address of each individual bit of an object.
3.4 1 byte
addressable unit of data storage large enough to hold any member of the basic character
set of the execution environment
2 NOTE 1 It is possible to express the address of each individual byte of an object uniquely.
3 NOTE 2 A byte is composed of a contiguous sequence of bits, the number of which is implementation-defined. The least significant bit is called the low-order bit; the most significant bit is called the high-order
bit.
3.5 1 character
bit representation that fits in a byte
3.6 1 constraints
restrictions, both syntactic and semantic, by which the exposition of language elements is
to be interpreted
3.7 1 correctly rounded result
a representation in the result format that is nearest in value, subject to the effective
rounding mode, to what the result would be given unlimited range and precision
3.8 1 diagnostic message
message belonging to an implementation-defined subset of the implementation's message
output
3.9 1 forward references
references to later subclauses of this International Standard that contain additional
information relevant to this subclause
3.10 1 implementation
a particular set of software, running in a particular translation environment under
particular control options, that performs translation of programs for, and supports
3.2 General 3.10
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execution of functions in, a particular execution environment
3.11 1 implementation-defined behavior
unspecified behavior where each implementation documents how the choice is made
2 EXAMPLE An example of implementation-defined behavior is the propagation of the high-order bit when a signed integer is shifted right.
3.12 1 implementation limits
restrictions imposed upon programs by the implementation
3.13 1 locale-specific behavior
behavior that depends on local conventions of nationality, culture, and language that each
implementation documents
2 EXAMPLE An example of locale-specific behavior is whether the islower function returns true for characters other than the 26 lowercase Latin letters.
3.14 1 multibyte character
sequence of one or more bytes representing a member of the extended character set of
either the source or the execution environment
2 NOTE The extended character set is a superset of the basic character set.
3.15 1 object
region of data storage in the execution environment, the contents of which can represent
values
2 NOTE When referenced, an object may be interpreted as having a particular type; see 6.3.2.1.
3.16 1 parameter
formal parameter
formal argument (deprecated)
object declared as part of a function declaration or definition that acquires a value on
entry to the function, or an identifier from the comma-separated list bounded by the
parentheses immediately following the macro name in a function-like macro definition
3.17 1 recommended practice
specifications that are strongly recommended as being in keeping with the intent of the
standard, but that may be impractical for some implementations
3.10 General 3.17
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3.18 1 undefined behavior
behavior, upon use of a nonportable or erroneous program construct, of erroneous data, or
of indeterminately valued objects, for which this International Standard imposes no
requirements
2 NOTE Possible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the
environment (with or without the issuance of a diagnostic message), to terminating a translation or
execution (with the issuance of a diagnostic message).
3 EXAMPLE An example of undefined behavior is the behavior on integer overflow.
3.19 1 unspecified behavior
behavior where this International Standard provides two or more possibilities and
imposes no requirements on which is chosen in any instance
2 EXAMPLE An example of unspecified behavior is the order in which the arguments to a function are evaluated.
Forward references: bitwise shift operators (6.5.7), expressions (6.5), function calls
(6.5.2.2), the islower function (7.4.1.6), localization (7.11).
4. Conformance
1 In this International Standard, '' shall'' is to be interpreted as a requirement on an
implementation or on a program; conversely, '' shall not'' is to be interpreted as a
prohibition.
2 If a '' shall'' or '' shall not'' requirement that appears outside of a constraint is violated, the
behavior is undefined. Undefined behavior is otherwise indicated in this International
Standard by the words '' undefined behavior'' or by the omission of any explicit definition
of behavior. There is no difference in emphasis among these three; they all describe
'' behavior that is undefined''.
3 A program that is correct in all other aspects, operating on correct data, containing
unspecified behavior shall be a correct program and act in accordance with 5.1.2.3.
4 The implementation shall not successfully translate a preprocessing translation unit
containing a #error preprocessing directive unless it is part of a group skipped by
conditional inclusion.
5 Astrictly conforming program shall use only those features of the language and library
specified in this International Standard. 2) It shall not produce output dependent on any
unspecified, undefined, or implementation-defined behavior, and shall not exceed any
minimum implementation limit.
3.17 General 4
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6 The two forms of conforming implementation are hosted and freestanding. A conforming
hosted implementation shall accept any strictly conforming program. A conforming
freestanding implementation shall accept any strictly conforming program that does not
use complex types and in which the use of the features specified in the library clause
(clause 7) is confined to the contents of the standard headers <float. h>,
<iso646. h>, <limits. h>, <stdarg. h>, <stdbool. h>, <stddef. h>, and
<stdint. h>. A conforming implementation may have extensions (including additional
library functions), provided they do not alter the behavior of any strictly conforming
program. 3)
7 Aconforming program is one that is acceptable to a conforming implementation. 4)
8 An implementation shall be accompanied by a document that defines all implementation-defined
and locale-specific characteristics and all extensions.
Forward references: conditional inclusion (6.10.1), characteristics of floating types
<float. h> (7.7), alternative spellings <iso646. h> (7.9), sizes of integer types
<limits. h> (7.10), variable arguments <stdarg. h> (7.15), boolean type and values
<stdbool. h> (7.16), common definitions <stddef. h> (7.17), integer types
<stdint. h> (7.18).
2) A strictly conforming program can use conditional features (such as those in annex F) provided the
use is guarded by a #ifdef directive with the appropriate macro. For example:
#ifdef _ _STDC_ IEC_ 559_ _ /* FE_ UPWARD defined */
/* ... */
fesetround( FE_ UPWARD);
/* ... */
#endif
3) This implies that a conforming implementation reserves no identifiers other than those explicitly
reserved in this International Standard.
4) Strictly conforming programs are intended to be maximally portable among conforming
implementations. Conforming programs may depend upon nonportable features of a conforming
implementation.
4 General 4
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5. Environment
1 An implementation translates C source files and executes C programs in two data-processing-
system environments, which will be called the translation environment and
the execution environment in this International Standard. Their characteristics define and
constrain the results of executing conforming C programs constructed according to the
syntactic and semantic rules for conforming implementations.
Forward references: In this clause, only a few of many possible forward references
have been noted.
5.1 Conceptual models
5.1.1 Translation environment
5.1.1.1 Program structure
1 A C program need not all be translated at the same time. The text of the program is kept
in units called source files, (or preprocessing files) in this International Standard. A
source file together with all the headers and source files included via the preprocessing
directive #include is known as a preprocessing translation unit. After preprocessing, a
preprocessing translation unit is called a translation unit. Previously translated translation
units may be preserved individually or in libraries. The separate translation units of a
program communicate by (for example) calls to functions whose identifiers have external
linkage, manipulation of objects whose identifiers have external linkage, or manipulation
of data files. Translation units may be separately translated and then later linked to
produce an executable program.
Forward references: conditional inclusion (6.10.1), linkages of identifiers (6.2.2),
source file inclusion (6.10.2), external definitions (6.9), preprocessing directives (6.10).
5.1.1.2 Translation phases
1 The precedence among the syntax rules of translation is specified by the following
phases. 5)
1. Physical source file multibyte characters are mapped to the source character set
(introducing new-line characters for end-of-line indicators) if necessary. Trigraph
sequences are replaced by corresponding single-character internal representations.
2. Each instance of a backslash character (\) immediately followed by a new-line
character is deleted, splicing physical source lines to form logical source lines. If,
as a result, a character sequence that matches the syntax of a universal character
name is produced, the behavior is undefined. Only the last backslash on any
5) Implementations shall behave as if these separate phases occur, even though many are typically folded
together in practice.
5 Environment 5.1.1.2
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physical source line shall be eligible for being part of such a splice. A source file
that is not empty shall end in a new-line character, which shall not be immediately
preceded by a backslash character before any such splicing takes place.
3. The source file is decomposed into preprocessing tokens 6) and sequences of
white-space characters (including comments). A source file shall not end in a
partial preprocessing token or in a partial comment. Each comment is replaced by
one space character. New-line characters are retained. Whether each nonempty
sequence of white-space characters other than new-line is retained or replaced by
one space character is implementation-defined.
4. Preprocessing directives are executed, macro invocations are expanded, and
_Pragma unary operator expressions are executed. If a character sequence that
matches the syntax of a universal character name is produced by token
concatenation (6.10.3.3), the behavior is undefined. A #include preprocessing
directive causes the named header or source file to be processed from phase 1
through phase 4, recursively. All preprocessing directives are then deleted.
5. Each source character set member and escape sequence in character constants and
string literals is converted to the corresponding member of the execution character
set; if there is no corresponding member, it is converted to an implementation-defined
member.
6. Adjacent string literal tokens are concatenated.
7. White-space characters separating tokens are no longer significant. Each
preprocessing token is converted into a token. The resulting tokens are
syntactically and semantically analyzed and translated as a translation unit.
8. All external object and function references are resolved. Library components are
linked to satisfy external references to functions and objects not defined in the
current translation. All such translator output is collected into a program image
which contains information needed for execution in its execution environment.
Forward references: universal character names (6.4.3), lexical elements (6.4),
preprocessing directives (6.10), trigraph sequences (5.2.1.1), external definitions (6.9).
6) As described in 6.4, the process of dividing a source file's characters into preprocessing tokens is
context-dependent. For example, see the handling of < within a #include preprocessing directive.
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5.1.1.3 Diagnostics
1 A conforming implementation shall produce at least one diagnostic message (identified in
an implementation-defined manner) if a preprocessing translation unit or translation unit
contains a violation of any syntax rule or constraint, even if the behavior is also explicitly
specified as undefined or implementation-defined. Diagnostic messages need not be
produced in other circumstances. 7)
2 EXAMPLE An implementation shall issue a diagnostic for the translation unit:
char i;
int i;
because in those cases where wording in this International Standard describes the behavior for a construct
as being both a constraint error and resulting in undefined behavior, the constraint error shall be diagnosed.
5.1.2 Execution environments
1 Tw o execution environments are defined: freestanding and hosted. In both cases,
program startup occurs when a designated C function is called by the execution
environment. All objects in static storage shall be initialized (set to their initial values)
before program startup. The manner and timing of such initialization are otherwise
unspecified. Program termination returns control to the execution environment.
Forward references: initialization (6.7.8).
5.1.2.1 Freestanding environment
1 In a freestanding environment (in which C program execution may take place without any
benefit of an operating system), the name and type of the function called at program
startup are implementation-defined. Any library facilities available to a freestanding
program, other than the minimal set required by clause 4, are implementation-defined.
2 The effect of program termination in a freestanding environment is implementation-defined.
5.1.2.2 Hosted environment
1 A hosted environment need not be provided, but shall conform to the following
specifications if present.
5.1.2.2.1 Program startup
1 The function called at program startup is named main. The implementation declares no
prototype for this function. It shall be defined with a return type of int and with no
parameters:
7) The intent is that an implementation should identify the nature of, and where possible localize, each
violation. Of course, an implementation is free to produce any number of diagnostics as long as a
valid program is still correctly translated. It may also successfully translate an invalid program.
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int main( void) { /* ... */ }
or with two parameters (referred to here as argc and argv, though any names may be
used, as they are local to the function in which they are declared):
int main( int argc, char *argv[]) { /* ... */ }
or equivalent; 8) or in some other implementation-defined manner.
2 If they are declared, the parameters to the main function shall obey the following
constraints:
The value of argc shall be nonnegative.
argv[ argc] shall be a null pointer.
If the value of argc is greater than zero, the array members argv[ 0] through
argv[ argc-1] inclusive shall contain pointers to strings, which are given
implementation-defined values by the host environment prior to program startup. The
intent is to supply to the program information determined prior to program startup
from elsewhere in the hosted environment. If the host environment is not capable of
supplying strings with letters in both uppercase and lowercase, the implementation
shall ensure that the strings are received in lowercase.
If the value of argc is greater than zero, the string pointed to by argv[ 0]
represents the program name; argv[ 0][ 0] shall be the null character if the
program name is not available from the host environment. If the value of argc is
greater than one, the strings pointed to by argv[ 1] through argv[ argc-1]
represent the program parameters.
The parameters argc and argv and the strings pointed to by the argv array shall
be modifiable by the program, and retain their last-stored values between program
startup and program termination.
5.1.2.2.2 Program execution
1 In a hosted environment, a program may use all the functions, macros, type definitions,
and objects described in the library clause (clause 7).
5.1.2.2.3 Program termination
1 If the return type of the main function is a type compatible with int, a return from the
initial call to the main function is equivalent to calling the exit function with the value
returned by the main function as its argument; 9) reaching the } that terminates the main
8) Thus, int can be replaced by a typedef name defined as int, or the type of argv can be written as
char ** argv, and so on.
9) In accordance with 6.2.4, objects with automatic storage duration declared in main will no longer
have storage guaranteed to be reserved in the former case even where they would in the latter.
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function returns a value of 0. If the return type is not compatible with int, the
termination status returned to the host environment is unspecified.
Forward references: definition of terms (7.1.1), the exit function (7.20.4.3).
5.1.2.3 Program execution
1 The semantic descriptions in this International Standard describe the behavior of an
abstract machine in which issues of optimization are irrelevant.
2 Accessing a volatile object, modifying an object, modifying a file, or calling a function
that does any of those operations are all side effects, 10) which are changes in the state of
the execution environment. Evaluation of an expression may produce side effects. At
certain specified points in the execution sequence called sequence points, all side effects
of previous evaluations shall be complete and no side effects of subsequent evaluations
shall have taken place. (A summary of the sequence points is given in annex C.)
3 In the abstract machine, all expressions are evaluated as specified by the semantics. An
actual implementation need not evaluate part of an expression if it can deduce that its
value is not used and that no needed side effects are produced (including any caused by
calling a function or accessing a volatile object).
4 When the processing of the abstract machine is interrupted by receipt of a signal, only the
values of objects as of the previous sequence point may be relied on. Objects that may be
modified between the previous sequence point and the next sequence point need not have
received their correct values yet.
5 An instance of each object with automatic storage duration is associated with each entry
into its block. Such an object exists and retains its last-stored value during the execution
of the block and while the block is suspended (by a call of a function or receipt of a
signal).
6 The least requirements on a conforming implementation are:
At sequence points, volatile objects are stable in the sense that previous accesses are
complete and subsequent accesses have not yet occurred.
At program termination, all data written into files shall be identical to the result that
execution of the program according to the abstract semantics would have produced.
10) The IEC 60559 standard for binary floating-point arithmetic requires certain user-accessible status
flags and control modes. Floating-point operations implicitly set the status flags; modes affect result
values of floating-point operations. Implementations that support such floating-point state are
required to regard changes to it as side effects see annex F for details. The floating-point
environment library <fenv. h> provides a programming facility for indicating when these side
effects matter, freeing the implementations in other cases.
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The input and output dynamics of interactive devices shall take place as specified in
7.19.3. The intent of these requirements is that unbuffered or line-buffered output
appear as soon as possible, to ensure that prompting messages actually appear prior to
a program waiting for input.
7 What constitutes an interactive device is implementation-defined.
8 More stringent correspondences between abstract and actual semantics may be defined by
each implementation.
9 EXAMPLE 1 An implementation might define a one-to-one correspondence between abstract and actual semantics: at every sequence point, the values of the actual objects would agree with those specified by the
abstract semantics. The keyword volatile would then be redundant.
10 Alternatively, an implementation might perform various optimizations within each translation unit, such that the actual semantics would agree with the abstract semantics only when making function calls across
translation unit boundaries. In such an implementation, at the time of each function entry and function
return where the calling function and the called function are in different translation units, the values of all
externally linked objects and of all objects accessible via pointers therein would agree with the abstract
semantics. Furthermore, at the time of each such function entry the values of the parameters of the called
function and of all objects accessible via pointers therein would agree with the abstract semantics. In this
type of implementation, objects referred to by interrupt service routines activated by the signal function
would require explicit specification of volatile storage, as well as other implementation-defined
restrictions.
11 EXAMPLE 2 In executing the fragment
char c1, c2;
/* ... */
c1 = c1 + c2;
the '' integer promotions'' require that the abstract machine promote the value of each variable to int size
and then add the two ints and truncate the sum. Provided the addition of two chars can be done without
overflow, or with overflow wrapping silently to produce the correct result, the actual execution need only
produce the same result, possibly omitting the promotions.
12 EXAMPLE 3 Similarly, in the fragment
float f1, f2;
double d;
/* ... */
f1 = f2 * d;
the multiplication may be executed using single-precision arithmetic if the implementation can ascertain
that the result would be the same as if it were executed using double-precision arithmetic (for example, if d
were replaced by the constant 2.0, which has type double).
13 EXAMPLE 4 Implementations employing wide registers have to take care to honor appropriate semantics. Values are independent of whether they are represented in a register or in memory. For
example, an implicit spilling of a register is not permitted to alter the value. Also, an explicit store and load
is required to round to the precision of the storage type. In particular, casts and assignments are required to
perform their specified conversion. For the fragment
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double d1, d2;
float f;
d1 = f = expression;
d2 = (float) expressions;
the values assigned to d1 and d2 are required to have been converted to float.
14 EXAMPLE 5 Rearrangement for floating-point expressions is often restricted because of limitations in precision as well as range. The implementation cannot generally apply the mathematical associative rules
for addition or multiplication, nor the distributive rule, because of roundoff error, even in the absence of
overflow and underflow. Likewise, implementations cannot generally replace decimal constants in order to
rearrange expressions. In the following fragment, rearrangements suggested by mathematical rules for real
numbers are often not valid (see F. 8).
double x, y, z;
/* ... */
x =(x *y)* z; // not equivalent to x *=y *z;
z =(x -y)+y ;// not equivalent to z =x;
z =x +x *y; // not equivalent to z =x *( 1.0 + y);
y =x /5.0; // not equivalent to y =x *0.2;
15 EXAMPLE 6 To illustrate the grouping behavior of expressions, in the following fragment
int a, b;
/* ... */
a =a +32760 + b + 5;
the expression statement behaves exactly the same as
a = ((( a + 32760) + b) + 5);
due to the associativity and precedence of these operators. Thus, the result of the sum (a + 32760) is
next added to b, and that result is then added to 5 which results in the value assigned to a. On a machine in
which overflows produce an explicit trap and in which the range of values representable by an int is
[32768, +32767], the implementation cannot rewrite this expression as
a = (( a + b) + 32765);
since if the values for a and b were, respectively, 32754 and 15, the sum a +bwould produce a trap
while the original expression would not; nor can the expression be rewritten either as
a = (( a + 32765) + b);
or
a = (a + (b + 32765));
since the values for a and b might have been, respectively, 4 and 8 or 17 and 12. However, on a machine
in which overflow silently generates some value and where positive and negative overflows cancel, the
above expression statement can be rewritten by the implementation in any of the above ways because the
same result will occur.
16 EXAMPLE 7 The grouping of an expression does not completely determine its evaluation. In the following fragment
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#include <stdio. h>
int sum;
char *p;
/* ... */
sum = sum * 10 -'0' + (* p++ = getchar());
the expression statement is grouped as if it were written as
sum = ((( sum * 10) -'0') + ((*( p++)) = (getchar())));
but the actual increment of p can occur at any time between the previous sequence point and the next
sequence point (the ;), and the call to getchar can occur at any point prior to the need of its returned
value.
Forward references: compound statement, or block (6.8.2), expressions (6.5), files
(7.19.3), sequence points (6.5, 6.8), the signal function (7.14), type qualifiers (6.7.3).
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5.2 Environmental considerations
5.2.1 Character sets
1 Tw o sets of characters and their associated collating sequences shall be defined: the set in
which source files are written, and the set interpreted in the execution environment. The
values of the members of the execution character set are implementation-defined; any
additional members beyond those required by this subclause are locale-specific.
2 In a character constant or string literal, members of the execution character set shall be
represented by corresponding members of the source character set or by escape
sequences consisting of the backslash \ followed by one or more characters. A byte with
all bits set to 0, called the null character, shall exist in the basic execution character set; it
is used to terminate a character string.
3 Both the basic source and basic execution character sets shall have at least the following
members: the 26 uppercase letters of the Latin alphabet
A B C D E F G H I J K L M
N O P Q R S T U V W X Y Z
the 26 lowercase letters of the Latin alphabet
a b c d e f g h i j k l m
n o p q r s t u v w x y z
the 10 decimal digits
0 1 2 3 4 5 6 7 8 9
the following 29 graphic characters
!"#%&' ()*+,-./:
;<=>?[\]^_{|}~
the space character, and control characters representing horizontal tab, vertical tab, and
form feed. The representation of each member of the source and execution basic
character sets shall fit in a byte. In both the source and execution basic character sets, the
value of each character after 0 in the above list of decimal digits shall be one greater than
the value of the previous. In source files, there shall be some way of indicating the end of
each line of text; this International Standard treats such an end-of-line indicator as if it
were a single new-line character. In the execution character set, there shall be control
characters representing alert, backspace, carriage return, and new line. If any other
characters are encountered in a source file (except in an identifier, a character constant, a
string literal, a header name, a comment, or a preprocessing token that is never converted
to a token), the behavior is undefined.
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4 The universal character name construct provides a way to name other characters.
Forward references: universal character names (6.4.3), character constants (6.4.4.4),
preprocessing directives (6.10), string literals (6.4.5), comments (6.4.9), string (7.1.1).
5.2.1.1 Trigraph sequences
1 All occurrences in a source file of the following sequences of three characters (called
trigraph sequences 11) ) are replaced with the corresponding single character.
??= #
??( [
??/ \
??) ]
?? ' ^
??< {
??! |
??> }
??-~
No other trigraph sequences exist. Each ? that does not begin one of the trigraphs listed
above is not changed.
2 EXAMPLE The following source line
printf(" Eh???/ n");
becomes (after replacement of the trigraph sequence ??/)
printf(" Eh?\ n");
5.2.1.2 Multibyte characters
1 The source character set may contain multibyte characters, used to represent members of
the extended character set. The execution character set may also contain multibyte
characters, which need not have the same encoding as for the source character set. For
both character sets, the following shall hold:
The single-byte characters defined in 5.2.1 shall be present.
The presence, meaning, and representation of any additional members is locale-specific.
A multibyte character set may have a state-dependent encoding, wherein each
sequence of multibyte characters begins in an initial shift state and enters other
locale-specific shift states when specific multibyte characters are encountered in the
sequence. While in the initial shift state, all single-byte characters retain their usual
interpretation and do not alter the shift state. The interpretation for subsequent bytes
in the sequence is a function of the current shift state.
A byte with all bits zero shall be interpreted as a null character independent of shift
state.
11) The trigraph sequences enable the input of characters that are not defined in the Invariant Code Set as
described in ISO/ IEC 646, which is a subset of the seven-bit US ASCII code set.
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A byte with all bits zero shall not occur in the second or subsequent bytes of a
multibyte character.
2 For source files, the following shall hold:
An identifier, comment, string literal, character constant, or header name shall begin
and end in the initial shift state.
An identifier, comment, string literal, character constant, or header name shall consist
of a sequence of valid multibyte characters.
5.2.2 Character display semantics
1 The active position is that location on a display device where the next character output by
the fputc or fputwc function would appear. The intent of writing a printing character
(as defined by the isprint or iswprint function) to a display device is to display a
graphic representation of that character at the active position and then advance the active
position to the next position on the current line. The direction of writing is locale-specific.
If the active position is at the final position of a line (if there is one), the
behavior is unspecified.
2 Alphabetic escape sequences representing nongraphic characters in the execution
character set are intended to produce actions on display devices as follows:
\a (alert) Produces an audible or visible alert. The active position shall not be changed.
\b (backspace) Moves the active position to the previous position on the current line. If
the active position is at the initial position of a line, the behavior is unspecified.
\f ( form feed) Moves the active position to the initial position at the start of the next
logical page.
\n (new line) Moves the active position to the initial position of the next line.
\r (carriage return) Moves the active position to the initial position of the current line.
\t (horizontal tab) Moves the active position to the next horizontal tabulation position
on the current line. If the active position is at or past the last defined horizontal
tabulation position, the behavior is unspecified.
\v (vertical tab) Moves the active position to the initial position of the next vertical
tabulation position. If the active position is at or past the last defined vertical
tabulation position, the behavior is unspecified.
3 Each of these escape sequences shall produce a unique implementation-defined value
which can be stored in a single char object. The external representations in a text file
need not be identical to the internal representations, and are outside the scope of this
International Standard.
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Forward references: the isprint function (7.4.1.7), the fputc function (7.19.7.3),
the fputwc functions (7.24.3.3), the iswprint function (7.25.2.1.7).
5.2.3 Signals and interrupts
1 Functions shall be implemented such that they may be interrupted at any time by a signal,
or may be called by a signal handler, or both, with no alteration to earlier, but still active,
invocations' control flow (after the interruption), function return values, or objects with
automatic storage duration. All such objects shall be maintained outside the function
image (the instructions that compose the executable representation of a function) on a
per-invocation basis.
5.2.4 Environmental limits
1 Both the translation and execution environments constrain the implementation of
language translators and libraries. The following summarizes the language-related
environmental limits on a conforming implementation; the library-related limits are
discussed in clause 7.
5.2.4.1 Translation limits
1 The implementation shall be able to translate and execute at least one program that
contains at least one instance of every one of the following limits: 12)
127 nesting levels of blocks
63 nesting levels of conditional inclusion
12 pointer, array, and function declarators (in any combinations) modifying an
arithmetic, structure, union, or incomplete type in a declaration
63 nesting levels of parenthesized declarators within a full declarator
63 nesting levels of parenthesized expressions within a full expression
63 significant initial characters in an internal identifier or a macro name (each
universal character name or extended source character is considered a single
character)
31 significant initial characters in an external identifier (each universal character name
specifying a character short identifier of 0000FFFF or less is considered 6 characters,
each universal character name specifying a character short identifier of 00010000 or
more is considered 10 characters, and each extended source character is considered
the same number of characters as the corresponding universal character name, if any)
12) Implementations should avoid imposing fixed translation limits whenever possible.
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4095 external identifiers in one translation unit
511 identifiers with block scope declared in one block
4095 macro identifiers simultaneously defined in one preprocessing translation unit
127 parameters in one function definition
127 arguments in one function call
127 parameters in one macro definition
127 arguments in one macro invocation
4095 characters in a logical source line
4095 characters in a character string literal or wide string literal (after concatenation)
65535 bytes in an object (in a hosted environment only)
15 nesting levels for #included files
1023 case labels for a switch statement (excluding those for any nested switch
statements)
1023 members in a single structure or union
1023 enumeration constants in a single enumeration
63 lev els of nested structure or union definitions in a single struct-declaration-list
5.2.4.2 Numerical limits
1 A conforming implementation shall document all the limits specified in this subclause,
which are specified in the headers <limits. h> and <float. h>. Additional limits are
specified in <stdint. h>.
5.2.4.2.1 Sizes of integer types <limits. h>
1 The values given below shall be replaced by constant expressions suitable for use in #if
preprocessing directives. Moreover, except for CHAR_ BIT and MB_ LEN_ MAX, the
following shall be replaced by expressions that have the same type as would an
expression that is an object of the corresponding type converted according to the integer
promotions. Their implementation-defined values shall be equal or greater in magnitude
(absolute value) to those shown, with the same sign.
number of bits for smallest object that is not a bit-field (byte)
CHAR_ BIT 8
minimum value for an object of type signed char
SCHAR_ MIN -127 // -( 2 7 -1)
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maximum value for an object of type signed char
SCHAR_ MAX +127 // 2 7 -1
maximum value for an object of type unsigned char
UCHAR_ MAX 255 // 2 8 -1
minimum value for an object of type char
CHAR_ MIN see below
maximum value for an object of type char
CHAR_ MAX see below
maximum number of bytes in a multibyte character, for any supported locale
MB_ LEN_ MAX 1
minimum value for an object of type short int
SHRT_ MIN -32767 // -( 2 15 -1)
maximum value for an object of type short int
SHRT_ MAX +32767 // 2 15 -1
maximum value for an object of type unsigned short int
USHRT_ MAX 65535 // 2 16 -1
minimum value for an object of type int
INT_ MIN -32767 // -( 2 15 -1)
maximum value for an object of type int
INT_ MAX +32767 // 2 15 -1
maximum value for an object of type unsigned int
UINT_ MAX 65535 // 2 16 -1
minimum value for an object of type long int
LONG_ MIN -2147483647 // -( 2 31 -1)
maximum value for an object of type long int
LONG_ MAX +2147483647 // 2 31 -1
maximum value for an object of type unsigned long int
ULONG_ MAX 4294967295 // 2 32 -1
minimum value for an object of type long long int
LLONG_ MIN -9223372036854775807 // -( 2 63 -1)
maximum value for an object of type long long int
LLONG_ MAX +9223372036854775807 // 2 63 -1
maximum value for an object of type unsigned long long int
ULLONG_ MAX 18446744073709551615 // 2 64 -1
5.2.4.2.1 Environment 5.2.4.2.1
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2 If the value of an object of type char is treated as a signed integer when used in an
expression, the value of CHAR_ MIN shall be the same as that of SCHAR_ MIN and the
value of CHAR_ MAX shall be the same as that of SCHAR_ MAX. Otherwise, the value of
CHAR_ MIN shall be 0 and the value of CHAR_ MAX shall be the same as that of
UCHAR_ MAX. 13) The value UCHAR_ MAX+ 1 shall equal 2 raised to the power
CHAR_ BIT.
5.2.4.2.2 Characteristics of floating types <float. h>
1 The characteristics of floating types are defined in terms of a model that describes a
representation of floating-point numbers and values that provide information about an
implementation's floating-point arithmetic. 14) The following parameters are used to
define the model for each floating-point type:
s sign (±1)
b base or radix of exponent representation (an integer > 1)
e exponent (an integer between a minimum emin and a maximum emax )
p precision (the number of base-b digits in the significand)
fk nonnegative integers less than b (the significand digits)
2 A normalized floating-point number x ( f1 >0 ifxΉ0) is defined by the following model:
x = s ΄ b e ΄ p k=1 S fk ΄ b -k , emin £ e £ emax
3 Floating types may include values that are not normalized floating-point numbers, for
example subnormal floating-point numbers (x Ή 0, e = emin , f1 = 0), infinities, and
NaNs. 15) A NaN is an encoding signifying Not-a-Number. A quiet NaN propagates
through almost every arithmetic operation without raising an exception; a signaling NaN
generally raises an exception when occurring as an arithmetic operand. 16)
4 The accuracy of the floating-point operations (+, -, *, /) and of the library functions in
<math. h> and <complex. h> that return floating-point results is implementation
defined. The implementation may state that the accuracy is unknown.
13) See 6.2.5.
14) The floating-point model is intended to clarify the description of each floating-point characteristic and
does not require the floating-point arithmetic of the implementation to be identical.
15) Although they are stored in floating types, infinities and NaNs are not floating-point numbers.
16) IEC 60559: 1989 specifies quiet and signaling NaNs. For implementations that do not support IEC
60559: 1989, the terms quiet NaN and signaling NaN are intended to apply to encodings with similar
behavior.
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5 All integer values in the <float. h> header, except FLT_ ROUNDS, shall be constant
expressions suitable for use in #if preprocessing directives; all floating values shall be
constant expressions. All except DECIMAL_ DIG, FLT_ EVAL_ METHOD, FLT_ RADIX,
and FLT_ ROUNDS have separate names for all three floating-point types. The floating-point
model representation is provided for all values except FLT_ EVAL_ METHOD and
FLT_ ROUNDS.
6 The rounding mode for floating-point addition is characterized by the value of
FLT_ ROUNDS: 17)
-1 indeterminable
0 toward zero
1 to nearest
2 toward positive infinity
3 toward negative infinity
All other values for FLT_ ROUNDS characterize implementation-defined rounding
behavior.
7 The values of operations with floating operands and values subject to the usual arithmetic
conversions and of floating constants are evaluated to a format whose range and precision
may be greater than required by the type. The use of evaluation formats is characterized
by the value of FLT_ EVAL_ METHOD: 18)
-1 indeterminable;
0 evaluate all operations and constants just to the range and precision of the
type;
1 evaluate operations and constants of type float and double to the
range and precision of the double type, evaluate long double
operations and constants to the range and precision of the long double
type;
2 evaluate all operations and constants to the range and precision of the
long double type.
All other negative values for FLT_ EVAL_ METHOD characterize implementation-defined
behavior.
17) Evaluation of FLT_ ROUNDS correctly reflects any execution-time change of rounding mode through
the function fesetround in <fenv. h>.
18) The evaluation method determines evaluation formats of expressions involving all floating types, not
just real types. For example, if FLT_ EVAL_ METHOD is 1, then the product of two float
_Complex operands is represented in the double _Complex format, and its parts are evaluated to
double.
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8 The values given in the following list shall be replaced by implementation-defined
constant expressions with values that are greater or equal in magnitude (absolute value) to
those shown, with the same sign:
radix of exponent representation, b
FLT_ RADIX 2
number of base-FLT_ RADIX digits in the floating-point significand, p
FLT_ MANT_ DIG
DBL_ MANT_ DIG
LDBL_ MANT_ DIG
number of decimal digits, n, such that any floating-point number in the widest
supported floating type with pmax radix b digits can be rounded to a floating-point
number with n decimal digits and back again without change to the value, pmax
΄ log 10 b
ι 1 + pmax ΄ log 10 b ω
if b is a power of 10
otherwise
DECIMAL_ DIG 10
number of decimal digits, q, such that any floating-point number with q decimal digits
can be rounded into a floating-point number with p radix b digits and back again
without change to the q decimal digits, p
΄ log 10 b
λ (p -1) ΄ log 10 b ϋ
if b is a power of 10
otherwise
FLT_ DIG 6
DBL_ DIG 10
LDBL_ DIG 10
minimum negative integer such that FLT_ RADIX raised to one less than that power is
a normalized floating-point number, emin
FLT_ MIN_ EXP
DBL_ MIN_ EXP
LDBL_ MIN_ EXP
minimum negative integer such that 10 raised to that power is in the range of
normalized floating-point numbers, ι κ log 10 b e min-1 ω ϊ
FLT_ MIN_ 10_ EXP -37
DBL_ MIN_ 10_ EXP -37
LDBL_ MIN_ 10_ EXP -37
maximum integer such that FLT_ RADIX raised to one less than that power is a
representable finite floating-point number, emax
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FLT_ MAX_ EXP
DBL_ MAX_ EXP
LDBL_ MAX_ EXP
maximum integer such that 10 raised to that power is in the range of representable
finite floating-point numbers, λ log 10 (( 1 -b -p ) ΄ b e max ) ϋ
FLT_ MAX_ 10_ EXP +37
DBL_ MAX_ 10_ EXP +37
LDBL_ MAX_ 10_ EXP +37
9 The values given in the following list shall be replaced by implementation-defined
constant expressions with values that are greater than or equal to those shown:
maximum representable finite floating-point number, (1 -b -p ) ΄ b e max
FLT_ MAX 1E+ 37
DBL_ MAX 1E+ 37
LDBL_ MAX 1E+ 37
10 The values given in the following list shall be replaced by implementation-defined
constant expressions with (positive) values that are less than or equal to those shown:
the difference between 1 and the least value greater than 1 that is representable in the
given floating point type, b 1-p
FLT_ EPSILON 1E-5
DBL_ EPSILON 1E-9
LDBL_ EPSILON 1E-9
minimum normalized positive floating-point number, b e min-1
FLT_ MIN 1E-37
DBL_ MIN 1E-37
LDBL_ MIN 1E-37
11 EXAMPLE 1 The following describes an artificial floating-point representation that meets the minimum requirements of this International Standard, and the appropriate values in a <float. h> header for type
float:
x = s ΄ 16 e ΄ 6 k=1 S fk ΄ 16 -k , -31 £ e £ +32
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FLT_ RADIX 16
FLT_ MANT_ DIG 6
FLT_ EPSILON 9.53674316E-07F
FLT_ DIG 6
FLT_ MIN_ EXP -31
FLT_ MIN 2.93873588E-39F
FLT_ MIN_ 10_ EXP -38
FLT_ MAX_ EXP +32
FLT_ MAX 3.40282347E+ 38F
FLT_ MAX_ 10_ EXP +38
12 EXAMPLE 2 The following describes floating-point representations that also meet the requirements for single-precision and double-precision normalized numbers in IEC 60559, 19) and the appropriate values in a
<float. h> header for types float and double:
x f = s ΄ 2 e ΄ 24 k=1 S fk ΄ 2 -k , -125 £ e £ +128
xd = s ΄ 2 e ΄ 53 k=1 S fk ΄ 2 -k , -1021 £ e £ +1024
FLT_ RADIX 2
DECIMAL_ DIG 17
FLT_ MANT_ DIG 24
FLT_ EPSILON 1.19209290E-07F // decimal constant
FLT_ EPSILON 0X1P-23F // hex constant
FLT_ DIG 6
FLT_ MIN_ EXP -125
FLT_ MIN 1.17549435E-38F // decimal constant
FLT_ MIN 0X1P-126F // hex constant
FLT_ MIN_ 10_ EXP -37
FLT_ MAX_ EXP +128
FLT_ MAX 3.40282347E+ 38F // decimal constant
FLT_ MAX 0X1. fffffeP127F // hex constant
FLT_ MAX_ 10_ EXP +38
DBL_ MANT_ DIG 53
DBL_ EPSILON 2.2204460492503131E-16 // decimal constant
DBL_ EPSILON 0X1P-52 // hex constant
DBL_ DIG 15
DBL_ MIN_ EXP -1021
DBL_ MIN 2.2250738585072014E-308 // decimal constant
DBL_ MIN 0X1P-1022 // hex constant
DBL_ MIN_ 10_ EXP -307
DBL_ MAX_ EXP +1024
DBL_ MAX 1.7976931348623157E+ 308 // decimal constant
DBL_ MAX 0X1. ffffffffffffeP1023 // hex constant
DBL_ MAX_ 10_ EXP +308
If a type wider than double were supported, then DECIMAL_ DIG would be greater than 17. For
19) The floating-point model in that standard sums powers of b from zero, so the values of the exponent
limits are one less than shown here.
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example, if the widest type were to use the minimal-width IEC 60559 double-extended format (64 bits of
precision), then DECIMAL_ DIG would be 21.
Forward references: conditional inclusion (6.10.1), complex arithmetic
<complex. h> (7.3), mathematics <math. h> (7.12), integer types <stdint. h>
(7.18).
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6. Language
6.1 Notation
1 In the syntax notation used in this clause, syntactic categories (nonterminals) are
indicated by italic type, and literal words and character set members (terminals) by bold
type. A colon (:) following a nonterminal introduces its definition. Alternative
definitions are listed on separate lines, except when prefaced by the words '' one of''. An
optional symbol is indicated by the subscript '' opt'', so that
{ expression opt }
indicates an optional expression enclosed in braces.
2 A summary of the language syntax is given in annex A.
6.2 Concepts
6.2.1 Scopes of identifiers
1 An identifier can denote an object; a function; a tag or a member of a structure, union, or
enumeration; a typedef name; a label name; a macro name; or a macro parameter. The
same identifier can denote different entities at different points in the program. A member
of an enumeration is called an enumeration constant. Macro names and macro
parameters are not considered further here, because prior to the semantic phase of
program translation any occurrences of macro names in the source file are replaced by the
preprocessing token sequences that constitute their macro definitions.
2 For each different entity that an identifier designates, the identifier is visible (i. e., can be
used) only within a region of program text called its scope. Different entities designated
by the same identifier either have different scopes, or are in different name spaces. There
are four kinds of scopes: function, file, block, and function prototype. (A function
prototype is a declaration of a function that declares the types of its parameters.)
3 A label name is the only kind of identifier that has function scope. It can be used (in a
goto statement) anywhere in the function in which it appears, and is declared implicitly
by its syntactic appearance (followed by a : and a statement). *
4 Every other identifier has scope determined by the placement of its declaration (in a
declarator or type specifier). If the declarator or type specifier that declares the identifier
appears outside of any block or list of parameters, the identifier has file scope, which
terminates at the end of the translation unit. If the declarator or type specifier that
declares the identifier appears inside a block or within the list of parameter declarations in
a function definition, the identifier has block scope, which terminates at the end of the
associated block. If the declarator or type specifier that declares the identifier appears
within the list of parameter declarations in a function prototype (not part of a function
definition), the identifier has function prototype scope, which terminates at the end of the
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function declarator. If an identifier designates two different entities in the same name
space, the scopes might overlap. If so, the scope of one entity (the inner scope) will be a
strict subset of the scope of the other entity (the outer scope). Within the inner scope, the
identifier designates the entity declared in the inner scope; the entity declared in the outer
scope is hidden (and not visible) within the inner scope.
5 Unless explicitly stated otherwise, where this International Standard uses the term
identifier to refer to some entity (as opposed to the syntactic construct), it refers to the
entity in the relevant name space whose declaration is visible at the point the identifier
occurs.
6 Tw o identifiers have the same scope if and only if their scopes terminate at the same
point.
7 Structure, union, and enumeration tags have scope that begins just after the appearance of
the tag in a type specifier that declares the tag. Each enumeration constant has scope that
begins just after the appearance of its defining enumerator in an enumerator list. Any
other identifier has scope that begins just after the completion of its declarator.
Forward references: compound statement, or block (6.8.2), declarations (6.7),
enumeration specifiers (6.7.2.2), function calls (6.5.2.2), function declarators (including
prototypes) (6.7.5.3), function definitions (6.9.1), the goto statement (6.8.6.1), labeled
statements (6.8.1), name spaces of identifiers (6.2.3), scope of macro definitions
(6.10.3.5), source file inclusion (6.10.2), tags (6.7.2.3), type specifiers (6.7.2).
6.2.2 Linkages of identifiers
1 An identifier declared in different scopes or in the same scope more than once can be
made to refer to the same object or function by a process called linkage. There are three
kinds of linkage: external, internal, and none.
2 In the set of translation units and libraries that constitutes an entire program, each
declaration of a particular identifier with external linkage denotes the same object or
function. Within one translation unit, each declaration of an identifier with internal
linkage denotes the same object or function. Each declaration of an identifier with no
linkage denotes a unique entity.
3 If the declaration of a file scope identifier for an object or a function contains the storage-class
specifier static, the identifier has internal linkage. 20)
4 For an identifier declared with the storage-class specifier extern in a scope in which a
prior declaration of that identifier is visible, 21) if the prior declaration specifies internal or
20) A function declaration can contain the storage-class specifier static only if it is at file scope; see
6.7.1.
21) As specified in 6.2.1, the later declaration might hide the prior declaration.
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external linkage, the linkage of the identifier at the later declaration is the same as the
linkage specified at the prior declaration. If no prior declaration is visible, or if the prior
declaration specifies no linkage, then the identifier has external linkage.
5 If the declaration of an identifier for a function has no storage-class specifier, its linkage
is determined exactly as if it were declared with the storage-class specifier extern. If
the declaration of an identifier for an object has file scope and no storage-class specifier,
its linkage is external.
6 The following identifiers have no linkage: an identifier declared to be anything other than
an object or a function; an identifier declared to be a function parameter; a block scope
identifier for an object declared without the storage-class specifier extern.
7 If, within a translation unit, the same identifier appears with both internal and external
linkage, the behavior is undefined.
Forward references: compound statement, or block (6.8.2), declarations (6.7),
expressions (6.5), external definitions (6.9).
6.2.3 Name spaces of identifiers
1 If more than one declaration of a particular identifier is visible at any point in a
translation unit, the syntactic context disambiguates uses that refer to different entities.
Thus, there are separate name spaces for various categories of identifiers, as follows:
label names (disambiguated by the syntax of the label declaration and use);
the tags of structures, unions, and enumerations (disambiguated by following any 22)
of the keywords struct, union, orenum);
the members of structures or unions; each structure or union has a separate name
space for its members (disambiguated by the type of the expression used to access the
member via the . or -> operator);
all other identifiers, called ordinary identifiers (declared in ordinary declarators or as
enumeration constants).
Forward references: enumeration specifiers (6.7.2.2), labeled statements (6.8.1),
structure and union specifiers (6.7.2.1), structure and union members (6.5.2.3), tags
(6.7.2.3).
22) There is only one name space for tags even though three are possible.
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6.2.4 Storage durations of objects
1 An object has a storage duration that determines its lifetime. There are three storage
durations: static, automatic, and allocated. Allocated storage is described in 7.20.3.
2 An object whose identifier is declared with external or internal linkage, or with the
storage-class specifier static has static storage duration. For such an object, storage is
reserved and its stored value is initialized only once, prior to program startup. The object
exists, has a constant address, and retains its last-stored value throughout the execution of
the entire program. 23)
3 An object whose identifier is declared with no linkage and without the storage-class
specifier static has automatic storage duration.
4 For such an object that does not have a variable length array type, storage is guaranteed to
be reserved for a new instance of the object on each entry into the block with which it is
associated; the initial value of the object is indeterminate. If an initialization is specified
for the object, it is performed each time the declaration is reached in the execution of the
block; otherwise, the value becomes indeterminate each time the declaration is reached.
Storage for the object is no longer guaranteed to be reserved when execution of the block
ends in any way. (Entering an enclosed block or calling a function suspends, but does not
end, execution of the current block.)
5 For such an object that does have a variable length array type, storage is guaranteed to be
reserved for a new instance of the object each time the declaration is reached in the
execution of the program. The initial value of the object is indeterminate. Storage for the
object is no longer guaranteed to be reserved when the execution of the program leaves
the scope of the declaration. 24)
6 If an object is referred to when storage is not reserved for it, the behavior is undefined.
The value of a pointer that referred to an object whose storage is no longer reserved is
indeterminate. During the time that its storage is reserved, an object has a constant
address.
Forward references: compound statement, or block (6.8.2), function calls (6.5.2.2),
declarators (6.7.5), array declarators (6.7.5.2), initialization (6.7.8).
23) The term constant address means that two pointers to the object constructed at possibly different times
will compare equal. The address may be different during two different executions of the same
program.
In the case of a volatile object, the last store need not be explicit in the program.
24) Leaving the innermost block containing the declaration, or jumping to a point in that block or an
embedded block prior to the declaration, leaves the scope of the declaration.
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6.2.5 Types
1 The meaning of a value stored in an object or returned by a function is determined by the
type of the expression used to access it. (An identifier declared to be an object is the
simplest such expression; the type is specified in the declaration of the identifier.) Types
are partitioned into object types (types that describe objects), function types (types that
describe functions), and incomplete types (types that describe objects but lack
information needed to determine their sizes).
2 An object declared as type _Bool is large enough to store the values 0 and 1.
3 An object declared as type char is large enough to store any member of the basic
execution character set. If a member of the required source character set enumerated in
5.2.1 is stored in a char object, its value is guaranteed to be positive. If any other
character is stored in a char object, the resulting value is implementation-defined but
shall be within the range of values that can be represented in that type.
4 There are five standard signed integer types, designated as signed char, short
int, int, long int, and long long int. (These and other types may be
designated in several additional ways, as described in 6.7.2.) There may also be
implementation-defined extended signed integer types. 25) The standard and extended
signed integer types are collectively called signed integer types. 26)
5 An object declared as type signed char occupies the same amount of storage as a
'' plain'' char object. A '' plain'' int object has the natural size suggested by the
architecture of the execution environment (large enough to contain any value in the range
INT_ MIN to INT_ MAX as defined in the header <limits. h>).
6 For each of the signed integer types, there is a corresponding (but different) unsigned
integer type (designated with the keyword unsigned) that uses the same amount of
storage (including sign information) and has the same alignment requirements. The type
_Bool and the unsigned integer types that correspond to the standard signed integer
types are the standard unsigned integer types. The unsigned integer types that
correspond to the extended signed integer types are the extended unsigned integer types.
The standard and extended unsigned integer types are collectively called unsigned integer
types. 27)
25) Implementation-defined keywords shall have the form of an identifier reserved for any use as
described in 7.1.3.
26) Therefore, any statement in this Standard about signed integer types also applies to the extended
signed integer types.
27) Therefore, any statement in this Standard about unsigned integer types also applies to the extended
unsigned integer types.
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7 The standard signed integer types and standard unsigned integer types are collectively
called the standard integer types, the extended signed integer types and extended
unsigned integer types are collectively called the extended integer types.
8 For any two types with the same signedness and different integer conversion rank (see
6.3.1.1), the range of values of the type with smaller integer conversion rank is a subrange
of the values of the other type.
9 The range of nonnegative values of a signed integer type is a subrange of the
corresponding unsigned integer type, and the representation of the same value in each
type is the same. 28) A computation involving unsigned operands can never overflow,
because a result that cannot be represented by the resulting unsigned integer type is
reduced modulo the number that is one greater than the largest value that can be
represented by the resulting type.
10 There are three real floating types, designated as float, double, and long
double. 29) The set of values of the type float is a subset of the set of values of the
type double; the set of values of the type double is a subset of the set of values of the
type long double.
11 There are three complex types, designated as float _Complex, double
_Complex, and long double _Complex. 30) The real floating and complex types
are collectively called the floating types.
12 For each floating type there is a corresponding real type, which is always a real floating
type. For real floating types, it is the same type. For complex types, it is the type given
by deleting the keyword _Complex from the type name.
13 Each complex type has the same representation and alignment requirements as an array
type containing exactly two elements of the corresponding real type; the first element is
equal to the real part, and the second element to the imaginary part, of the complex
number.
14 The type char, the signed and unsigned integer types, and the floating types are
collectively called the basic types. Even if the implementation defines two or more basic
types to have the same representation, they are nevertheless different types. 31)
15 The three types char, signed char, and unsigned char are collectively called
the character types. The implementation shall define char to have the same range,
representation, and behavior as either signed char or unsigned char. 32)
28) The same representation and alignment requirements are meant to imply interchangeability as
arguments to functions, return values from functions, and members of unions.
29) See '' future language directions'' (6.11.1).
30) A specification for imaginary types is in informative annex G.
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16 An enumeration comprises a set of named integer constant values. Each distinct
enumeration constitutes a different enumerated type.
17 The type char, the signed and unsigned integer types, and the enumerated types are
collectively called integer types. The integer and real floating types are collectively called
real types.
18 The void type comprises an empty set of values; it is an incomplete type that cannot be
completed.
19 Any number of derived types can be constructed from the object, function, and
incomplete types, as follows:
Anarray type describes a contiguously allocated nonempty set of objects with a
particular member object type, called the element type. 33) Array types are
characterized by their element type and by the number of elements in the array. An
array type is said to be derived from its element type, and if its element type is T, the
array type is sometimes called '' array of T''. The construction of an array type from
an element type is called '' array type derivation''.
Astructure type describes a sequentially allocated nonempty set of member objects
(and, in certain circumstances, an incomplete array), each of which has an optionally
specified name and possibly distinct type.
Aunion type describes an overlapping nonempty set of member objects, each of
which has an optionally specified name and possibly distinct type.
Afunction type describes a function with specified return type. A function type is
characterized by its return type and the number and types of its parameters. A
function type is said to be derived from its return type, and if its return type is T, the
function type is sometimes called '' function returning T''. The construction of a
function type from a return type is called '' function type derivation''.
Apointer type may be derived from a function type, an object type, or an incomplete
type, called the referenced type. A pointer type describes an object whose value
provides a reference to an entity of the referenced type. A pointer type derived from
31) An implementation may define new keywords that provide alternative ways to designate a basic (or
any other) type; this does not violate the requirement that all basic types be different.
Implementation-defined keywords shall have the form of an identifier reserved for any use as
described in 7.1.3.
32) CHAR_ MIN, defined in <limits. h>, will have one of the values 0 or SCHAR_ MIN, and this can be
used to distinguish the two options. Irrespective of the choice made, char is a separate type from the
other two and is not compatible with either.
33) Since object types do not include incomplete types, an array of incomplete type cannot be constructed.
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the referenced type T is sometimes called '' pointer to T''. The construction of a
pointer type from a referenced type is called '' pointer type derivation''.
20 These methods of constructing derived types can be applied recursively.
21 Integer and floating types are collectively called arithmetic types. Arithmetic types and
pointer types are collectively called scalar types. Array and structure types are
collectively called aggregate types. 34)
22 Each arithmetic type belongs to one type domain. The real type domain comprises the
real types. The complex type domain comprises the complex types.
23 An array type of unknown size is an incomplete type. It is completed, for an identifier of
that type, by specifying the size in a later declaration (with internal or external linkage).
A structure or union type of unknown content (as described in 6.7.2.3) is an incomplete
type. It is completed, for all declarations of that type, by declaring the same structure or
union tag with its defining content later in the same scope. A structure type containing a
flexible array member is an incomplete type that cannot be completed.
24 Array, function, and pointer types are collectively called derived declarator types. A
declarator type derivation from a type T is the construction of a derived declarator type
from T by the application of an array-type, a function-type, or a pointer-type derivation to
T.
25 A type is characterized by its type category, which is either the outermost derivation of a
derived type (as noted above in the construction of derived types), or the type itself if the
type consists of no derived types.
26 Any type so far mentioned is an unqualified type. Each unqualified type has several
qualified versions of its type, 35) corresponding to the combinations of one, two, or all
three of the const, volatile, and restrict qualifiers. The qualified or unqualified
versions of a type are distinct types that belong to the same type category and have the
same representation and alignment requirements. 28) A derived type is not qualified by the
qualifiers (if any) of the type from which it is derived.
27 A pointer to void shall have the same representation and alignment requirements as a
pointer to a character type. Similarly, pointers to qualified or unqualified versions of
compatible types shall have the same representation and alignment requirements. 28) All
pointers to structure types shall have the same representation and alignment requirements
as each other. All pointers to union types shall have the same representation and
alignment requirements as each other. Pointers to other types need not have the same
representation or alignment requirements.
34) Note that aggregate type does not include union type because an object with union type can only
contain one member at a time.
35) See 6.7.3 regarding qualified array and function types.
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28 EXAMPLE 1 The type designated as '' float *'' has type '' pointer to float''. Its type category is pointer, not a floating type. The const-qualified version of this type is designated as '' float * const''
whereas the type designated as '' const float *'' is not a qualified type its type is '' pointer to const-qualified
float'' and is a pointer to a qualified type.
29 EXAMPLE 2 The type designated as '' struct tag (*[ 5])( float) '' has type '' array of pointer to function returning struct tag''. The array has length five and the function has a single parameter of type
float. Its type category is array.
Forward references: character constants (6.4.4.4), compatible type and composite type
(6.2.7), declarations (6.7), tags (6.7.2.3), type qualifiers (6.7.3).
6.2.6 Representations of types
6.2.6.1 General
1 The representations of all types are unspecified except as stated in this subclause.
2 Except for bit-fields, objects are composed of contiguous sequences of one or more bytes,
the number, order, and encoding of which are either explicitly specified or
implementation-defined.
3 Values stored in objects of type unsigned char shall be represented using a pure
binary notation. 36)
4 Values stored in objects of any other object type consist of n ΄ CHAR_ BIT bits, where n
is the size of an object of that type, in bytes. The value may be copied into an object of
type unsigned char [n] (e. g., by memcpy); the resulting set of bytes is called the
object representation of the value. Two values (other than NaNs) with the same object
representation compare equal, but values that compare equal may have different object
representations.
5 Certain object representations need not represent a value of the object type. If the stored
value of an object has such a representation and is accessed by an lvalue expression that
does not have character type, the behavior is undefined. If such a representation is
produced by a side effect that modifies all or any part of the object by an lvalue
expression that does not have character type, the behavior is undefined. 37) Such a
representation is called a trap representation.
36) A positional representation for integers that uses the binary digits 0 and 1, in which the values
represented by successive bits are additive, begin with 1, and are multiplied by successive integral
powers of 2, except perhaps the bit with the highest position. (Adapted from the American National
Dictionary for Information Processing Systems.) A byte contains CHAR_ BIT bits, and the values of
type unsigned char range from 0 to 2 CHAR_ BIT -1.
37) Thus, an automatic variable can be initialized to a trap representation without causing undefined
behavior, but the value of the variable cannot be used until a proper value is stored in it.
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6 When a value is stored in an object of structure or union type, including in a member
object, the bytes of the object representation that correspond to any padding bytes take
unspecified values. 38) The values of padding bytes shall not affect whether the value of
such an object is a trap representation. Those bits of a structure or union object that are
in the same byte as a bit-field member, but are not part of that member, shall similarly not
affect whether the value of such an object is a trap representation.
7 When a value is stored in a member of an object of union type, the bytes of the object
representation that do not correspond to that member but do correspond to other members
take unspecified values, but the value of the union object shall not thereby become a trap
representation.
8 Where an operator is applied to a value which has more than one object representation,
which object representation is used shall not affect the value of the result. Where a value
is stored in an object using a type that has more than one object representation for that
value, it is unspecified which representation is used, but a trap representation shall not be
generated.
6.2.6.2 Integer types
1 For unsigned integer types other than unsigned char, the bits of the object
representation shall be divided into two groups: value bits and padding bits (there need
not be any of the latter). If there are N value bits, each bit shall represent a different
power of 2 between 1 and 2 N-1 , so that objects of that type shall be capable of
representing values from 0 to 2 N -1 using a pure binary representation; this shall be
known as the value representation. The values of any padding bits are unspecified. 39)
2 For signed integer types, the bits of the object representation shall be divided into three
groups: value bits, padding bits, and the sign bit. There need not be any padding bits;
there shall be exactly one sign bit. Each bit that is a value bit shall have the same value as
the same bit in the object representation of the corresponding unsigned type (if there are
M value bits in the signed type and N in the unsigned type, then M £ N). If the sign bit
is zero, it shall not affect the resulting value. If the sign bit is one, then the value shall be
modified in one of the following ways:
the corresponding value with sign bit 0 is negated;
38) Thus, for example, structure assignment may be implemented element-at-a-time or via memcpy.
39) Some combinations of padding bits might generate trap representations, for example, if one padding
bit is a parity bit. Regardless, no arithmetic operation on valid values can generate a trap
representation other than as part of an exception such as an overflow, and this cannot occur with
unsigned types. All other combinations of padding bits are alternative object representations of the
value specified by the value bits.
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the sign bit has the value -2 N ;
the sign bit has the value 1 -2 N .
3 The values of any padding bits are unspecified. 39) A valid (non-trap) object
representation of a signed integer type where the sign bit is zero is a valid object
representation of the corresponding unsigned type, and shall represent the same value.
4 The precision of an integer type is the number of bits it uses to represent values,
excluding any sign and padding bits. The width of an integer type is the same but
including any sign bit; thus for unsigned integer types the two values are the same, while
for signed integer types the width is one greater than the precision.
6.2.7 Compatible type and composite type
1 Tw o types have compatible type if their types are the same. Additional rules for
determining whether two types are compatible are described in 6.7.2 for type specifiers,
in 6.7.3 for type qualifiers, and in 6.7.5 for declarators. 40) Moreover, two structure,
union, or enumerated types declared in separate translation units are compatible if their
tags and members satisfy the following requirements: If one is declared with a tag, the
other shall be declared with the same tag. If both are completed types, then the following
additional requirements apply: there shall be a one-to-one correspondence between their
members such that each pair of corresponding members are declared with compatible
types, and such that if one member of a corresponding pair is declared with a name, the
other member is declared with the same name. For two structures, corresponding
members shall be declared in the same order. For two structures or unions, corresponding
bit-fields shall have the same widths. For two enumerations, corresponding members
shall have the same values.
2 All declarations that refer to the same object or function shall have compatible type;
otherwise, the behavior is undefined.
3 Acomposite type can be constructed from two types that are compatible; it is a type that
is compatible with both of the two types and satisfies the following conditions:
If one type is an array of known constant size, the composite type is an array of that
size; otherwise, if one type is a variable length array, the composite type is that type.
If only one type is a function type with a parameter type list (a function prototype),
the composite type is a function prototype with the parameter type list.
If both types are function types with parameter type lists, the type of each parameter
in the composite parameter type list is the composite type of the corresponding
parameters.
These rules apply recursively to the types from which the two types are derived.
40) Tw o types need not be identical to be compatible.
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4 For an identifier with internal or external linkage declared in a scope in which a prior
declaration of that identifier is visible, 41) if the prior declaration specifies internal or
external linkage, the type of the identifier at the later declaration becomes the composite
type.
5 EXAMPLE Given the following two file scope declarations:
int f( int (*)(), double (*)[ 3]);
int f( int (*)( char *), double (*)[]);
The resulting composite type for the function is:
int f( int (*)( char *), double (*)[ 3]);
Forward references: declarators (6.7.5), enumeration specifiers (6.7.2.2), structure and
union specifiers (6.7.2.1), type definitions (6.7.7), type qualifiers (6.7.3), type specifiers
(6.7.2).
41) As specified in 6.2.1, the later declaration might hide the prior declaration.
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6.3 Conversions
1 Several operators convert operand values from one type to another automatically. This
subclause specifies the result required from such an implicit conversion, as well as those
that result from a cast operation (an explicit conversion). The list in 6.3.1.8 summarizes
the conversions performed by most ordinary operators; it is supplemented as required by
the discussion of each operator in 6.5.
2 Conversion of an operand value to a compatible type causes no change to the value or the
representation.
Forward references: cast operators (6.5.4).
6.3.1 Arithmetic operands
6.3.1.1 Boolean, characters, and integers
1 Every integer type has an integer conversion rank defined as follows:
No two signed integer types shall have the same rank, even if they hav e the same
representation.
The rank of a signed integer type shall be greater than the rank of any signed integer
type with less precision.
The rank of long long int shall be greater than the rank of long int, which
shall be greater than the rank of int, which shall be greater than the rank of short
int, which shall be greater than the rank of signed char.
The rank of any unsigned integer type shall equal the rank of the corresponding
signed integer type, if any.
The rank of any standard integer type shall be greater than the rank of any extended
integer type with the same width.
The rank of char shall equal the rank of signed char and unsigned char.
The rank of _Bool shall be less than the rank of all other standard integer types.
The rank of any enumerated type shall equal the rank of the compatible integer type
(see 6.7.2.2).
The rank of any extended signed integer type relative to another extended signed
integer type with the same precision is implementation-defined, but still subject to the
other rules for determining the integer conversion rank.
For all integer types T1, T2, and T3, ifT1 has greater rank than T2 and T2 has
greater rank than T3, then T1 has greater rank than T3.
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2 The following may be used in an expression wherever an int or unsigned int may
be used:
An object or expression with an integer type whose integer conversion rank is less
than the rank of int and unsigned int.
A bit-field of type _Bool, int, signed int, orunsigned int.
If an int can represent all values of the original type, the value is converted to an int;
otherwise, it is converted to an unsigned int. These are called the integer
promotions. 42) All other types are unchanged by the integer promotions.
3 The integer promotions preserve value including sign. As discussed earlier, whether a
'' plain'' char is treated as signed is implementation-defined.
Forward references: enumeration specifiers (6.7.2.2), structure and union specifiers
(6.7.2.1).
6.3.1.2 Boolean type
1 When any scalar value is converted to _Bool, the result is 0 if the value compares equal
to 0; otherwise, the result is 1.
6.3.1.3 Signed and unsigned integers
1 When a value with integer type is converted to another integer type other than _Bool, if
the value can be represented by the new type, it is unchanged.
2 Otherwise, if the new type is unsigned, the value is converted by repeatedly adding or
subtracting one more than the maximum value that can be represented in the new type
until the value is in the range of the new type.
3 Otherwise, the new type is signed and the value cannot be represented in it; the result is
implementation-defined.
6.3.1.4 Real floating and integer
1 When a finite value of real floating type is converted to an integer type other than _Bool,
the fractional part is discarded (i. e., the value is truncated toward zero). If the value of
the integral part cannot be represented by the integer type, the behavior is undefined. 43)
42) The integer promotions are applied only: as part of the usual arithmetic conversions, to certain
argument expressions, to the operands of the unary +, -, and ~ operators, and to both operands of the
shift operators, as specified by their respective subclauses.
43) The remaindering operation performed when a value of integer type is converted to unsigned type
need not be performed when a value of real floating type is converted to unsigned type. Thus, the
range of portable real floating values is (1, Utype_ MAX+ 1).
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2 When a value of integer type is converted to a real floating type, if the value being
converted is in the range of values that can be represented but cannot be represented
exactly, the result is either the nearest higher or nearest lower value, chosen in an
implementation-defined manner. If the value being converted is outside the range of
values that can be represented, the behavior is undefined.
6.3.1.5 Real floating types
1 When a float is promoted to double or long double, or adouble is promoted
to long double, its value is unchanged.
2 When a double is demoted to float, along double is demoted to double or
float, or a value being represented in greater precision and range than required by its
semantic type (see 6.3.1.8) is explicitly converted to its semantic type, if the value being
converted is outside the range of values that can be represented, the behavior is
undefined. If the value being converted is in the range of values that can be represented
but cannot be represented exactly, the result is either the nearest higher or nearest lower
representable value, chosen in an implementation-defined manner.
6.3.1.6 Complex types
1 When a value of complex type is converted to another complex type, both the real and
imaginary parts follow the conversion rules for the corresponding real types.
6.3.1.7 Real and complex
1 When a value of real type is converted to a complex type, the real part of the complex
result value is determined by the rules of conversion to the corresponding real type and
the imaginary part of the complex result value is a positive zero or an unsigned zero.
2 When a value of complex type is converted to a real type, the imaginary part of the
complex value is discarded and the value of the real part is converted according to the
conversion rules for the corresponding real type.
6.3.1.8 Usual arithmetic conversions
1 Many operators that expect operands of arithmetic type cause conversions and yield result
types in a similar way. The purpose is to determine a common real type for the operands
and result. For the specified operands, each operand is converted, without change of type
domain, to a type whose corresponding real type is the common real type. Unless
explicitly stated otherwise, the common real type is also the corresponding real type of
the result, whose type domain is the type domain of the operands if they are the same,
and complex otherwise. This pattern is called the usual arithmetic conversions:
First, if the corresponding real type of either operand is long double, the other
operand is converted, without change of type domain, to a type whose
corresponding real type is long double.
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Otherwise, if the corresponding real type of either operand is double, the other
operand is converted, without change of type domain, to a type whose
corresponding real type is double.
Otherwise, if the corresponding real type of either operand is float, the other
operand is converted, without change of type domain, to a type whose
corresponding real type is float. 44)
Otherwise, the integer promotions are performed on both operands. Then the
following rules are applied to the promoted operands:
If both operands have the same type, then no further conversion is needed.
Otherwise, if both operands have signed integer types or both have unsigned
integer types, the operand with the type of lesser integer conversion rank is
converted to the type of the operand with greater rank.
Otherwise, if the operand that has unsigned integer type has rank greater or
equal to the rank of the type of the other operand, then the operand with
signed integer type is converted to the type of the operand with unsigned
integer type.
Otherwise, if the type of the operand with signed integer type can represent
all of the values of the type of the operand with unsigned integer type, then
the operand with unsigned integer type is converted to the type of the
operand with signed integer type.
Otherwise, both operands are converted to the unsigned integer type
corresponding to the type of the operand with signed integer type.
2 The values of floating operands and of the results of floating expressions may be
represented in greater precision and range than that required by the type; the types are not
changed thereby. 45)
44) For example, addition of a double _Complex and a float entails just the conversion of the
float operand to double (and yields a double _Complex result).
45) The cast and assignment operators are still required to perform their specified conversions as
described in 6.3.1.4 and 6.3.1.5.
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6.3.2 Other operands
6.3.2.1 Lvalues and function designators
1 Anlvalue is an expression with an object type or an incomplete type other than void; 46)
if an lvalue does not designate an object when it is evaluated, the behavior is undefined.
When an object is said to have a particular type, the type is specified by the lvalue used to
designate the object. A modifiable lvalue is an lvalue that does not have array type, does
not have an incomplete type, does not have a const-qualified type, and if it is a structure
or union, does not have any member (including, recursively, any member or element of
all contained aggregates or unions) with a const-qualified type.
2 Except when it is the operand of the sizeof operator, the unary & operator, the ++
operator, the --operator, or the left operand of the . operator or an assignment operator,
an lvalue that does not have array type is converted to the value stored in the designated
object (and is no longer an lvalue). If the lvalue has qualified type, the value has the
unqualified version of the type of the lvalue; otherwise, the value has the type of the
lvalue. If the lvalue has an incomplete type and does not have array type, the behavior is
undefined.
3 Except when it is the operand of the sizeof operator or the unary & operator, or is a
string literal used to initialize an array, an expression that has type '' array of type'' is
converted to an expression with type '' pointer to type'' that points to the initial element of
the array object and is not an lvalue. If the array object has register storage class, the
behavior is undefined.
4 Afunction designator is an expression that has function type. Except when it is the
operand of the sizeof operator 47) or the unary & operator, a function designator with
type '' function returning type'' is converted to an expression that has type '' pointer to
function returning type'' .
Forward references: address and indirection operators (6.5.3.2), assignment operators
(6.5.16), common definitions <stddef. h> (7.17), initialization (6.7.8), postfix
increment and decrement operators (6.5.2.4), prefix increment and decrement operators
(6.5.3.1), the sizeof operator (6.5.3.4), structure and union members (6.5.2.3).
46) The name '' lvalue'' comes originally from the assignment expression E1 = E2, in which the left
operand E1 is required to be a (modifiable) lvalue. It is perhaps better considered as representing an
object '' locator value''. What is sometimes called '' rvalue'' is in this International Standard described
as the '' value of an expression''.
An obvious example of an lvalue is an identifier of an object. As a further example, if E is a unary
expression that is a pointer to an object, *E is an lvalue that designates the object to which E points.
47) Because this conversion does not occur, the operand of the sizeof operator remains a function
designator and violates the constraint in 6.5.3.4.
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6.3.2.2 void
1 The (nonexistent) value of a void expression (an expression that has type void) shall not
be used in any way, and implicit or explicit conversions (except to void) shall not be
applied to such an expression. If an expression of any other type is evaluated as a void
expression, its value or designator is discarded. (A void expression is evaluated for its
side effects.)
6.3.2.3 Pointers
1 A pointer to void may be converted to or from a pointer to any incomplete or object
type. A pointer to any incomplete or object type may be converted to a pointer to void
and back again; the result shall compare equal to the original pointer.
2 For any qualifier q, a pointer to a non-q-qualified type may be converted to a pointer to
the q-qualified version of the type; the values stored in the original and converted pointers
shall compare equal.
3 An integer constant expression with the value 0, or such an expression cast to type void
*, is called a null pointer constant. 48) If a null pointer constant is converted to a pointer
type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a
pointer to any object or function.
4 Conversion of a null pointer to another pointer type yields a null pointer of that type.
Any two null pointers shall compare equal.
5 An integer may be converted to any pointer type. Except as previously specified, the
result is implementation-defined, might not be properly aligned, and might not point to an
entity of the referenced type. 49)
6 Any pointer type may be converted to an integer type. Except as previously specified, the
result is implementation-defined. If the result cannot be represented in the integer type,
the behavior is undefined. The result need not be in the range of values of any integer
type.
7 A pointer to an object or incomplete type may be converted to a pointer to a different
object or incomplete type. If the resulting pointer is not correctly aligned 50) for the
pointed-to type, the behavior is undefined. Otherwise, when converted back again, the
result shall compare equal to the original pointer. When a pointer to an object is
48) The macro NULL is defined in <stddef. h> as a null pointer constant; see 7.17.
49) The mapping functions for converting a pointer to an integer or an integer to a pointer are intended to
be consistent with the addressing structure of the execution environment.
50) In general, the concept '' correctly aligned'' is transitive: if a pointer to type A is correctly aligned for a
pointer to type B, which in turn is correctly aligned for a pointer to type C, then a pointer to type A is
correctly aligned for a pointer to type C.
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converted to a pointer to a character type, the result points to the lowest addressed byte of
the object. Successive increments of the result, up to the size of the object, yield pointers
to the remaining bytes of the object.
8 A pointer to a function of one type may be converted to a pointer to a function of another
type and back again; the result shall compare equal to the original pointer. If a converted
pointer is used to call a function whose type is not compatible with the pointed-to type,
the behavior is undefined.
Forward references: cast operators (6.5.4), equality operators (6.5.9), simple
assignment (6.5.16.1).
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6.4 Lexical elements
Syntax
1 token:
keyword
identifier
constant
string-literal
punctuator
preprocessing-token:
header-name
identifier
pp-number
character-constant
string-literal
punctuator
each non-white-space character that cannot be one of the above *
Constraints
2 Each preprocessing token that is converted to a token shall have the lexical form of a
keyword, an identifier, a constant, a string literal, or a punctuator.
Semantics
3 Atoken is the minimal lexical element of the language in translation phases 7 and 8. The
categories of tokens are: keywords, identifiers, constants, string literals, and punctuators.
A preprocessing token is the minimal lexical element of the language in translation
phases 3 through 6. The categories of preprocessing token are: header names, identifiers,
preprocessing numbers, character constants, string literals, punctuators, and single non-white-
space characters that do not lexically match the other preprocessing token
categories. 51) If a ' or a " character matches the last category, the behavior is undefined.
Preprocessing tokens can be separated by white space; this consists of comments
(described later), or white-space characters (space, horizontal tab, new-line, vertical tab,
and form-feed), or both. As described in 6.10, in certain circumstances during translation
phase 4, white space (or the absence thereof) serves as more than preprocessing token
separation. White space may appear within a preprocessing token only as part of a
header name or between the quotation characters in a character constant or string literal.
51) An additional category, placemarkers, is used internally in translation phase 4 (see 6.10.3.3); it cannot
occur in source files.
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4 If the input stream has been parsed into preprocessing tokens up to a given character, the
next preprocessing token is the longest sequence of characters that could constitute a
preprocessing token. There is one exception to this rule: a header name preprocessing
token is only recognized within a #include preprocessing directive, and within such a
directive, a sequence of characters that could be either a header name or a string literal is
recognized as the former.
5 EXAMPLE 1 The program fragment 1Ex is parsed as a preprocessing number token (one that is not a valid floating or integer constant token), even though a parse as the pair of preprocessing tokens 1 and Ex
might produce a valid expression (for example, if Ex were a macro defined as +1). Similarly, the program
fragment 1E1 is parsed as a preprocessing number (one that is a valid floating constant token), whether or
not E is a macro name.
6 EXAMPLE 2 The program fragment x+++++ y is parsed as x +++++ y, which violates a constraint on increment operators, even though the parse x +++++ ymight yield a correct expression.
Forward references: character constants (6.4.4.4), comments (6.4.9), expressions (6.5),
floating constants (6.4.4.2), header names (6.4.7), macro replacement (6.10.3), postfix
increment and decrement operators (6.5.2.4), prefix increment and decrement operators
(6.5.3.1), preprocessing directives (6.10), preprocessing numbers (6.4.8), string literals
(6.4.5).
6.4.1 Keywords
Syntax
1 keyword: one of
auto
break
case
char
const
continue
default
do
double
else
enum
extern
float
for
goto
if
inline
int
long
register
restrict
return
short
signed
sizeof
static
struct
switch
typedef
union
unsigned
void
volatile
while
_Bool
_Complex
_Imaginary
Semantics
2 The above tokens (case sensitive) are reserved (in translation phases 7 and 8) for use as
keywords, and shall not be used otherwise.
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6.4.2 Identifiers
6.4.2.1 General
Syntax
1 identifier:
identifier-nondigit
identifier identifier-nondigit
identifier digit
identifier-nondigit:
nondigit
universal-character-name
other implementation-defined characters
nondigit: one of
_a b c d e f g h i j k l m
n o p q r s t u v w x y z
A B C D E F G H I J K L M
N O P Q R S T U V W X Y Z
digit: one of
0 1 2 3 4 5 6 7 8 9
Semantics
2 An identifier is a sequence of nondigit characters (including the underscore _, the
lowercase and uppercase Latin letters, and other characters) and digits, which designates
one or more entities as described in 6.2.1. Lowercase and uppercase letters are distinct.
There is no specific limit on the maximum length of an identifier.
3 Each universal character name in an identifier shall designate a character whose encoding
in ISO/ IEC 10646 falls into one of the ranges specified in annex D. 52) The initial
character shall not be a universal character name designating a digit. An implementation
may allow multibyte characters that are not part of the required source character set to
appear in identifiers; which characters and their correspondence to universal character
names is implementation defined.
4 When preprocessing tokens are converted to tokens during translation phase 7, if a
preprocessing token could be converted to either a keyword or an identifier, it is converted
to a keyword.
52) On systems in which linkers cannot accept extended characters, an encoding of the universal character
name may be used in forming valid external identifiers. For example, some otherwise unused
character or sequence of characters may be used to encode the \u in a universal character name.
Extended characters may produce a long external identifier.
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Implementation limits
5 As discussed in 5.2.4.1, an implementation may limit the number of significant initial
characters in an identifier; the limit for an external name (an identifier that has external
linkage) may be more restrictive than that for an internal name (a macro name or an
identifier that does not have external linkage). The number of significant characters in an
identifier is implementation-defined.
6 Any identifiers that differ in a significant character are different identifiers. If two
identifiers differ only in nonsignificant characters, the behavior is undefined.
Forward references: universal character names (6.4.3), macro replacement (6.10.3).
6.4.2.2 Predefined identifiers
Semantics
1 The identifier _ _func_ _ shall be implicitly declared by the translator as if,
immediately following the opening brace of each function definition, the declaration
static const char _ _func_ _[]=" function-name";
appeared, where function-name is the name of the lexically-enclosing function. 53)
2 This name is encoded as if the implicit declaration had been written in the source
character set and then translated into the execution character set as indicated in translation
phase 5.
3 EXAMPLE Consider the code fragment:
#include <stdio. h>
void myfunc( void)
{
printf("% s\ n", _ _func_ _);
/* ... */
}
Each time the function is called, it will print to the standard output stream:
myfunc
Forward references: function definitions (6.9.1).
53) Note that since the name _ _func_ _ is reserved for any use by the implementation (7.1.3), if any
other identifier is explicitly declared using the name _ _func_ _, the behavior is undefined.
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6.4.3 Universal character names
Syntax
1 universal-character-name:
\u hex-quad
\U hex-quad hex-quad
hex-quad:
hexadecimal-digit hexadecimal-digit
hexadecimal-digit hexadecimal-digit
Constraints
2 A universal character name shall not specify a character short identifier in the range
00000000 through 00000020, 0000007F through 0000009F, or 0000D800 through
0000DFFF inclusive. A universal character name shall not designate a character in the
required character set.
Description
3 Universal character names may be used in identifiers, character constants, and string
literals to designate characters that are not in the required character set.
Semantics
4 The universal character name \Unnnnnnnn designates the character whose character short
identifier (as specified by ISO/ IEC 10646) is nnnnnnnn. Similarly, the universal
character name \unnnn designates the character whose character short identifier is
0000nnnn.
6.4.4 Constants
Syntax
1 constant:
integer-constant
floating-constant
enumeration-constant
character-constant
Constraints
2 The value of a constant shall be in the range of representable values for its type.
Semantics
3 Each constant has a type, determined by its form and value, as detailed later.
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6.4.4.1 Integer constants
Syntax
1 integer-constant:
decimal-constant integer-suffix opt
octal-constant integer-suffix opt
hexadecimal-constant integer-suffix opt
decimal-constant:
nonzero-digit
decimal-constant digit
octal-constant:
0
octal-constant octal-digit
hexadecimal-constant:
hexadecimal-prefix hexadecimal-digit
hexadecimal-constant hexadecimal-digit
hexadecimal-prefix: one of
0x 0X
nonzero-digit: one of
1 2 3 4 5 6 7 8 9
octal-digit: one of
0 1 2 3 4 5 6 7
hexadecimal-digit: one of
0 1 2 3 4 5 6 7 8 9
a b c d e f
A B C D E F
integer-suffix:
unsigned-suffix long-suffix opt
unsigned-suffix long-long-suffix
long-suffix unsigned-suffix opt
long-long-suffix unsigned-suffix opt
unsigned-suffix: one of
u U
long-suffix: one of
l L
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long-long-suffix: one of
ll LL
Description
2 An integer constant begins with a digit, but has no period or exponent part. It may have a
prefix that specifies its base and a suffix that specifies its type.
3 A decimal constant begins with a nonzero digit and consists of a sequence of decimal
digits. An octal constant consists of the prefix 0 optionally followed by a sequence of the
digits 0 through 7 only. A hexadecimal constant consists of the prefix 0x or 0X followed
by a sequence of the decimal digits and the letters a (or A) through f (or F) with values
10 through 15 respectively.
Semantics
4 The value of a decimal constant is computed base 10; that of an octal constant, base 8;
that of a hexadecimal constant, base 16. The lexically first digit is the most significant.
5 The type of an integer constant is the first of the corresponding list in which its value can
be represented.
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Octal or Hexadecimal
Suffix DecimalConstant Constant
int int
long int unsigned int
long int
unsigned long int
long long int
unsigned long long int
long long int
none
unsigned int unsigned int
unsigned long int unsigned long int
unsigned long long int unsigned long long int
u or U
long int long int
unsigned long int
long long int
unsigned long long int
long long int
l or L
Both u or U unsigned long int unsigned long int
and l or L unsigned long long int unsigned long long int
long long int
unsigned long long int
ll or LL long long int
Both u or U
and ll or LL
unsigned long long int unsigned long long int
If an integer constant cannot be represented by any type in its list, it may have an
extended integer type, if the extended integer type can represent its value. If all of the
types in the list for the constant are signed, the extended integer type shall be signed. If
all of the types in the list for the constant are unsigned, the extended integer type shall be
unsigned. If the list contains both signed and unsigned types, the extended integer type
may be signed or unsigned.
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6.4.4.2 Floating constants
Syntax
1 floating-constant:
decimal-floating-constant
hexadecimal-floating-constant
decimal-floating-constant:
fractional-constant exponent-part opt floating-suffix opt
digit-sequence exponent-part floating-suffix opt
hexadecimal-floating-constant:
hexadecimal-prefix hexadecimal-fractional-constant
binary-exponent-part floating-suffix opt
hexadecimal-prefix hexadecimal-digit-sequence
binary-exponent-part floating-suffix opt
fractional-constant:
digit-sequence opt . digit-sequence
digit-sequence .
exponent-part:
e sign opt digit-sequence
E sign opt digit-sequence
sign: one of
+-digit-
sequence:
digit
digit-sequence digit
hexadecimal-fractional-constant:
hexadecimal-digit-sequence opt .
hexadecimal-digit-sequence
hexadecimal-digit-sequence .
binary-exponent-part:
p sign opt digit-sequence
P sign opt digit-sequence
hexadecimal-digit-sequence:
hexadecimal-digit
hexadecimal-digit-sequence hexadecimal-digit
floating-suffix: one of
f l F L
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Description
2 A floating constant has a significand part that may be followed by an exponent part and a
suffix that specifies its type. The components of the significand part may include a digit
sequence representing the whole-number part, followed by a period (.), followed by a
digit sequence representing the fraction part. The components of the exponent part are an
e, E, p, orPfollowed by an exponent consisting of an optionally signed digit sequence.
Either the whole-number part or the fraction part has to be present; for decimal floating
constants, either the period or the exponent part has to be present.
Semantics
3 The significand part is interpreted as a (decimal or hexadecimal) rational number; the
digit sequence in the exponent part is interpreted as a decimal integer. For decimal
floating constants, the exponent indicates the power of 10 by which the significand part is
to be scaled. For hexadecimal floating constants, the exponent indicates the power of 2
by which the significand part is to be scaled. For decimal floating constants, and also for
hexadecimal floating constants when FLT_ RADIX is not a power of 2, the result is either
the nearest representable value, or the larger or smaller representable value immediately
adjacent to the nearest representable value, chosen in an implementation-defined manner.
For hexadecimal floating constants when FLT_ RADIX is a power of 2, the result is
correctly rounded.
4 An unsuffixed floating constant has type double. If suffixed by the letter f or F, it has
type float. If suffixed by the letter l or L, it has type long double.
Recommended practice
5 The implementation should produce a diagnostic message if a hexadecimal constant
cannot be represented exactly in its evaluation format; the implementation should then
proceed with the translation of the program.
6 The translation-time conversion of floating constants should match the execution-time
conversion of character strings by library functions, such as strtod, giv en matching
inputs suitable for both conversions, the same result format, and default execution-time
rounding. 54)
54) The specification for the library functions recommends more accurate conversion than required for
floating constants (see 7.20.1.3).
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6.4.4.3 Enumeration constants
Syntax
1 enumeration-constant:
identifier
Semantics
2 An identifier declared as an enumeration constant has type int.
Forward references: enumeration specifiers (6.7.2.2).
6.4.4.4 Character constants
Syntax
1 character-constant:
'c-char-sequence'
L'c-char-sequence'
c-char-sequence:
c-char
c-char-sequence c-char
c-char:
any member of the source character set except
the single-quote ', backslash \, or new-line character
escape-sequence
escape-sequence:
simple-escape-sequence
octal-escape-sequence
hexadecimal-escape-sequence
universal-character-name
simple-escape-sequence: one of
\' \" \? \\
\a \b \f \n \r \t \v
octal-escape-sequence:
\ octal-digit
\ octal-digit octal-digit
\ octal-digit octal-digit octal-digit
hexadecimal-escape-sequence:
\x hexadecimal-digit
hexadecimal-escape-sequence hexadecimal-digit
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Description
2 An integer character constant is a sequence of one or more multibyte characters enclosed
in single-quotes, as in 'x' or 'ab'. A wide character constant is the same, except
prefixed by the letter L. With a few exceptions detailed later, the elements of the sequence
are any members of the source character set; they are mapped in an implementation-defined
manner to members of the execution character set.
3 The single-quote ', the double-quote ", the question-mark ?, the backslash \, and
arbitrary integer values, are representable according to the following table of escape
sequences:
single quote ' \'
double quote "\"
question mark ?\?
backslash \\\
octal character \octal digits
hexadecimal character \xhexadecimal digits
4 The double-quote " and question-mark ? are representable either by themselves or by the
escape sequences \" and \?, respectively, but the single-quote ' and the backslash \
shall be represented, respectively, by the escape sequences \' and \\.
5 The octal digits that follow the backslash in an octal escape sequence are taken to be part
of the construction of a single character for an integer character constant or of a single
wide character for a wide character constant. The numerical value of the octal integer so
formed specifies the value of the desired character or wide character.
6 The hexadecimal digits that follow the backslash and the letter x in a hexadecimal escape
sequence are taken to be part of the construction of a single character for an integer
character constant or of a single wide character for a wide character constant. The
numerical value of the hexadecimal integer so formed specifies the value of the desired
character or wide character.
7 Each octal or hexadecimal escape sequence is the longest sequence of characters that can
constitute the escape sequence.
8 In addition, characters not in the required character set are representable by universal
character names and certain nongraphic characters are representable by escape sequences
consisting of the backslash \ followed by a lowercase letter: \a, \b, \f, \n, \r, \t,
and \v. 55)
55) The semantics of these characters were discussed in 5.2.2. If any other character follows a backslash,
the result is not a token and a diagnostic is required. See '' future language directions'' (6.11.2).
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Constraints
9 The value of an octal or hexadecimal escape sequence shall be in the range of
representable values for the type unsigned char for an integer character constant, or
the unsigned type corresponding to wchar_ t for a wide character constant.
Semantics
10 An integer character constant has type int. The value of an integer character constant
containing a single character that maps to a member of the basic execution character set is
the numerical value of the representation of the mapped character interpreted as an
integer. The value of an integer character constant containing more than one character, or
containing a character or escape sequence not represented in the basic execution character
set, is implementation-defined. If an integer character constant contains a single
character or escape sequence, its value is the one that results when an object with type
char whose value is that of the single character or escape sequence is converted to type
int.
11 A wide character constant has type wchar_ t, an integer type defined in the
<stddef. h> header. The value of a wide character constant containing a single
multibyte character that maps to a member of the extended execution character set is the
wide character (code) corresponding to that multibyte character, as defined by the
mbtowc function, with an implementation-defined current locale. The value of a wide
character constant containing more than one multibyte character, or containing a
multibyte character or escape sequence not represented in the extended execution
character set, is implementation-defined.
12 EXAMPLE 1 The construction '\ 0' is commonly used to represent the null character.
13 EXAMPLE 2 Consider implementations that use two's-complement representation for integers and eight bits for objects that have type char. In an implementation in which type char has the same range of
values as signed char, the integer character constant '\ xFF' has the value 1; if type char has the
same range of values as unsigned char, the character constant '\ xFF' has the value +255 .
14 EXAMPLE 3 Even if eight bits are used for objects that have type char, the construction '\ x123' specifies an integer character constant containing only one character, since a hexadecimal escape sequence
is terminated only by a non-hexadecimal character. To specify an integer character constant containing the
two characters whose values are '\ x12' and '3', the construction '\ 0223' may be used, since an octal
escape sequence is terminated after three octal digits. (The value of this two-character integer character
constant is implementation-defined.)
15 EXAMPLE 4 Even if 12 or more bits are used for objects that have type wchar_ t, the construction L'\ 1234' specifies the implementation-defined value that results from the combination of the values
0123 and '4'.
Forward references: common definitions <stddef. h> (7.17), the mbtowc function
(7.20.7.2).
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6.4.5 String literals
Syntax
1 string-literal:
"s-char-sequence opt "
L" s-char-sequence opt "
s-char-sequence:
s-char
s-char-sequence s-char
s-char:
any member of the source character set except
the double-quote ", backslash \, or new-line character
escape-sequence
Description
2 Acharacter string literal is a sequence of zero or more multibyte characters enclosed in
double-quotes, as in "xyz". Awide string literal is the same, except prefixed by the
letter L.
3 The same considerations apply to each element of the sequence in a character string
literal or a wide string literal as if it were in an integer character constant or a wide
character constant, except that the single-quote ' is representable either by itself or by the
escape sequence \', but the double-quote " shall be represented by the escape sequence
\".
Semantics
4 In translation phase 6, the multibyte character sequences specified by any sequence of
adjacent character and wide string literal tokens are concatenated into a single multibyte
character sequence. If any of the tokens are wide string literal tokens, the resulting
multibyte character sequence is treated as a wide string literal; otherwise, it is treated as a
character string literal.
5 In translation phase 7, a byte or code of value zero is appended to each multibyte
character sequence that results from a string literal or literals. 56) The multibyte character
sequence is then used to initialize an array of static storage duration and length just
sufficient to contain the sequence. For character string literals, the array elements have
type char, and are initialized with the individual bytes of the multibyte character
sequence; for wide string literals, the array elements have type wchar_ t, and are
initialized with the sequence of wide characters corresponding to the multibyte character
56) A character string literal need not be a string (see 7.1.1), because a null character may be embedded in
it by a \0 escape sequence.
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sequence, as defined by the mbstowcs function with an implementation-defined current
locale. The value of a string literal containing a multibyte character or escape sequence
not represented in the execution character set is implementation-defined.
6 It is unspecified whether these arrays are distinct provided their elements have the
appropriate values. If the program attempts to modify such an array, the behavior is
undefined.
7 EXAMPLE This pair of adjacent character string literals
"\ x12" "3"
produces a single character string literal containing the two characters whose values are '\ x12' and '3',
because escape sequences are converted into single members of the execution character set just prior to
adjacent string literal concatenation.
Forward references: common definitions <stddef. h> (7.17).
6.4.6 Punctuators
Syntax
1 punctuator: one of
[](){}.->
++ --& * + -~ !
/%<<>><><=>===!=^|&&||
?:;...
= *= /= %= += -= <<= >>= &= ^= |=
,###
<: :> <% %> %: %:%:
Semantics
2 A punctuator is a symbol that has independent syntactic and semantic significance.
Depending on context, it may specify an operation to be performed (which in turn may
yield a value or a function designator, produce a side effect, or some combination thereof)
in which case it is known as an operator (other forms of operator also exist in some
contexts). An operand is an entity on which an operator acts.
3 In all aspects of the language, these six tokens
<: :> <% %> %: %:%:
behave, respectively, the same as these six tokens
[]{}###
except for their spelling. 57)
57) Thus [ and <: behave differently when '' stringized'' (see 6.10.3.2), but can otherwise be freely
interchanged.
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Forward references: expressions (6.5), declarations (6.7), preprocessing directives
(6.10), statements (6.8).
6.4.7 Header names
Syntax
1 header-name:
<h-char-sequence>
"q-char-sequence"
h-char-sequence:
h-char
h-char-sequence h-char
h-char:
any member of the source character set except
the new-line character and >
q-char-sequence:
q-char
q-char-sequence q-char
q-char:
any member of the source character set except
the new-line character and "
Semantics
2 The sequences in both forms of header names are mapped in an implementation-defined
manner to headers or external source file names as specified in 6.10.2.
3 If the characters ', \, ", //, or/* occur in the sequence between the < and > delimiters,
the behavior is undefined. Similarly, if the characters ', \, //, or/* occur in the
sequence between the " delimiters, the behavior is undefined. 58) A header name
preprocessing token is recognized only within a #include preprocessing directive.
4 EXAMPLE The following sequence of characters:
0x3< 1/ a. h> 1e2
#include <1/ a. h>
#define const. member@$
forms the following sequence of preprocessing tokens (with each individual preprocessing token delimited
by a { on the left and a } on the right).
58) Thus, sequences of characters that resemble escape sequences cause undefined behavior.
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{0x3}{<}{ 1}{/}{ a}{.}{ h}{>}{ 1e2}
{#}{ include}{< 1/ a. h>}
{#}{ define}{ const}{.}{ member}{@}{$}
Forward references: source file inclusion (6.10.2).
6.4.8 Preprocessing numbers
Syntax
1 pp-number:
digit
. digit
pp-number digit
pp-number identifier-nondigit
pp-number e sign
pp-number E sign
pp-number p sign
pp-number P sign
pp-number .
Description
2 A preprocessing number begins with a digit optionally preceded by a period (.) and may
be followed by letters, underscores, digits, periods, and e+, e-, E+, E-, p+, p-, P+, or
P-character sequences.
3 Preprocessing number tokens lexically include all floating and integer constant tokens.
Semantics
4 A preprocessing number does not have type or a value; it acquires both after a successful
conversion (as part of translation phase 7) to a floating constant token or an integer
constant token.
6.4.9 Comments
1 Except within a character constant, a string literal, or a comment, the characters /*
introduce a comment. The contents of a comment are examined only to identify
multibyte characters and to find the characters */ that terminate it. 59)
2 Except within a character constant, a string literal, or a comment, the characters //
introduce a comment that includes all multibyte characters up to, but not including, the
next new-line character. The contents of such a comment are examined only to identify
multibyte characters and to find the terminating new-line character.
59) Thus, /* ... */ comments do not nest.
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3 EXAMPLE 1
"a// b" // four-character string literal
#include "// e" // undefined behavior
// */ // comment, not syntax error
f = g/**// h; // equivalent to f =g /h;
//\
i(); // part of a two-line comment
/\
/ j(); // part of a two-line comment
#define glue( x, y) x## y
glue(/,/) k(); // syntax error, not comment
/*//*/ l(); // equivalent to l();
m = n//**/ o
+p; // equivalent to m =n +p;
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6.5 Expressions
1 Anexpression is a sequence of operators and operands that specifies computation of a
value, or that designates an object or a function, or that generates side effects, or that
performs a combination thereof.
2 Between the previous and next sequence point an object shall have its stored value
modified at most once by the evaluation of an expression. Furthermore, the prior value
shall be accessed only to determine the value to be stored. 60) *
3 The grouping of operators and operands is indicated by the syntax. 61) Except as specified
later (for the function-call (), &&, ||, ?:, and comma operators), the order of evaluation
of subexpressions and the order in which side effects take place are both unspecified.
4 Some operators (the unary operator ~, and the binary operators <<, >>, &, ^, and |,
collectively described as bitwise operators) are required to have operands that have
integer type. These operators return values that depend on the internal representations of
integers, and have implementation-defined and undefined aspects for signed types.
5 If an exception occurs during the evaluation of an expression (that is, if the result is not
mathematically defined or not in the range of representable values for its type), the
behavior is undefined.
6 The effective type of an object for an access to its stored value is the declared type of the
object, if any. 62) If a value is stored into an object having no declared type through an
60) This paragraph renders undefined statement expressions such as
i = ++ i + 1;
a[ i++] = i;
while allowing
i =i +1;
a[ i] = i;
61) The syntax specifies the precedence of operators in the evaluation of an expression, which is the same
as the order of the major subclauses of this subclause, highest precedence first. Thus, for example, the
expressions allowed as the operands of the binary + operator (6.5.6) are those expressions defined in
6.5.1 through 6.5.6. The exceptions are cast expressions (6.5.4) as operands of unary operators
(6.5.3), and an operand contained between any of the following pairs of operators: grouping
parentheses () (6.5.1), subscripting brackets [] (6.5.2.1), function-call parentheses () (6.5.2.2), and
the conditional operator ?: (6.5.15).
Within each major subclause, the operators have the same precedence. Left-or right-associativity is
indicated in each subclause by the syntax for the expressions discussed therein.
62) Allocated objects have no declared type.
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lvalue having a type that is not a character type, then the type of the lvalue becomes the
effective type of the object for that access and for subsequent accesses that do not modify
the stored value. If a value is copied into an object having no declared type using
memcpy or memmove, or is copied as an array of character type, then the effective type
of the modified object for that access and for subsequent accesses that do not modify the
value is the effective type of the object from which the value is copied, if it has one. For
all other accesses to an object having no declared type, the effective type of the object is
simply the type of the lvalue used for the access.
7 An object shall have its stored value accessed only by an lvalue expression that has one of
the following types: 63)
a type compatible with the effective type of the object,
a qualified version of a type compatible with the effective type of the object,
a type that is the signed or unsigned type corresponding to the effective type of the
object,
a type that is the signed or unsigned type corresponding to a qualified version of the
effective type of the object,
an aggregate or union type that includes one of the aforementioned types among its
members (including, recursively, a member of a subaggregate or contained union), or
a character type.
8 A floating expression may be contracted, that is, evaluated as though it were an atomic
operation, thereby omitting rounding errors implied by the source code and the
expression evaluation method. 64) The FP_ CONTRACT pragma in <math. h> provides a
way to disallow contracted expressions. Otherwise, whether and how expressions are
contracted is implementation-defined. 65)
63) The intent of this list is to specify those circumstances in which an object may or may not be aliased.
64) A contracted expression might also omit the raising of floating-point exception flags.
65) This license is specifically intended to allow implementations to exploit fast machine instructions that
combine multiple C operators. As contractions potentially undermine predictability, and can even
decrease accuracy for containing expressions, their use needs to be well-defined and clearly
documented.
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6.5.1 Primary expressions
Syntax
1 primary-expression:
identifier
constant
string-literal
( expression )
Semantics
2 An identifier is a primary expression, provided it has been declared as designating an
object (in which case it is an lvalue) or a function (in which case it is a function
designator). 66)
3 A constant is a primary expression. Its type depends on its form and value, as detailed in
6.4.4.
4 A string literal is a primary expression. It is an lvalue with type as detailed in 6.4.5.
5 A parenthesized expression is a primary expression. Its type and value are identical to
those of the unparenthesized expression. It is an lvalue, a function designator, or a void
expression if the unparenthesized expression is, respectively, an lvalue, a function
designator, or a void expression.
Forward references: declarations (6.7).
6.5.2 Postfix operators
Syntax
1 postfix-expression:
primary-expression
postfix-expression [ expression ]
postfix-expression ( argument-expression-list opt )
postfix-expression . identifier
postfix-expression -> identifier
postfix-expression ++
postfix-expression --
( type-name ){ initializer-list }
( type-name ){ initializer-list ,}
66) Thus, an undeclared identifier is a violation of the syntax.
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argument-expression-list:
assignment-expression
argument-expression-list , assignment-expression
6.5.2.1 Array subscripting
Constraints
1 One of the expressions shall have type '' pointer to object type'', the other expression shall
have integer type, and the result has type '' type'' .
Semantics
2 A postfix expression followed by an expression in square brackets [] is a subscripted
designation of an element of an array object. The definition of the subscript operator []
is that E1[ E2] is identical to (*(( E1)+( E2))). Because of the conversion rules that
apply to the binary + operator, if E1 is an array object (equivalently, a pointer to the
initial element of an array object) and E2 is an integer, E1[ E2] designates the E2-th
element of E1 (counting from zero).
3 Successive subscript operators designate an element of a multidimensional array object.
If E is an n-dimensional array (n³2) with dimensions i΄ j΄ ... ΄k, then E (used as other
than an lvalue) is converted to a pointer to an (n1)-dimensional array with dimensions
j΄ ... ΄k. If the unary * operator is applied to this pointer explicitly, or implicitly as a
result of subscripting, the result is the pointed-to (n1)-dimensional array, which itself is
converted into a pointer if used as other than an lvalue. It follows from this that arrays
are stored in row-major order (last subscript varies fastest).
4 EXAMPLE Consider the array object defined by the declaration
int x[ 3][ 5];
Here x is a 3΄5 array of ints; more precisely, x is an array of three element objects, each of which is an
array of five ints. In the expression x[ i], which is equivalent to (*(( x)+( i))), x is first converted to
a pointer to the initial array of five ints. Then i is adjusted according to the type of x, which conceptually
entails multiplying i by the size of the object to which the pointer points, namely an array of five int
objects. The results are added and indirection is applied to yield an array of five ints. When used in the
expression x[ i][ j], that array is in turn converted to a pointer to the first of the ints, so x[ i][ j]
yields an int.
Forward references: additive operators (6.5.6), address and indirection operators
(6.5.3.2), array declarators (6.7.5.2).
6.5.2.2 Function calls
Constraints
1 The expression that denotes the called function 67) shall have type pointer to function
returning void or returning an object type other than an array type.
67) Most often, this is the result of converting an identifier that is a function designator.
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2 If the expression that denotes the called function has a type that includes a prototype, the
number of arguments shall agree with the number of parameters. Each argument shall
have a type such that its value may be assigned to an object with the unqualified version
of the type of its corresponding parameter.
Semantics
3 A postfix expression followed by parentheses () containing a possibly empty, comma-separated
list of expressions is a function call. The postfix expression denotes the called
function. The list of expressions specifies the arguments to the function.
4 An argument may be an expression of any object type. In preparing for the call to a
function, the arguments are evaluated, and each parameter is assigned the value of the
corresponding argument. 68)
5 If the expression that denotes the called function has type pointer to function returning an
object type, the function call expression has the same type as that object type, and has the
value determined as specified in 6.8.6.4. Otherwise, the function call has type void. If
an attempt is made to modify the result of a function call or to access it after the next
sequence point, the behavior is undefined.
6 If the expression that denotes the called function has a type that does not include a
prototype, the integer promotions are performed on each argument, and arguments that
have type float are promoted to double. These are called the default argument
promotions. If the number of arguments does not agree with the number of parameters,
the behavior is undefined. If the function is defined with a type that includes a prototype,
and either the prototype ends with an ellipsis (, ...) or the types of the arguments after
promotion are not compatible with the types of the parameters, the behavior is undefined.
If the function is defined with a type that does not include a prototype, and the types of
the arguments after promotion are not compatible with those of the parameters after
promotion, the behavior is undefined, except for the following cases:
one promoted type is a signed integer type, the other promoted type is the
corresponding unsigned integer type, and the value is representable in both types;
one type is pointer to void and the other is a pointer to a character type.
7 If the expression that denotes the called function has a type that does include a prototype,
the arguments are implicitly converted, as if by assignment, to the types of the
corresponding parameters, taking the type of each parameter to be the unqualified version
of its declared type. The ellipsis notation in a function prototype declarator causes
68) A function may change the values of its parameters, but these changes cannot affect the values of the
arguments. On the other hand, it is possible to pass a pointer to an object, and the function may
change the value of the object pointed to. A parameter declared to have array or function type is
converted to a parameter with a pointer type as described in 6.9.1.
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argument type conversion to stop after the last declared parameter. The default argument
promotions are performed on trailing arguments.
8 No other conversions are performed implicitly; in particular, the number and types of
arguments are not compared with those of the parameters in a function definition that
does not include a function prototype declarator.
9 If the function is defined with a type that is not compatible with the type (of the
expression) pointed to by the expression that denotes the called function, the behavior is
undefined.
10 The order of evaluation of the function designator, the actual arguments, and
subexpressions within the actual arguments is unspecified, but there is a sequence point
before the actual call.
11 Recursive function calls shall be permitted, both directly and indirectly through any chain
of other functions.
12 EXAMPLE In the function call
(* pf[ f1()]) (f2(), f3() + f4())
the functions f1, f2, f3, and f4 may be called in any order. All side effects have to be completed before
the function pointed to by pf[ f1()] is called.
Forward references: function declarators (including prototypes) (6.7.5.3), function
definitions (6.9.1), the return statement (6.8.6.4), simple assignment (6.5.16.1).
6.5.2.3 Structure and union members
Constraints
1 The first operand of the . operator shall have a qualified or unqualified structure or union
type, and the second operand shall name a member of that type.
2 The first operand of the -> operator shall have type '' pointer to qualified or unqualified
structure'' or '' pointer to qualified or unqualified union'', and the second operand shall
name a member of the type pointed to.
Semantics
3 A postfix expression followed by the . operator and an identifier designates a member of
a structure or union object. The value is that of the named member, and is an lvalue if the
first expression is an lvalue. If the first expression has qualified type, the result has the
so-qualified version of the type of the designated member.
4 A postfix expression followed by the -> operator and an identifier designates a member
of a structure or union object. The value is that of the named member of the object to
which the first expression points, and is an lvalue. 69) If the first expression is a pointer to
a qualified type, the result has the so-qualified version of the type of the designated
member.
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5 With one exception, if the value of a member of a union object is used when the most
recent store to the object was to a different member, the behavior is
implementation-defined. 70) One special guarantee is made in order to simplify the use of
unions: If a union contains several structures that share a common initial sequence (see
below), and if the union object currently contains one of these structures, it is permitted to
inspect the common initial part of any of them anywhere that a declaration of the
completed type of the union is visible. Tw o structures share a common initial sequence if
corresponding members have compatible types (and, for bit-fields, the same widths) for a
sequence of one or more initial members.
6 EXAMPLE 1 If f is a function returning a structure or union, and x is a member of that structure or union, f(). x is a valid postfix expression but is not an lvalue.
7 EXAMPLE 2 In:
struct s { int i; const int ci; };
struct s s;
const struct s cs;
volatile struct s vs;
the various members have the types:
s. i int
s. ci const int
cs. i const int
cs. ci const int
vs. i volatile int
vs. ci volatile const int
8 EXAMPLE 3 The following is a valid fragment:
69) If &E is a valid pointer expression (where & is the '' address-of '' operator, which generates a pointer to
its operand), the expression (& E)-> MOS is the same as E. MOS.
70) The '' byte orders'' for scalar types are invisible to isolated programs that do not indulge in type
punning (for example, by assigning to one member of a union and inspecting the storage by accessing
another member that is an appropriately sized array of character type), but have to be accounted for
when conforming to externally imposed storage layouts.
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union {
struct {
int alltypes;
}n;
struct {
int type;
int intnode;
} ni;
struct {
int type;
double doublenode;
} nf;
}u;
u. nf. type = 1;
u. nf. doublenode = 3.14;
/* ... */
if (u. n. alltypes == 1)
if (sin( u. nf. doublenode) == 0.0)
/* ... */
The following is not a valid fragment (because the union type is not visible within function f):
struct t1 { int m; };
struct t2 { int m; };
int f( struct t1 * p1, struct t2 * p2)
{
if (p1-> m < 0)
p2-> m = -p2-> m;
return p1-> m;
}
int g()
{
union {
struct t1 s1;
struct t2 s2;
}u;
/* ... */
return f(& u. s1, &u. s2);
}
Forward references: address and indirection operators (6.5.3.2), structure and union
specifiers (6.7.2.1).
6.5.2.4 Postfix increment and decrement operators
Constraints
1 The operand of the postfix increment or decrement operator shall have qualified or
unqualified real or pointer type and shall be a modifiable lvalue.
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Semantics
2 The result of the postfix ++ operator is the value of the operand. After the result is
obtained, the value of the operand is incremented. (That is, the value 1 of the appropriate
type is added to it.) See the discussions of additive operators and compound assignment
for information on constraints, types, and conversions and the effects of operations on
pointers. The side effect of updating the stored value of the operand shall occur between
the previous and the next sequence point.
3 The postfix --operator is analogous to the postfix ++ operator, except that the value of
the operand is decremented (that is, the value 1 of the appropriate type is subtracted from
it).
Forward references: additive operators (6.5.6), compound assignment (6.5.16.2).
6.5.2.5 Compound literals
Constraints
1 The type name shall specify an object type or an array of unknown size, but not a variable
length array type.
2 No initializer shall attempt to provide a value for an object not contained within the entire
unnamed object specified by the compound literal.
3 If the compound literal occurs outside the body of a function, the initializer list shall
consist of constant expressions.
Semantics
4 A postfix expression that consists of a parenthesized type name followed by a brace-enclosed
list of initializers is a compound literal. It provides an unnamed object whose
value is given by the initializer list. 71)
5 If the type name specifies an array of unknown size, the size is determined by the
initializer list as specified in 6.7.7, and the type of the compound literal is that of the
completed array type. Otherwise (when the type name specifies an object type), the type
of the compound literal is that specified by the type name. In either case, the result is an
lvalue.
6 The value of the compound literal is that of an unnamed object initialized by the
initializer list. If the compound literal occurs outside the body of a function, the object
has static storage duration; otherwise, it has automatic storage duration associated with
the enclosing block.
71) Note that this differs from a cast expression. For example, a cast specifies a conversion to scalar types
or void only, and the result of a cast expression is not an lvalue.
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7 All the semantic rules and constraints for initializer lists in 6.7.8 are applicable to
compound literals. 72)
8 String literals, and compound literals with const-qualified types, need not designate
distinct objects. 73)
9 EXAMPLE 1 The file scope definition
int *p = (int []){ 2, 4};
initializes p to point to the first element of an array of two ints, the first having the value two and the
second, four. The expressions in this compound literal are required to be constant. The unnamed object
has static storage duration.
10 EXAMPLE 2 In contrast, in
void f( void)
{
int *p;
/*...*/
p = (int [2]){* p};
/*...*/
}
p is assigned the address of the first element of an array of two ints, the first having the value previously
pointed to by p and the second, zero. The expressions in this compound literal need not be constant. The
unnamed object has automatic storage duration.
11 EXAMPLE 3 Initializers with designations can be combined with compound literals. Structure objects created using compound literals can be passed to functions without depending on member order:
drawline(( struct point){. x= 1, .y= 1},
(struct point){. x= 3, .y= 4});
Or, if drawline instead expected pointers to struct point:
drawline(&( struct point){. x= 1, .y= 1},
&( struct point){. x= 3, .y= 4});
12 EXAMPLE 4 A read-only compound literal can be specified through constructions like:
(const float []){ 1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6}
13 EXAMPLE 5 The following three expressions have different meanings:
"/ tmp/ fileXXXXXX"
(char []){"/ tmp/ fileXXXXXX"}
(const char []){"/ tmp/ fileXXXXXX"}
The first always has static storage duration and has type array of char, but need not be modifiable; the last
two hav e automatic storage duration when they occur within the body of a function, and the first of these
two is modifiable.
72) For example, subobjects without explicit initializers are initialized to zero.
73) This allows implementations to share storage for string literals and constant compound literals with
the same or overlapping representations.
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14 EXAMPLE 6 Like string literals, const-qualified compound literals can be placed into read-only memory and can even be shared. For example,
(const char []){" abc"} == "abc"
might yield 1 if the literals' storage is shared.
15 EXAMPLE 7 Since compound literals are unnamed, a single compound literal cannot specify a circularly linked object. For example, there is no way to write a self-referential compound literal that could be used
as the function argument in place of the named object endless_ zeros below:
struct int_ list { int car; struct int_ list *cdr; };
struct int_ list endless_ zeros = {0, &endless_ zeros};
eval( endless_ zeros);
16 EXAMPLE 8 Each compound literal creates only a single object in a given scope:
struct s { int i; };
int f (void)
{
struct s *p = 0, *q;
int j = 0;
again:
q =p,p =&(( struct s){ j++ });
if (j < 2) goto again;
return p == q && q-> i == 1;
}
The function f() always returns the value 1.
17 Note that if an iteration statement were used instead of an explicit goto and a labeled statement, the lifetime of the unnamed object would be the body of the loop only, and on entry next time around p would
be pointing to an object which is no longer guaranteed to exist, which would result in undefined behavior.
6.5.3 Unary operators
Syntax
1 unary-expression:
postfix-expression
++ unary-expression
--unary-expression
unary-operator cast-expression
sizeof unary-expression
sizeof ( type-name )
unary-operator: one of
&*+-~!
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6.5.3.1 Prefix increment and decrement operators
Constraints
1 The operand of the prefix increment or decrement operator shall have qualified or
unqualified real or pointer type and shall be a modifiable lvalue.
Semantics
2 The value of the operand of the prefix ++ operator is incremented. The result is the new
value of the operand after incrementation. The expression ++ E is equivalent to (E+= 1).
See the discussions of additive operators and compound assignment for information on
constraints, types, side effects, and conversions and the effects of operations on pointers.
3 The prefix --operator is analogous to the prefix ++ operator, except that the value of the
operand is decremented.
Forward references: additive operators (6.5.6), compound assignment (6.5.16.2).
6.5.3.2 Address and indirection operators
Constraints
1 The operand of the unary & operator shall be either a function designator, the result of a
[] or unary * operator, or an lvalue that designates an object that is not a bit-field and is
not declared with the register storage-class specifier.
2 The operand of the unary * operator shall have pointer type.
Semantics
3 The unary & operator returns the address of its operand. If the operand has type '' type'' ,
the result has type '' pointer to type''. If the operand is the result of a unary * operator,
neither that operator nor the & operator is evaluated and the result is as if both were
omitted, except that the constraints on the operators still apply and the result is not an
lvalue. Similarly, if the operand is the result of a [] operator, neither the & operator nor
the unary * that is implied by the [] is evaluated and the result is as if the & operator
were removed and the [] operator were changed to a + operator. Otherwise, the result is
a pointer to the object or function designated by its operand.
4 The unary * operator denotes indirection. If the operand points to a function, the result is
a function designator; if it points to an object, the result is an lvalue designating the
object. If the operand has type '' pointer to type'', the result has type '' type'' . I f an
invalid value has been assigned to the pointer, the behavior of the unary * operator is
undefined. 74)
Forward references: storage-class specifiers (6.7.1), structure and union specifiers
(6.7.2.1).
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6.5.3.3 Unary arithmetic operators
Constraints
1 The operand of the unary + or -operator shall have arithmetic type; of the ~ operator,
integer type; of the ! operator, scalar type.
Semantics
2 The result of the unary + operator is the value of its (promoted) operand. The integer
promotions are performed on the operand, and the result has the promoted type.
3 The result of the unary -operator is the negative of its (promoted) operand. The integer
promotions are performed on the operand, and the result has the promoted type.
4 The result of the ~ operator is the bitwise complement of its (promoted) operand (that is,
each bit in the result is set if and only if the corresponding bit in the converted operand is
not set). The integer promotions are performed on the operand, and the result has the
promoted type. If the promoted type is an unsigned type, the expression ~E is equivalent
to the maximum value representable in that type minus E.
5 The result of the logical negation operator ! is 0 if the value of its operand compares
unequal to 0, 1 if the value of its operand compares equal to 0. The result has type int.
The expression !E is equivalent to (0= =E).
Forward references: characteristics of floating types <float. h> (7.7), sizes of
integer types <limits. h> (7.10).
74) Thus, &* E is equivalent to E (even if E is a null pointer), and &( E1[ E2]) to (( E1)+( E2)). It is
always true that if E is a function designator or an lvalue that is a valid operand of the unary &
operator, *& E is a function designator or an lvalue equal to E. If* P is an lvalue and T is the name of
an object pointer type, *( T) P is an lvalue that has a type compatible with that to which T points.
Among the invalid values for dereferencing a pointer by the unary * operator are a null pointer, an
address inappropriately aligned for the type of object pointed to, and the address of an automatic
storage duration object when execution of the block with which the object is associated has
terminated.
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6.5.3.4 The sizeof operator
Constraints
1 The sizeof operator shall not be applied to an expression that has function type or an
incomplete type, to the parenthesized name of such a type, or to an expression that
designates a bit-field member.
Semantics
2 The sizeof operator yields the size (in bytes) of its operand, which may be an
expression or the parenthesized name of a type. The size is determined from the type of
the operand. The result is an integer. If the type of the operand is a variable length array
type, the operand is evaluated; otherwise, the operand is not evaluated and the result is an
integer constant.
3 When applied to an operand that has type char, unsigned char, orsigned char,
(or a qualified version thereof) the result is 1. When applied to an operand that has array
type, the result is the total number of bytes in the array. 75) When applied to an operand
that has structure or union type, the result is the total number of bytes in such an object,
including internal and trailing padding.
4 The value of the result is implementation-defined, and its type (an unsigned integer type)
is size_ t, defined in the <stddef. h> header.
5 EXAMPLE 1 A principal use of the sizeof operator is in communication with routines such as storage allocators and I/ O systems. A storage-allocation function might accept a size (in bytes) of an object to
allocate and return a pointer to void. For example:
extern void *alloc( size_ t);
double *dp = alloc( sizeof *dp);
The implementation of the alloc function should ensure that its return value is aligned suitably for
conversion to a pointer to double.
6 EXAMPLE 2 Another use of the sizeof operator is to compute the number of elements in an array:
sizeof array / sizeof array[ 0]
7 EXAMPLE 3 In this example, the size of a variable-length array is computed and returned from a function:
75) When applied to a parameter declared to have array or function type, the sizeof operator yields the
size of the adjusted (pointer) type (see 6.9.1).
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size_ t fsize3 (int n)
{
char b[ n+ 3]; // Variable length array.
return sizeof b; // Execution time sizeof.
}
int main()
{
size_ t size;
size = fsize3( 10); // fsize3 returns 13.
return 0;
}
Forward references: common definitions <stddef. h> (7.17), declarations (6.7),
structure and union specifiers (6.7.2.1), type names (6.7.6), array declarators (6.7.5.2).
6.5.4 Cast operators
Syntax
1 cast-expression:
unary-expression
( type-name ) cast-expression
Constraints
2 Unless the type name specifies a void type, the type name shall specify qualified or
unqualified scalar type and the operand shall have scalar type.
3 Conversions that involve pointers, other than where permitted by the constraints of
6.5.16.1, shall be specified by means of an explicit cast.
Semantics
4 Preceding an expression by a parenthesized type name converts the value of the
expression to the named type. This construction is called a cast. 76) A cast that specifies
no conversion has no effect on the type or value of an expression. 77)
Forward references: equality operators (6.5.9), function declarators (including
prototypes) (6.7.5.3), simple assignment (6.5.16.1), type names (6.7.6).
76) A cast does not yield an lvalue. Thus, a cast to a qualified type has the same effect as a cast to the
unqualified version of the type.
77) If the value of the expression is represented with greater precision or range than required by the type
named by the cast (6.3.1.8), then the cast specifies a conversion even if the type of the expression is
the same as the named type.
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6.5.5 Multiplicative operators
Syntax
1 multiplicative-expression:
cast-expression
multiplicative-expression * cast-expression
multiplicative-expression / cast-expression
multiplicative-expression % cast-expression
Constraints
2 Each of the operands shall have arithmetic type. The operands of the % operator shall
have integer type.
Semantics
3 The usual arithmetic conversions are performed on the operands. *
4 The result of the binary * operator is the product of the operands.
5 The result of the / operator is the quotient from the division of the first operand by the
second; the result of the % operator is the remainder. In both operations, if the value of
the second operand is zero, the behavior is undefined.
6 When integers are divided, the result of the / operator is the algebraic quotient with any
fractional part discarded. 78) If the quotient a/ b is representable, the expression
(a/ b)* b + a% b shall equal a.
6.5.6 Additive operators
Syntax
1 additive-expression:
multiplicative-expression
additive-expression + multiplicative-expression
additive-expression -multiplicative-expression
Constraints
2 For addition, either both operands shall have arithmetic type, or one operand shall be a
pointer to an object type and the other shall have integer type. (Incrementing is
equivalent to adding 1.)
3 For subtraction, one of the following shall hold:
both operands have arithmetic type;
78) This is often called '' truncation toward zero''.
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both operands are pointers to qualified or unqualified versions of compatible object
types; or
the left operand is a pointer to an object type and the right operand has integer type.
(Decrementing is equivalent to subtracting 1.)
Semantics
4 If both operands have arithmetic type, the usual arithmetic conversions are performed on
them. *
5 The result of the binary + operator is the sum of the operands.
6 The result of the binary -operator is the difference resulting from the subtraction of the
second operand from the first.
7 For the purposes of these operators, a pointer to a nonarray object behaves the same as a
pointer to the first element of an array of length one with the type of the object as its
element type.
8 When an expression that has integer type is added to or subtracted from a pointer, the
result has the type of the pointer operand. If the pointer operand points to an element of
an array object, and the array is large enough, the result points to an element offset from
the original element such that the difference of the subscripts of the resulting and original
array elements equals the integer expression. In other words, if the expression P points to
the i-th element of an array object, the expressions (P)+ N (equivalently, N+( P)) and
(P)-N (where N has the value n) point to, respectively, the i+ n-th and in-th elements of
the array object, provided they exist. Moreover, if the expression P points to the last
element of an array object, the expression (P)+ 1 points one past the last element of the
array object, and if the expression Q points one past the last element of an array object,
the expression (Q)-1 points to the last element of the array object. If both the pointer
operand and the result point to elements of the same array object, or one past the last
element of the array object, the evaluation shall not produce an overflow; otherwise, the
behavior is undefined. If the result points one past the last element of the array object, it
shall not be used as the operand of a unary * operator that is evaluated.
9 When two pointers are subtracted, both shall point to elements of the same array object,
or one past the last element of the array object; the result is the difference of the
subscripts of the two array elements. The size of the result is implementation-defined,
and its type (a signed integer type) is ptrdiff_ t defined in the <stddef. h> header.
If the result is not representable in an object of that type, the behavior is undefined. In
other words, if the expressions P and Q point to, respectively, the i-th and j-th elements of
an array object, the expression (P)-( Q) has the value ij provided the value fits in an
object of type ptrdiff_ t. Moreover, if the expression P points either to an element of
an array object or one past the last element of an array object, and the expression Q points
to the last element of the same array object, the expression (( Q)+ 1)-( P) has the same
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value as (( Q)-( P))+ 1 and as -(( P)-(( Q)+ 1)), and has the value zero if the
expression P points one past the last element of the array object, even though the
expression (Q)+ 1 does not point to an element of the array object. 79)
10 EXAMPLE Pointer arithmetic is well defined with pointers to variable length array types.
{
int n = 4, m = 3;
int a[ n][ m];
int (* p)[ m] = a; // p == &a[ 0]
p += 1; // p == &a[ 1]
(* p)[ 2] = 99; // a[ 1][ 2] == 99
n =p -a; //n == 1
}
11 If array a in the above example were declared to be an array of known constant size, and pointer p were declared to be a pointer to an array of the same known constant size (pointing to a), the results would be
the same.
Forward references: array declarators (6.7.5.2), common definitions <stddef. h>
(7.17).
6.5.7 Bitwise shift operators
Syntax
1 shift-expression:
additive-expression
shift-expression << additive-expression
shift-expression >> additive-expression
Constraints
2 Each of the operands shall have integer type.
Semantics
3 The integer promotions are performed on each of the operands. The type of the result is
that of the promoted left operand. If the value of the right operand is negative or is
greater than or equal to the width of the promoted left operand, the behavior is undefined.
79) Another way to approach pointer arithmetic is first to convert the pointer( s) to character pointer( s): In
this scheme the integer expression added to or subtracted from the converted pointer is first multiplied
by the size of the object originally pointed to, and the resulting pointer is converted back to the
original type. For pointer subtraction, the result of the difference between the character pointers is
similarly divided by the size of the object originally pointed to.
When viewed in this way, an implementation need only provide one extra byte (which may overlap
another object in the program) just after the end of the object in order to satisfy the '' one past the last
element'' requirements.
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4 The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits are filled with
zeros. If E1 has an unsigned type, the value of the result is E1 ΄ 2 E2 , reduced modulo
one more than the maximum value representable in the result type. If E1 has a signed
type and nonnegative value, and E1 ΄ 2 E2 is representable in the result type, then that is
the resulting value; otherwise, the behavior is undefined.
5 The result of E1 >> E2 is E1 right-shifted E2 bit positions. If E1 has an unsigned type
or if E1 has a signed type and a nonnegative value, the value of the result is the integral
part of the quotient of E1 divided by the quantity, 2 raised to the power E2. IfE1 has a
signed type and a negative value, the resulting value is implementation-defined.
6.5.8 Relational operators
Syntax
1 relational-expression:
shift-expression
relational-expression < shift-expression
relational-expression > shift-expression
relational-expression <= shift-expression
relational-expression >= shift-expression
Constraints
2 One of the following shall hold:
both operands have real type;
both operands are pointers to qualified or unqualified versions of compatible object
types; or
both operands are pointers to qualified or unqualified versions of compatible
incomplete types.
Semantics
3 If both of the operands have arithmetic type, the usual arithmetic conversions are
performed.
4 For the purposes of these operators, a pointer to a nonarray object behaves the same as a
pointer to the first element of an array of length one with the type of the object as its
element type.
5 When two pointers are compared, the result depends on the relative locations in the
address space of the objects pointed to. If two pointers to object or incomplete types both
point to the same object, or both point one past the last element of the same array object,
they compare equal. If the objects pointed to are members of the same aggregate object,
pointers to structure members declared later compare greater than pointers to members
declared earlier in the structure, and pointers to array elements with larger subscript
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values compare greater than pointers to elements of the same array with lower subscript
values. All pointers to members of the same union object compare equal. If the
expression P points to an element of an array object and the expression Q points to the
last element of the same array object, the pointer expression Q+ 1 compares greater than
P. In all other cases, the behavior is undefined.
6 Each of the operators < (less than), > (greater than), <= (less than or equal to), and >=
(greater than or equal to) shall yield 1 if the specified relation is true and 0 if it is false. 80)
The result has type int.
6.5.9 Equality operators
Syntax
1 equality-expression:
relational-expression
equality-expression == relational-expression
equality-expression != relational-expression
Constraints
2 One of the following shall hold:
both operands have arithmetic type;
both operands are pointers to qualified or unqualified versions of compatible types;
one operand is a pointer to an object or incomplete type and the other is a pointer to a
qualified or unqualified version of void; or
one operand is a pointer and the other is a null pointer constant.
Semantics
3 The == (equal to) and != (not equal to) operators are analogous to the relational
operators except for their lower precedence. 81) Each of the operators yields 1 if the
specified relation is true and 0 if it is false. The result has type int. For any pair of
operands, exactly one of the relations is true.
4 If both of the operands have arithmetic type, the usual arithmetic conversions are
performed. Values of complex types are equal if and only if both their real parts are equal
and also their imaginary parts are equal. Any two values of arithmetic types from
different type domains are equal if and only if the results of their conversions to the
(complex) result type determined by the usual arithmetic conversions are equal.
80) The expression a< b< c is not interpreted as in ordinary mathematics. As the syntax indicates, it
means (a< b)< c; in other words, '' if a is less than b, compare 1 to c; otherwise, compare 0 to c'' .
81) Because of the precedences, a< b == c< d is 1 whenever a< b and c< d have the same truth-value.
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5 Otherwise, at least one operand is a pointer. If one operand is a null pointer constant, it is
converted to the type of the other operand. If one operand is a pointer to an object or
incomplete type and the other is a pointer to a qualified or unqualified version of void,
the former is converted to the type of the latter.
6 Two pointers compare equal if and only if both are null pointers, both are pointers to the
same object (including a pointer to an object and a subobject at its beginning) or function,
both are pointers to one past the last element of the same array object, or one is a pointer
to one past the end of one array object and the other is a pointer to the start of a different
array object that happens to immediately follow the first array object in the address
space. 82)
6.5.10 Bitwise AND operator
Syntax
1 AND-expression:
equality-expression
AND-expression & equality-expression
Constraints
2 Each of the operands shall have integer type.
Semantics
3 The usual arithmetic conversions are performed on the operands.
4 The result of the binary & operator is the bitwise AND of the operands (that is, each bit in
the result is set if and only if each of the corresponding bits in the converted operands is
set).
82) Tw o objects may be adjacent in memory because they are adjacent elements of a larger array or
adjacent members of a structure with no padding between them, or because the implementation chose
to place them so, even though they are unrelated. If prior invalid pointer operations (such as accesses
outside array bounds) produced undefined behavior, subsequent comparisons also produce undefined
behavior.
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6.5.11 Bitwise exclusive OR operator
Syntax
1 exclusive-OR-expression:
AND-expression
exclusive-OR-expression ^ AND-expression
Constraints
2 Each of the operands shall have integer type.
Semantics
3 The usual arithmetic conversions are performed on the operands.
4 The result of the ^ operator is the bitwise exclusive OR of the operands (that is, each bit
in the result is set if and only if exactly one of the corresponding bits in the converted
operands is set).
6.5.12 Bitwise inclusive OR operator
Syntax
1 inclusive-OR-expression:
exclusive-OR-expression
inclusive-OR-expression | exclusive-OR-expression
Constraints
2 Each of the operands shall have integer type.
Semantics
3 The usual arithmetic conversions are performed on the operands.
4 The result of the | operator is the bitwise inclusive OR of the operands (that is, each bit in
the result is set if and only if at least one of the corresponding bits in the converted
operands is set).
6.5.13 Logical AND operator
Syntax
1 logical-AND-expression:
inclusive-OR-expression
logical-AND-expression && inclusive-OR-expression
Constraints
2 Each of the operands shall have scalar type.
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Semantics
3 The && operator shall yield 1 if both of its operands compare unequal to 0; otherwise, it
yields 0. The result has type int.
4 Unlike the bitwise binary & operator, the && operator guarantees left-to-right evaluation;
there is a sequence point after the evaluation of the first operand. If the first operand
compares equal to 0, the second operand is not evaluated.
6.5.14 Logical OR operator
Syntax
1 logical-OR-expression:
logical-AND-expression
logical-OR-expression || logical-AND-expression
Constraints
2 Each of the operands shall have scalar type.
Semantics
3 The || operator shall yield 1 if either of its operands compare unequal to 0; otherwise, it
yields 0. The result has type int.
4 Unlike the bitwise | operator, the || operator guarantees left-to-right evaluation; there is
a sequence point after the evaluation of the first operand. If the first operand compares
unequal to 0, the second operand is not evaluated.
6.5.15 Conditional operator
Syntax
1 conditional-expression:
logical-OR-expression
logical-OR-expression ? expression : conditional-expression
Constraints
2 The first operand shall have scalar type.
3 One of the following shall hold for the second and third operands:
both operands have arithmetic type;
both operands have compatible structure or union types;
both operands have void type;
both operands are pointers to qualified or unqualified versions of compatible types;
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one operand is a pointer and the other is a null pointer constant; or
one operand is a pointer to an object or incomplete type and the other is a pointer to a
qualified or unqualified version of void.
Semantics
4 The first operand is evaluated; there is a sequence point after its evaluation. The second
operand is evaluated only if the first compares unequal to 0; the third operand is evaluated
only if the first compares equal to 0; the result is the value of the second or third operand
(whichever is evaluated), converted to the type described below. 83) If an attempt is made
to modify the result of a conditional operator or to access it after the next sequence point,
the behavior is undefined.
5 If both the second and third operands have arithmetic type, the result type that would be
determined by the usual arithmetic conversions, were they applied to those two operands,
is the type of the result. If both the operands have structure or union type, the result has
that type. If both operands have void type, the result has void type.
6 If both the second and third operands are pointers or one is a null pointer constant and the
other is a pointer, the result type is a pointer to a type qualified with all the type qualifiers
of the types pointed-to by both operands. Furthermore, if both operands are pointers to
compatible types or to differently qualified versions of compatible types, the result type is
a pointer to an appropriately qualified version of the composite type; if one operand is a
null pointer constant, the result has the type of the other operand; otherwise, one operand
is a pointer to void or a qualified version of void, in which case the result type is a
pointer to an appropriately qualified version of void.
7 EXAMPLE The common type that results when the second and third operands are pointers is determined in two independent stages. The appropriate qualifiers, for example, do not depend on whether the two
pointers have compatible types.
8 Given the declarations
const void *c_ vp;
void *vp;
const int *c_ ip;
volatile int *v_ ip;
int *ip;
const char *c_ cp;
the third column in the following table is the common type that is the result of a conditional expression in
which the first two columns are the second and third operands (in either order):
83) A conditional expression does not yield an lvalue.
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c_ vp c_ ip const void *
v_ ip 0 volatile int *
c_ ip v_ ip const volatile int *
vp c_ cp const void *
ip c_ ip const int *
vp ip void *
6.5.16 Assignment operators
Syntax
1 assignment-expression:
conditional-expression
unary-expression assignment-operator assignment-expression
assignment-operator: one of
= *= /= %= += -= <<= >>= &= ^= |=
Constraints
2 An assignment operator shall have a modifiable lvalue as its left operand.
Semantics
3 An assignment operator stores a value in the object designated by the left operand. An
assignment expression has the value of the left operand after the assignment, but is not an
lvalue. The type of an assignment expression is the type of the left operand unless the
left operand has qualified type, in which case it is the unqualified version of the type of
the left operand. The side effect of updating the stored value of the left operand shall
occur between the previous and the next sequence point.
4 The order of evaluation of the operands is unspecified. If an attempt is made to modify
the result of an assignment operator or to access it after the next sequence point, the
behavior is undefined.
6.5.16.1 Simple assignment
Constraints
1 One of the following shall hold: 84)
the left operand has qualified or unqualified arithmetic type and the right has
arithmetic type;
the left operand has a qualified or unqualified version of a structure or union type
compatible with the type of the right;
84) The asymmetric appearance of these constraints with respect to type qualifiers is due to the conversion
(specified in 6.3.2.1) that changes lvalues to '' the value of the expression'' which removes any type
qualifiers from the type category of the expression.
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both operands are pointers to qualified or unqualified versions of compatible types,
and the type pointed to by the left has all the qualifiers of the type pointed to by the
right;
one operand is a pointer to an object or incomplete type and the other is a pointer to a
qualified or unqualified version of void, and the type pointed to by the left has all
the qualifiers of the type pointed to by the right; or
the left operand is a pointer and the right is a null pointer constant.
the left operand has type _Bool and the right is a pointer.
Semantics
2 Insimple assignment (=), the value of the right operand is converted to the type of the
assignment expression and replaces the value stored in the object designated by the left
operand.
3 If the value being stored in an object is accessed from another object that overlaps in any
way the storage of the first object, then the overlap shall be exact and the two objects
shall have qualified or unqualified versions of a compatible type; otherwise, the behavior
is undefined.
4 EXAMPLE 1 In the program fragment
int f( void);
char c;
/* ... */
if (( c = f()) == -1)
/* ... */
the int value returned by the function may be truncated when stored in the char, and then converted back
to int width prior to the comparison. In an implementation in which '' plain'' char has the same range of
values as unsigned char (and char is narrower than int), the result of the conversion cannot be
negative, so the operands of the comparison can never compare equal. Therefore, for full portability, the
variable c should be declared as int.
5 EXAMPLE 2 In the fragment:
char c;
int i;
long l;
l =(c =i);
the value of i is converted to the type of the assignment expression c =i, that is, char type. The value
of the expression enclosed in parentheses is then converted to the type of the outer assignment expression,
that is, long int type.
6 EXAMPLE 3 Consider the fragment:
const char ** cpp;
char *p;
const char c = 'A';
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cpp = &p; // constraint violation
*cpp = &c; // valid
*p = 0; // valid
The first assignment is unsafe because it would allow the following valid code to attempt to change the
value of the const object c.
6.5.16.2 Compound assignment
Constraints
1 For the operators += and -= only, either the left operand shall be a pointer to an object
type and the right shall have integer type, or the left operand shall have qualified or
unqualified arithmetic type and the right shall have arithmetic type.
2 For the other operators, each operand shall have arithmetic type consistent with those
allowed by the corresponding binary operator.
Semantics
3 Acompound assignment of the form E1 op =E2differs from the simple assignment
expression E1 = E1 op (E2) only in that the lvalue E1 is evaluated only once.
6.5.17 Comma operator
Syntax
1 expression:
assignment-expression
expression , assignment-expression
Semantics
2 The left operand of a comma operator is evaluated as a void expression; there is a
sequence point after its evaluation. Then the right operand is evaluated; the result has its
type and value. 85) If an attempt is made to modify the result of a comma operator or to
access it after the next sequence point, the behavior is undefined.
3 EXAMPLE As indicated by the syntax, the comma operator (as described in this subclause) cannot appear in contexts where a comma is used to separate items in a list (such as arguments to functions or lists
of initializers). On the other hand, it can be used within a parenthesized expression or within the second
expression of a conditional operator in such contexts. In the function call
f( a, (t= 3, t+ 2), c)
the function has three arguments, the second of which has the value 5.
Forward references: initialization (6.7.8).
85) A comma operator does not yield an lvalue.
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6.6 Constant expressions
Syntax
1 constant-expression:
conditional-expression
Description
2 Aconstant expression can be evaluated during translation rather than runtime, and
accordingly may be used in any place that a constant may be.
Constraints
3 Constant expressions shall not contain assignment, increment, decrement, function-call,
or comma operators, except when they are contained within a subexpression that is not
evaluated. 86)
4 Each constant expression shall evaluate to a constant that is in the range of representable
values for its type.
Semantics
5 An expression that evaluates to a constant is required in several contexts. If a floating
expression is evaluated in the translation environment, the arithmetic precision and range
shall be at least as great as if the expression were being evaluated in the execution
environment.
6 Aninteger constant expression 87) shall have integer type and shall only have operands
that are integer constants, enumeration constants, character constants, sizeof
expressions whose results are integer constants, and floating constants that are the
immediate operands of casts. Cast operators in an integer constant expression shall only
convert arithmetic types to integer types, except as part of an operand to the sizeof
operator.
7 More latitude is permitted for constant expressions in initializers. Such a constant
expression shall be, or evaluate to, one of the following:
an arithmetic constant expression,
a null pointer constant,
86) The operand of a sizeof operator is usually not evaluated (6.5.3.4).
87) An integer constant expression is used to specify the size of a bit-field member of a structure, the
value of an enumeration constant, the size of an array, or the value of a case constant. Further
constraints that apply to the integer constant expressions used in conditional-inclusion preprocessing
directives are discussed in 6.10.1.
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an address constant, or
an address constant for an object type plus or minus an integer constant expression.
8 Anarithmetic constant expression shall have arithmetic type and shall only have
operands that are integer constants, floating constants, enumeration constants, character
constants, and sizeof expressions. Cast operators in an arithmetic constant expression
shall only convert arithmetic types to arithmetic types, except as part of an operand to the
sizeof operator.
9 Anaddress constant is a null pointer, a pointer to an lvalue designating an object of static
storage duration, or to a function designator; it shall be created explicitly using the unary
& operator or an integer constant cast to pointer type, or implicitly by the use of an
expression of array or function type. The array-subscript [] and member-access . and
-> operators, the address & and indirection * unary operators, and pointer casts may be
used in the creation of an address constant, but the value of an object shall not be
accessed by use of these operators.
10 An implementation may accept other forms of constant expressions.
11 The semantic rules for the evaluation of a constant expression are the same as for
nonconstant expressions. 88)
Forward references: array declarators (6.7.5.2), initialization (6.7.8).
88) Thus, in the following initialization,
static int i = 2 || 1 / 0;
the expression is a valid integer constant expression with value one.
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6.7 Declarations
Syntax
1 declaration:
declaration-specifiers init-declarator-list opt ;
declaration-specifiers:
storage-class-specifier declaration-specifiers opt
type-specifier declaration-specifiers opt
type-qualifier declaration-specifiers opt
function-specifier declaration-specifiers opt
init-declarator-list:
init-declarator
init-declarator-list , init-declarator
init-declarator:
declarator
declarator = initializer
Constraints
2 A declaration shall declare at least a declarator (other than the parameters of a function or
the members of a structure or union), a tag, or the members of an enumeration.
3 If an identifier has no linkage, there shall be no more than one declaration of the identifier
(in a declarator or type specifier) with the same scope and in the same name space, except
for tags as specified in 6.7.2.3.
4 All declarations in the same scope that refer to the same object or function shall specify
compatible types.
Semantics
5 Adeclaration specifies the interpretation and attributes of a set of identifiers. A
definition of an identifier is a declaration for that identifier that:
for an object, causes storage to be reserved for that object;
for a function, includes the function body; 89)
for an enumeration constant or typedef name, is the (only) declaration of the
identifier.
6 The declaration specifiers consist of a sequence of specifiers that indicate the linkage,
storage duration, and part of the type of the entities that the declarators denote. The init-declarator-
list is a comma-separated sequence of declarators, each of which may have
89) Function definitions have a different syntax, described in 6.9.1.
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additional type information, or an initializer, or both. The declarators contain the
identifiers (if any) being declared.
7 If an identifier for an object is declared with no linkage, the type for the object shall be
complete by the end of its declarator, or by the end of its init-declarator if it has an
initializer.
Forward references: declarators (6.7.5), enumeration specifiers (6.7.2.2), initialization
(6.7.8), tags (6.7.2.3).
6.7.1 Storage-class specifiers
Syntax
1 storage-class-specifier:
typedef
extern
static
auto
register
Constraints
2 At most, one storage-class specifier may be given in the declaration specifiers in a
declaration. 90)
Semantics
3 The typedef specifier is called a '' storage-class specifier'' for syntactic convenience
only; it is discussed in 6.7.7. The meanings of the various linkages and storage durations
were discussed in 6.2.2 and 6.2.4.
4 A declaration of an identifier for an object with storage-class specifier register
suggests that access to the object be as fast as possible. The extent to which such
suggestions are effective is implementation-defined. 91)
5 The declaration of an identifier for a function that has block scope shall have no explicit
storage-class specifier other than extern.
90) See '' future language directions'' (6.11.3).
91) The implementation may treat any register declaration simply as an auto declaration. However,
whether or not addressable storage is actually used, the address of any part of an object declared with
storage-class specifier register cannot be computed, either explicitly (by use of the unary &
operator as discussed in 6.5.3.2) or implicitly (by converting an array name to a pointer as discussed in
6.3.2.1). Thus, the only operator that can be applied to an array declared with storage-class specifier
register is sizeof.
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6 If an aggregate or union object is declared with a storage-class specifier other than
typedef, the properties resulting from the storage-class specifier, except with respect to
linkage, also apply to the members of the object, and so on recursively for any aggregate
or union member objects.
Forward references: type definitions (6.7.7).
6.7.2 Type specifiers
Syntax
1 type-specifier:
void
char
short
int
long
float
double
signed
unsigned
_Bool
_Complex
_Imaginary
struct-or-union-specifier
enum-specifier
typedef-name
Constraints
2 At least one type specifier shall be given in the declaration specifiers in each declaration,
and in the specifier-qualifier list in each struct declaration and type name. Each list of
type specifiers shall be one of the following sets (delimited by commas, when there is
more than one set on a line); the type specifiers may occur in any order, possibly
intermixed with the other declaration specifiers.
void
char
signed char
unsigned char
short, signed short, short int, orsigned short int
unsigned short, orunsigned short int
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int, signed, orsigned int
unsigned, orunsigned int
long, signed long, long int, orsigned long int
unsigned long, orunsigned long int
long long, signed long long, long long int, orsigned long
long int
unsigned long long, orunsigned long long int
float
double
long double
_Bool
float _Complex
double _Complex
long double _Complex
float _Imaginary
double _Imaginary
long double _Imaginary
struct or union specifier
enum specifier
typedef name
3 The type specifiers _Complex and _Imaginary shall not be used if the
implementation does not provide those types. 92)
Semantics
4 Specifiers for structures, unions, and enumerations are discussed in 6.7.2.1 through
6.7.2.3. Declarations of typedef names are discussed in 6.7.7. The characteristics of the
other types are discussed in 6.2.5.
5 Each of the comma-separated sets designates the same type, except that for bit-fields, it is
implementation-defined whether the specifier int designates the same type as signed
int or the same type as unsigned int.
92) Implementations are not required to provide imaginary types. Freestanding implementations are not
required to provide complex types.
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Forward references: enumeration specifiers (6.7.2.2), structure and union specifiers
(6.7.2.1), tags (6.7.2.3), type definitions (6.7.7).
6.7.2.1 Structure and union specifiers
Syntax
1 struct-or-union-specifier:
struct-or-union identifier opt { struct-declaration-list }
struct-or-union identifier
struct-or-union:
struct
union
struct-declaration-list:
struct-declaration
struct-declaration-list struct-declaration
struct-declaration:
specifier-qualifier-list struct-declarator-list ;
specifier-qualifier-list:
type-specifier specifier-qualifier-list opt
type-qualifier specifier-qualifier-list opt
struct-declarator-list:
struct-declarator
struct-declarator-list , struct-declarator
struct-declarator:
declarator
declarator opt : constant-expression
Constraints
2 A structure or union shall not contain a member with incomplete or function type (hence,
a structure shall not contain an instance of itself, but may contain a pointer to an instance
of itself), except that the last member of a structure with more than one named member
may have incomplete array type; such a structure (and any union containing, possibly
recursively, a member that is such a structure) shall not be a member of a structure or an
element of an array.
3 The expression that specifies the width of a bit-field shall be an integer constant
expression that has nonnegative value that shall not exceed the number of bits in an object
of the type that is specified if the colon and expression are omitted. If the value is zero,
the declaration shall have no declarator.
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Semantics
4 As discussed in 6.2.5, a structure is a type consisting of a sequence of members, whose
storage is allocated in an ordered sequence, and a union is a type consisting of a sequence
of members whose storage overlap.
5 Structure and union specifiers have the same form.
6 The presence of a struct-declaration-list in a struct-or-union-specifier declares a new type,
within a translation unit. The struct-declaration-list is a sequence of declarations for the
members of the structure or union. If the struct-declaration-list contains no named
members, the behavior is undefined. The type is incomplete until after the } that
terminates the list.
7 A member of a structure or union may have any object type other than a variably
modified type. 93) In addition, a member may be declared to consist of a specified number
of bits (including a sign bit, if any). Such a member is called a bit-field; 94) its width is
preceded by a colon.
8 A bit-field shall have a type that is a qualified or unqualified version of _Bool, signed
int, orunsigned int. A bit-field is interpreted as a signed or unsigned integer type
consisting of the specified number of bits. 95) If the value 0 or 1 is stored into a nonzero-width
bit-field of type _Bool, the value of the bit-field shall compare equal to the value
stored.
9 An implementation may allocate any addressable storage unit large enough to hold a bit-field.
If enough space remains, a bit-field that immediately follows another bit-field in a
structure shall be packed into adjacent bits of the same unit. If insufficient space remains,
whether a bit-field that does not fit is put into the next unit or overlaps adjacent units is
implementation-defined. The order of allocation of bit-fields within a unit (high-order to
low-order or low-order to high-order) is implementation-defined. The alignment of the
addressable storage unit is unspecified.
10 A bit-field declaration with no declarator, but only a colon and a width, indicates an
unnamed bit-field. 96) As a special case, a bit-field structure member with a width of 0
indicates that no further bit-field is to be packed into the unit in which the previous bit-93)
A structure or union can not contain a member with a variably modified type because member names
are not ordinary identifiers as defined in 6.2.3.
94) The unary & (address-of) operator cannot be applied to a bit-field object; thus, there are no pointers to
or arrays of bit-field objects.
95) As specified in 6.7.2 above, if the actual type specifier used is int or a typedef-name defined as int,
then it is implementation-defined whether the bit-field is signed or unsigned.
96) An unnamed bit-field structure member is useful for padding to conform to externally imposed
layouts.
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field, if any, was placed.
11 Each non-bit-field member of a structure or union object is aligned in an implementation-defined
manner appropriate to its type.
12 Within a structure object, the non-bit-field members and the units in which bit-fields
reside have addresses that increase in the order in which they are declared. A pointer to a
structure object, suitably converted, points to its initial member (or if that member is a
bit-field, then to the unit in which it resides), and vice versa. There may be unnamed
padding within a structure object, but not at its beginning.
13 The size of a union is sufficient to contain the largest of its members. The value of at
most one of the members can be stored in a union object at any time. A pointer to a
union object, suitably converted, points to each of its members (or if a member is a bit-field,
then to the unit in which it resides), and vice versa.
14 There may be unnamed padding at the end of a structure or union.
15 As a special case, the last element of a structure with more than one named member may
have an incomplete array type. This is called a flexible array member, and the size of the
structure shall be equal to the offset of the last element of an otherwise identical structure
that replaces the flexible array member with an array of unspecified length. 97) When an
lvalue whose type is a structure with a flexible array member is used to access an object,
it behaves as if that member were replaced with the longest array, with the same element
type, that would not make the structure larger than the object being accessed; the offset of
the array shall remain that of the flexible array member, even if this would differ from
that of the replacement array. If this array would have no elements, then it behaves as if it
had one element, but the behavior is undefined if any attempt is made to access that
element or to generate a pointer one past it.
16 EXAMPLE Assuming that all array members are aligned the same, after the declarations:
struct s { int n; double d[]; };
struct ss { int n; double d[ 1]; };
the three expressions:
sizeof (struct s)
offsetof( struct s, d)
offsetof( struct ss, d)
have the same value. The structure struct s has a flexible array member d.
17 If sizeof (double) is 8, then after the following code is executed:
97) The length is unspecified to allow for the fact that implementations may give array members different
alignments according to their lengths.
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struct s *s1;
struct s *s2;
s1 = malloc( sizeof (struct s) + 64);
s2 = malloc( sizeof (struct s) + 46);
and assuming that the calls to malloc succeed, the objects pointed to by s1 and s2 behave as if the
identifiers had been declared as:
struct { int n; double d[ 8]; } *s1;
struct { int n; double d[ 5]; } *s2;
18 Following the further successful assignments:
s1 = malloc( sizeof (struct s) + 10);
s2 = malloc( sizeof (struct s) + 6);
they then behave as if the declarations were:
struct { int n; double d[ 1]; } *s1, *s2;
and:
double *dp;
dp = &( s1-> d[ 0]); // Permitted
*dp = 42; // Permitted
dp = &( s2-> d[ 0]); // Permitted
*dp = 42; // Undefined behavior
Forward references: tags (6.7.2.3).
6.7.2.2 Enumeration specifiers
Syntax
1 enum-specifier:
enum identifier opt { enumerator-list }
enum identifier opt { enumerator-list ,}
enum identifier
enumerator-list:
enumerator
enumerator-list , enumerator
enumerator:
enumeration-constant
enumeration-constant = constant-expression
Constraints
2 The expression that defines the value of an enumeration constant shall be an integer
constant expression that has a value representable as an int.
Semantics
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3 The identifiers in an enumerator list are declared as constants that have type int and
may appear wherever such are permitted. 98) An enumerator with = defines its
enumeration constant as the value of the constant expression. If the first enumerator has
no =, the value of its enumeration constant is 0. Each subsequent enumerator with no =
defines its enumeration constant as the value of the constant expression obtained by
adding 1 to the value of the previous enumeration constant. (The use of enumerators with
= may produce enumeration constants with values that duplicate other values in the same
enumeration.) The enumerators of an enumeration are also known as its members.
4 Each enumerated type shall be compatible with an integer type. The choice of type is
implementation-defined, 99) but shall be capable of representing the values of all the
members of the enumeration. The enumerated type is incomplete until after the } that
terminates the list of enumerator declarations.
5 EXAMPLE The following fragment:
enum hue { chartreuse, burgundy, claret= 20, winedark };
enum hue col, *cp;
col = claret;
cp = &col;
if (* cp != burgundy)
/* ... */
makes hue the tag of an enumeration, and then declares col as an object that has that type and cp as a
pointer to an object that has that type. The enumerated values are in the set { 0, 1, 20, 21 }.
Forward references: tags (6.7.2.3).
6.7.2.3 Tags
Constraints
1 A specific type shall have its content defined at most once.
2 A type specifier of the form
enum identifier
without an enumerator list shall only appear after the type it specifies is completed.
Semantics
3 All declarations of structure, union, or enumerated types that have the same scope and
use the same tag declare the same type. The type is incomplete 100) until the closing
brace of the list defining the content, and complete thereafter.
98) Thus, the identifiers of enumeration constants declared in the same scope shall all be distinct from
each other and from other identifiers declared in ordinary declarators.
99) An implementation may delay the choice of which integer type until all enumeration constants have
been seen.
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4 Tw o declarations of structure, union, or enumerated types which are in different scopes or
use different tags declare distinct types. Each declaration of a structure, union, or
enumerated type which does not include a tag declares a distinct type.
5 A type specifier of the form
struct-or-union identifier opt { struct-declaration-list }
or
enum identifier { enumerator-list }
or
enum identifier { enumerator-list ,}
declares a structure, union, or enumerated type. The list defines the structure content,
union content, orenumeration content. If an identifier is provided, 101) the type specifier
also declares the identifier to be the tag of that type.
6 A declaration of the form
struct-or-union identifier ;
specifies a structure or union type and declares the identifier as a tag of that type. 102)
7 If a type specifier of the form
struct-or-union identifier
occurs other than as part of one of the above forms, and no other declaration of the
identifier as a tag is visible, then it declares an incomplete structure or union type, and
declares the identifier as the tag of that type. 102)
8 If a type specifier of the form
struct-or-union identifier
or
enum identifier
occurs other than as part of one of the above forms, and a declaration of the identifier as a
tag is visible, then it specifies the same type as that other declaration, and does not
100) An incomplete type may only by used when the size of an object of that type is not needed. It is not
needed, for example, when a typedef name is declared to be a specifier for a structure or union, or
when a pointer to or a function returning a structure or union is being declared. (See incomplete types
in 6.2.5.) The specification has to be complete before such a function is called or defined.
101) If there is no identifier, the type can, within the translation unit, only be referred to by the declaration
of which it is a part. Of course, when the declaration is of a typedef name, subsequent declarations
can make use of that typedef name to declare objects having the specified structure, union, or
enumerated type.
102) A similar construction with enum does not exist.
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redeclare the tag.
9 EXAMPLE 1 This mechanism allows declaration of a self-referential structure.
struct tnode {
int count;
struct tnode *left, *right;
};
specifies a structure that contains an integer and two pointers to objects of the same type. Once this
declaration has been given, the declaration
struct tnode s, *sp;
declares s to be an object of the given type and sp to be a pointer to an object of the given type. With
these declarations, the expression sp-> left refers to the left struct tnode pointer of the object to
which sp points; the expression s. right-> count designates the count member of the right struct
tnode pointed to from s.
10 The following alternative formulation uses the typedef mechanism:
typedef struct tnode TNODE;
struct tnode {
int count;
TNODE *left, *right;
};
TNODE s, *sp;
11 EXAMPLE 2 To illustrate the use of prior declaration of a tag to specify a pair of mutually referential structures, the declarations
struct s1 { struct s2 *s2p; /* ... */ }; // D1
struct s2 { struct s1 *s1p; /* ... */ }; // D2
specify a pair of structures that contain pointers to each other. Note, however, that if s2 were already
declared as a tag in an enclosing scope, the declaration D1 would refer to it, not to the tag s2 declared in
D2. To eliminate this context sensitivity, the declaration
struct s2;
may be inserted ahead of D1. This declares a new tag s2 in the inner scope; the declaration D2 then
completes the specification of the new type.
Forward references: declarators (6.7.5), array declarators (6.7.5.2), type definitions
(6.7.7).
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6.7.3 Type qualifiers
Syntax
1 type-qualifier:
const
restrict
volatile
Constraints
2 Types other than pointer types derived from object or incomplete types shall not be
restrict-qualified.
Semantics
3 The properties associated with qualified types are meaningful only for expressions that
are lvalues. 103)
4 If the same qualifier appears more than once in the same specifier-qualifier-list, either
directly or via one or more typedefs, the behavior is the same as if it appeared only
once.
5 If an attempt is made to modify an object defined with a const-qualified type through use
of an lvalue with non-const-qualified type, the behavior is undefined. If an attempt is
made to refer to an object defined with a volatile-qualified type through use of an lvalue
with non-volatile-qualified type, the behavior is undefined. 104)
6 An object that has volatile-qualified type may be modified in ways unknown to the
implementation or have other unknown side effects. Therefore any expression referring
to such an object shall be evaluated strictly according to the rules of the abstract machine,
as described in 5.1.2.3. Furthermore, at every sequence point the value last stored in the
object shall agree with that prescribed by the abstract machine, except as modified by the
unknown factors mentioned previously. 105) What constitutes an access to an object that
has volatile-qualified type is implementation-defined.
103) The implementation may place a const object that is not volatile in a read-only region of
storage. Moreover, the implementation need not allocate storage for such an object if its address is
never used.
104) This applies to those objects that behave as if they were defined with qualified types, even if they are
never actually defined as objects in the program (such as an object at a memory-mapped input/ output
address).
105) A volatile declaration may be used to describe an object corresponding to a memory-mapped
input/ output port or an object accessed by an asynchronously interrupting function. Actions on
objects so declared shall not be '' optimized out'' by an implementation or reordered except as
permitted by the rules for evaluating expressions.
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7 An object that is accessed through a restrict-qualified pointer has a special association
with that pointer. This association, defined in 6.7.3.1 below, requires that all accesses to
that object use, directly or indirectly, the value of that particular pointer. 106) The intended
use of the restrict qualifier (like the register storage class) is to promote
optimization, and deleting all instances of the qualifier from a conforming program does
not change its meaning (i. e., observable behavior).
8 If the specification of an array type includes any type qualifiers, the element type is so-qualified,
not the array type. If the specification of a function type includes any type
qualifiers, the behavior is undefined. 107)
9 For two qualified types to be compatible, both shall have the identically qualified version
of a compatible type; the order of type qualifiers within a list of specifiers or qualifiers
does not affect the specified type.
10 EXAMPLE 1 An object declared
extern const volatile int real_ time_ clock;
may be modifiable by hardware, but cannot be assigned to, incremented, or decremented.
11 EXAMPLE 2 The following declarations and expressions illustrate the behavior when type qualifiers modify an aggregate type:
const struct s { int mem; } cs = { 1 };
struct s ncs; // the object ncs is modifiable
typedef int A[ 2][ 3];
const A a = {{ 4, 5, 6}, {7, 8, 9}}; // array of array of
// const int
int *pi;
const int *pci;
ncs = cs; // valid
cs = ncs; // violates modifiable lvalue constraint for =
pi = &ncs. mem; // valid
pi = &cs. mem; // violates type constraints for =
pci = &cs. mem; // valid
pi = a[ 0]; // invalid: a[ 0] has type '' const int *''
6.7.3.1 Formal definition of restrict
1 Let D be a declaration of an ordinary identifier that provides a means of designating an
object P as a restrict-qualified pointer.
2 IfDappears inside a block and does not have storage class extern, let B denote the
block. If D appears in the list of parameter declarations of a function definition, let B
denote the associated block. Otherwise, let B denote the block of main (or the block of
106) For example, a statement that assigns a value returned by malloc to a single pointer establishes this
association between the allocated object and the pointer.
107) Both of these can occur through the use of typedefs.
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whatever function is called at program startup in a freestanding environment).
3 In what follows, a pointer expression E is said to be based on object P if (at some
sequence point in the execution of B prior to the evaluation of E) modifying P to point to
a copy of the array object into which it formerly pointed would change the value of E. 108)
Note that '' based'' is defined only for expressions with pointer types.
4 During each execution of B, let A be the array object that is determined dynamically by
all accesses through pointer expressions based on P. Then all accesses to values of A shall
be through pointer expressions based on P. Furthermore, if P is assigned the value of a
pointer expression E that is based on another restricted pointer object P2, associated with
block B2, then either the execution of B2 shall begin before the execution of B, or the
execution of B2 shall end prior to the assignment. If these requirements are not met, then
the behavior is undefined.
5 Here an execution of B means that portion of the execution of the program during which
storage is guaranteed to be reserved for an instance of an object that is associated with B
and that has automatic storage duration. An access to a value means either fetching it or
modifying it; expressions that are not evaluated do not access values.
6 A translator is free to ignore any or all aliasing implications of uses of restrict.
7 EXAMPLE 1 The file scope declarations
int * restrict a;
int * restrict b;
extern int c[];
assert that if an object is accessed using the value of one of a, b, orc, then it is never accessed using the
value of either of the other two.
8 EXAMPLE 2 The function parameter declarations in the following example
void f( int n, int * restrict p, int * restrict q)
{
while (n--> 0)
*p++ = *q++;
}
assert that, during each execution of the function, if an object is accessed through one of the pointer
parameters, then it is not also accessed through the other.
9 The benefit of the restrict qualifiers is that they enable a translator to make an effective dependence analysis of function f without examining any of the calls of f in the program. The cost is that the
programmer has to examine all of those calls to ensure that none give undefined behavior. For example, the
second call of f in g has undefined behavior because each of d[ 1] through d[ 49] is accessed through
108) In other words, E depends on the value of P itself rather than on the value of an object referenced
indirectly through P. For example, if identifier p has type (int ** restrict), then the pointer
expressions p and p+ 1 are based on the restricted pointer object designated by p, but the pointer
expressions *p and p[ 1] are not.
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both p and q.
void g( void)
{
extern int d[ 100];
f( 50, d + 50, d); // ok
f( 50, d + 1, d); // undefined behavior
}
10 EXAMPLE 3 The function parameter declarations
void h( int n, int * const restrict p,
int * const q, int * const r)
{
int i;
for (i = 0; i < n; i++)
p[ i] = q[ i] + r[ i];
}
show how const can be used in conjunction with restrict. The const qualifiers imply, without the
need to examine the body of h, that q and r cannot become based on p. The fact that p is restrict-qualified
therefore implies that an object accessed through p is never accessed through either of q or r. This is the
precise assertion required to optimize the loop. Note that a call of the form h( 100, a, b, b) has
defined behavior, which would not be true if all three of p, q, and r were restrict-qualified.
11 EXAMPLE 4 The rule limiting assignments between restricted pointers does not distinguish between a function call and an equivalent nested block. With one exception, only '' outer-to-inner'' assignments
between restricted pointers declared in nested blocks have defined behavior.
{
int * restrict p1;
int * restrict q1;
p1 = q1; // undefined behavior
{
int * restrict p2 = p1; // ok
int * restrict q2 = q1; // ok
p1 = q2; // undefined behavior
p2 = q2; // undefined behavior
}
}
The exception allows the value of a restricted pointer to be carried out of the block in which it (or, more
precisely, the ordinary identifier used to designate it) is declared when that block finishes execution. For
example, this permits new_ vector to return a vector.
typedef struct { int n; float * restrict v; } vector;
vector new_ vector( int n)
{
vector t;
t. n = n;
t. v = malloc( n * sizeof (float));
return t;
}
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6.7.4 Function specifiers
Syntax
1 function-specifier:
inline
Constraints
2 Function specifiers shall be used only in the declaration of an identifier for a function.
3 An inline definition of a function with external linkage shall not contain a definition of a
modifiable object with static storage duration, and shall not contain a reference to an
identifier with internal linkage.
4 The inline function specifier shall not appear in a declaration of main.
Semantics
5 A function declared with an inline function specifier is an inline function. The
function specifier may appear more than once; the behavior is the same as if it appeared
only once. Making a function an inline function suggests that calls to the function be as
fast as possible. 109) The extent to which such suggestions are effective is
implementation-defined. 110)
6 Any function with internal linkage can be an inline function. For a function with external
linkage, the following restrictions apply: If a function is declared with an inline
function specifier, then it shall also be defined in the same translation unit. If all of the
file scope declarations for a function in a translation unit include the inline function
specifier without extern, then the definition in that translation unit is an inline
definition. An inline definition does not provide an external definition for the function,
and does not forbid an external definition in another translation unit. An inline definition
provides an alternative to an external definition, which a translator may use to implement
any call to the function in the same translation unit. It is unspecified whether a call to the
function uses the inline definition or the external definition. 111)
109) By using, for example, an alternative to the usual function call mechanism, such as '' inline
substitution''.
Inline substitution is not textual substitution, nor does it create a new function. Therefore, for
example, the expansion of a macro used within the body of the function uses the definition it had at
the point the function body appears, and not where the function is called; and identifiers refer to the
declarations in scope where the body occurs. Similarly, the address of the function is not affected by
the function's being inlined.
110) For example, an implementation might never perform inline substitution, or might only perform inline
substitutions to calls in the scope of an inline declaration.
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7 EXAMPLE The declaration of an inline function with external linkage can result in either an external definition, or a definition available for use only within the translation unit. A file scope declaration with
extern creates an external definition. The following example shows an entire translation unit.
inline double fahr( double t)
{
return (9.0 * t) / 5.0 + 32.0;
}
inline double cels( double t)
{
return (5.0 * (t -32.0)) / 9.0;
}
extern double fahr( double); // creates an external definition
double convert( int is_ fahr, double temp)
{
/* A translator may perform inline substitutions. */
return is_ fahr ? cels( temp) : fahr( temp);
}
8 Note that the definition of fahr is an external definition because fahr is also declared with extern, but the definition of cels is an inline definition. Because cels has external linkage and is referenced, an
external definition has to appear in another translation unit (see 6.9); the inline definition and the external
definition are distinct and either may be used for the call.
6.7.5 Declarators
Syntax
1 declarator:
pointer opt direct-declarator
direct-declarator:
identifier
( declarator )
direct-declarator [ assignment-expression opt ]
direct-declarator [*]
direct-declarator ( parameter-type-list )
direct-declarator ( identifier-list opt )
pointer:
* type-qualifier-list opt
* type-qualifier-list opt pointer
111) Since an inline definition is distinct from the corresponding external definition and from any other
corresponding inline definitions in other translation units, all corresponding objects with static storage
duration are also distinct in each of the definitions.
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type-qualifier-list:
type-qualifier
type-qualifier-list type-qualifier
parameter-type-list:
parameter-list
parameter-list , ...
parameter-list:
parameter-declaration
parameter-list , parameter-declaration
parameter-declaration:
declaration-specifiers declarator
declaration-specifiers abstract-declarator opt
identifier-list:
identifier
identifier-list , identifier
Semantics
2 Each declarator declares one identifier, and asserts that when an operand of the same
form as the declarator appears in an expression, it designates a function or object with the
scope, storage duration, and type indicated by the declaration specifiers.
3 Afull declarator is a declarator that is not part of another declarator. The end of a full
declarator is a sequence point. If the nested sequence of declarators in a full declarator
contains a variable length array type, the type specified by the full declarator is said to be
variably modified.
4 In the following subclauses, consider a declaration
T D1
where T contains the declaration specifiers that specify a type T (such as int) and D1 is
a declarator that contains an identifier ident. The type specified for the identifier ident in
the various forms of declarator is described inductively using this notation.
5 If, in the declaration ''T D1'' , D1 has the form
identifier
then the type specified for ident is T.
6 If, in the declaration ''T D1'' , D1 has the form
(D )
then ident has the type specified by the declaration ''T D''. Thus, a declarator in
parentheses is identical to the unparenthesized declarator, but the binding of complicated
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declarators may be altered by parentheses.
Implementation limits
7 As discussed in 5.2.4.1, an implementation may limit the number of pointer, array, and
function declarators that modify an arithmetic, structure, union, or incomplete type, either
directly or via one or more typedefs.
Forward references: array declarators (6.7.5.2), type definitions (6.7.7).
6.7.5.1 Pointer declarators
Semantics
1 If, in the declaration ''T D1'' , D1 has the form
* type-qualifier-list opt D
and the type specified for ident in the declaration ''T D'' is '' derived-declarator-type-list
T'', then the type specified for ident is '' derived-declarator-type-list type-qualifier-list
pointer to T''. For each type qualifier in the list, ident is a so-qualified pointer.
2 For two pointer types to be compatible, both shall be identically qualified and both shall
be pointers to compatible types.
3 EXAMPLE The following pair of declarations demonstrates the difference between a '' variable pointer to a constant value'' and a '' constant pointer to a variable value''.
const int *ptr_ to_ constant;
int *const constant_ ptr;
The contents of any object pointed to by ptr_ to_ constant shall not be modified through that pointer,
but ptr_ to_ constant itself may be changed to point to another object. Similarly, the contents of the
int pointed to by constant_ ptr may be modified, but constant_ ptr itself shall always point to the
same location.
4 The declaration of the constant pointer constant_ ptr may be clarified by including a definition for the type '' pointer to int'' .
typedef int *int_ ptr;
const int_ ptr constant_ ptr;
declares constant_ ptr as an object that has type '' const-qualified pointer to int'' .
6.7.5.2 Array declarators
Constraints
1 The [ and ] may delimit an expression or *. If[ and ] delimit an expression (which
specifies the size of an array), it shall have an integer type. If the expression is a constant
expression then it shall have a value greater than zero. The element type shall not be an
incomplete or function type.
2 Only ordinary identifiers (as defined in 6.2.3) with both block scope or function prototype
scope and no linkage shall have a variably modified type. If an identifier is declared to be
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an object with static storage duration, it shall not have a variable length array type.
Semantics
3 If, in the declaration ''T D1'' , D1 has the form
D[ assignment-expression opt ]
or
D[*]
and the type specified for ident in the declaration ''T D'' is '' derived-declarator-type-list
T'', then the type specified for ident is '' derived-declarator-type-list array of T'' . 112) If
the size is not present, the array type is an incomplete type. If * is used instead of a size
expression, the array type is a variable length array type of unspecified size, which can
only be used in declarations with function prototype scope. 113) If the size expression is
an integer constant expression and the element type has a known constant size, the array
type is not a variable length array type; otherwise, the array type is a variable length array
type. If the size expression is not a constant expression, and it is evaluated at program
execution time, it shall evaluate to a value greater than zero. It is unspecified whether
side effects are produced when the size expression is evaluated. The size of each instance
of a variable length array type does not change during its lifetime.
4 For two array types to be compatible, both shall have compatible element types, and if
both size specifiers are present, and are integer constant expressions, then both size
specifiers shall have the same constant value. If the two array types are used in a context
which requires them to be compatible, it is undefined behavior if the two size specifiers
evaluate to unequal values.
5 EXAMPLE 1
float fa[ 11], *afp[ 17];
declares an array of float numbers and an array of pointers to float numbers.
6 EXAMPLE 2 Note the distinction between the declarations
extern int *x;
extern int y[];
The first declares x to be a pointer to int; the second declares y to be an array of int of unspecified size
(an incomplete type), the storage for which is defined elsewhere.
7 EXAMPLE 3 The following declarations demonstrate the compatibility rules for variably modified types.
112) When several '' array of'' specifications are adjacent, a multidimensional array is declared.
113) Thus, * can be used only in function declarations that are not definitions (see 6.7.5.3).
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extern int n;
extern int m;
void fcompat( void)
{
int a[ n][ 6][ m];
int (* p)[ 4][ n+ 1];
int c[ n][ n][ 6][ m];
int (* r)[ n][ n][ n+ 1];
p =a;// Error -not compatible because 4 != 6.
r =c;// Compatible, but defined behavior
// only if n == 6and m == n+ 1.
}
8 EXAMPLE 4 All declarations of variably modified (VM) types have to be at either block scope or function prototype scope. Array objects declared with the static or extern storage class specifier
cannot have a variable length array (VLA) type. However, an object declared with the static storage
class specifier can have a VM type (that is, a pointer to a VLA type). Finally, all identifiers declared with a
VM type have to be ordinary identifiers and cannot, therefore, be members of structures or unions.
extern int n;
int A[ n]; // Error -file scope VLA.
extern int (* p2)[ n]; // Error -file scope VM.
int B[ 100]; // OK -file scope but not VM.
void fvla( int m, int C[ m][ m]) // OK -VLA with prototype scope.
{
typedef int VLA[ m][ m] // OK -block scope typedef VLA.
struct tag {
int (* y)[ n]; // Error -y not ordinary identifier.
int z[ n]; // Error -z not ordinary identifier.
};
int D[ m]; // OK -auto VLA.
static int E[ m]; // Error -static block scope VLA.
extern int F[ m]; // Error -F has linkage and is VLA.
int (* s)[ m]; // OK -auto pointer to VLA.
extern int (* r)[ m]; // Error -r had linkage and is
// a pointer to VLA.
static int (* q)[ m] = &B; // OK -q is a static block
// pointer to VLA.
}
Forward references: function declarators (6.7.5.3), function definitions (6.9.1),
initialization (6.7.8).
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6.7.5.3 Function declarators (including prototypes)
Constraints
1 A function declarator shall not specify a return type that is a function type or an array
type.
2 The only storage-class specifier that shall occur in a parameter declaration is register.
3 An identifier list in a function declarator that is not part of a definition of that function
shall be empty.
4 After adjustment, the parameters in a parameter type list in a function declarator that is
part of a definition of that function shall not have incomplete type. *
Semantics
5 If, in the declaration ''T D1'' , D1 has the form
D( parameter-type-list)
or
D( identifier-list opt )
and the type specified for ident in the declaration ''T D'' is '' derived-declarator-type-list
T'', then the type specified for ident is '' derived-declarator-type-list function returning
T'' .
6 A parameter type list specifies the types of, and may declare identifiers for, the
parameters of the function. A declaration of a parameter as '' array of type'' shall be
adjusted to '' pointer to type'', and a declaration of a parameter as '' function returning
type'' shall be adjusted to '' pointer to function returning type'', as in 6.3.2.1. If the list
terminates with an ellipsis (, ...), no information about the number or types of the
parameters after the comma is supplied. 114) The special case of an unnamed parameter of
type void as the only item in the list specifies that the function has no parameters.
7 In a parameter declaration, a single typedef name in parentheses is taken to be an abstract
declarator that specifies a function with a single parameter, not as redundant parentheses
around the identifier for a declarator.
8 If the function declarator is not part of a definition of that function, parameters may have
incomplete type and may use the [*] notation in their sequences of declarator specifiers
to specify variable length array types.
9 The storage-class specifier in the declaration specifiers for a parameter declaration, if
present, is ignored unless the declared parameter is one of the members of the parameter
type list for a function definition.
114) The macros defined in the <stdarg. h> header (7.15) may be used to access arguments that
correspond to the ellipsis.
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10 An identifier list declares only the identifiers of the parameters of the function. An empty
list in a function declarator that is part of a definition of that function specifies that the
function has no parameters. The empty list in a function declarator that is not part of a
definition of that function specifies that no information about the number or types of the
parameters is supplied. 115)
11 For two function types to be compatible, both shall specify compatible return types. 116)
Moreover, the parameter type lists, if both are present, shall agree in the number of
parameters and in use of the ellipsis terminator; corresponding parameters shall have
compatible types. If one type has a parameter type list and the other type is specified by a
function declarator that is not part of a function definition and that contains an empty
identifier list, the parameter list shall not have an ellipsis terminator and the type of each
parameter shall be compatible with the type that results from the application of the
default argument promotions. If one type has a parameter type list and the other type is
specified by a function definition that contains a (possibly empty) identifier list, both shall
agree in the number of parameters, and the type of each prototype parameter shall be
compatible with the type that results from the application of the default argument
promotions to the type of the corresponding identifier. (In the determination of type
compatibility and of a composite type, each parameter declared with function or array
type is taken as having the adjusted type and each parameter declared with qualified type
is taken as having the unqualified version of its declared type.)
12 EXAMPLE 1 The declaration
int f( void), *fip(), (* pfi)();
declares a function f with no parameters returning an int, a function fip with no parameter specification
returning a pointer to an int, and a pointer pfi to a function with no parameter specification returning an
int. It is especially useful to compare the last two. The binding of *fip() is *( fip()), so that the
declaration suggests, and the same construction in an expression requires, the calling of a function fip,
and then using indirection through the pointer result to yield an int. In the declarator (* pfi)(), the
extra parentheses are necessary to indicate that indirection through a pointer to a function yields a function
designator, which is then used to call the function; it returns an int.
13 If the declaration occurs outside of any function, the identifiers have file scope and external linkage. If the declaration occurs inside a function, the identifiers of the functions f and fip have block scope and either
internal or external linkage (depending on what file scope declarations for these identifiers are visible), and
the identifier of the pointer pfi has block scope and no linkage.
14 EXAMPLE 2 The declaration
int (* apfi[ 3])( int *x, int *y);
declares an array apfi of three pointers to functions returning int. Each of these functions has two
parameters that are pointers to int. The identifiers x and y are declared for descriptive purposes only and
go out of scope at the end of the declaration of apfi.
115) See '' future language directions'' (6.11.4).
116) If both function types are '' old style'', parameter types are not compared.
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15 EXAMPLE 3 The declaration
int (* fpfi( int (*)( long), int))( int, ...);
declares a function fpfi that returns a pointer to a function returning an int. The function fpfi has two
parameters: a pointer to a function returning an int (with one parameter of type long int), and an int.
The pointer returned by fpfi points to a function that has one int parameter and accepts zero or more
additional arguments of any type.
16 EXAMPLE 4 The following prototype has a variably modified parameter.
void addscalar( int n, int m,
double a[ n][ n* m+ 300], double x);
int main()
{
double b[ 4][ 308];
addscalar( 4, 2, b, 2.17);
return 0;
}
void addscalar( int n, int m,
double a[ n][ n* m+ 300], double x)
{
for (int i = 0; i < n; i++)
for (int j = 0, k = n* m+ 300; j < k; j++)
// a is a pointer to a VLA
// with n* m+ 300 elements
a[ i][ j] += x;
}
17 EXAMPLE 5 The following are all compatible function prototype declarators.
double maximum( int n, int m, double a[ n][ m]);
double maximum( int n, int m, double a[*][*]);
double maximum( int n, int m, double a[ ][*]);
double maximum( int n, int m, double a[ ][ m]);
Forward references: function definitions (6.9.1), type names (6.7.6).
6.7.6 Type names
Syntax
1 type-name:
specifier-qualifier-list abstract-declarator opt
abstract-declarator:
pointer
pointer opt direct-abstract-declarator
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direct-abstract-declarator:
( abstract-declarator )
direct-abstract-declarator opt [ assignment-expression opt ]
direct-abstract-declarator opt [*]
direct-abstract-declarator opt ( parameter-type-list opt )
Semantics
2 In several contexts, it is necessary to specify a type. This is accomplished using a type
name, which is syntactically a declaration for a function or an object of that type that
omits the identifier. 117)
3 EXAMPLE The constructions
(a) int
(b) int *
(c) int *[ 3]
(d) int (*)[ 3]
(e) int (*)[*]
(f) int *()
(g) int (*)( void)
(h) int (* const [])( unsigned int, ...)
name respectively the types (a) int, (b) pointer to int, (c) array of three pointers to int, (d) pointer to an
array of three ints, (e) pointer to a variable length array of an unspecified number of ints, (f) function
with no parameter specification returning a pointer to int, (g) pointer to function with no parameters
returning an int, and (h) array of an unspecified number of constant pointers to functions, each with one
parameter that has type unsigned int and an unspecified number of other parameters, returning an
int.
6.7.7 Type definitions
Syntax
1 typedef-name:
identifier
Constraints
2 If a typedef name specifies a variably modified type then it shall have block scope.
Semantics
3 In a declaration whose storage-class specifier is typedef, each declarator defines an
identifier to be a typedef name that denotes the type specified for the identifier in the way
described in 6.7.5. Any array size expressions associated with variable length array
declarators are evaluated each time the declaration of the typedef name is reached in the
order of execution. A typedef declaration does not introduce a new type, only a
117) As indicated by the syntax, empty parentheses in a type name are interpreted as '' function with no
parameter specification'', rather than redundant parentheses around the omitted identifier.
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synonym for the type so specified. That is, in the following declarations:
typedef T type_ ident;
type_ ident D;
type_ ident is defined as a typedef name with the type specified by the declaration
specifiers in T (known as T), and the identifier in D has the type '' derived-declarator-type-
list T '' where the derived-declarator-type-list is specified by the declarators of D. A
typedef name shares the same name space as other identifiers declared in ordinary
declarators.
4 EXAMPLE 1 After
typedef int MILES, KLICKSP();
typedef struct { double hi, lo; } range;
the constructions
MILES distance;
extern KLICKSP *metricp;
range x;
range z, *zp;
are all valid declarations. The type of distance is int, that of metricp is '' pointer to function with no
parameter specification returning int'', and that of x and z is the specified structure; zp is a pointer to
such a structure. The object distance has a type compatible with any other int object.
5 EXAMPLE 2 After the declarations
typedef struct s1 { int x; } t1, *tp1;
typedef struct s2 { int x; } t2, *tp2;
type t1 and the type pointed to by tp1 are compatible. Type t1 is also compatible with type struct
s1, but not compatible with the types struct s2, t2, the type pointed to by tp2, orint.
6 EXAMPLE 3 The following obscure constructions
typedef signed int t;
typedef int plain;
struct tag {
unsigned t: 4;
const t: 5;
plain r: 5;
};
declare a typedef name t with type signed int, a typedef name plain with type int, and a structure
with three bit-field members, one named t that contains values in the range [0, 15], an unnamed const-qualified
bit-field which (if it could be accessed) would contain values in at least the range [15, +15], and
one named r that contains values in the range [0, 31] or values in at least the range [15, +15]. (The choice
of range is implementation-defined.) The first two bit-field declarations differ in that unsigned is a type
specifier (which forces t to be the name of a structure member), while const is a type qualifier (which
modifies t which is still visible as a typedef name). If these declarations are followed in an inner scope by
t f( t (t));
long t;
then a function f is declared with type '' function returning signed int with one unnamed parameter
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with type pointer to function returning signed int with one unnamed parameter with type signed
int'', and an identifier t with type long int.
7 EXAMPLE 4 On the other hand, typedef names can be used to improve code readability. All three of the following declarations of the signal function specify exactly the same type, the first without making use
of any typedef names.
typedef void fv( int), (* pfv)( int);
void (* signal( int, void (*)( int)))( int);
fv *signal( int, fv *);
pfv signal( int, pfv);
8 EXAMPLE 5 If a typedef name denotes a variable length array type, the length of the array is fixed at the time the typedef name is defined, not each time it is used:
void copyt( int n)
{
typedef int B[ n]; // B is n ints, n evaluated now.
n += 1;
B a; // ais n ints, n without += 1.
int b[ n]; // a and b are different sizes
for (int i = 1; i < n; i++)
a[ i-1] = b[ i];
}
Forward references: the signal function (7.14.1.1).
6.7.8 Initialization
Syntax
1 initializer:
assignment-expression
{ initializer-list }
{ initializer-list ,}
initializer-list:
designation opt initializer
initializer-list , designation opt initializer
designation:
designator-list =
designator-list:
designator
designator-list designator
designator:
[ constant-expression ]
. identifier
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Constraints
2 No initializer shall attempt to provide a value for an object not contained within the entity
being initialized.
3 The type of the entity to be initialized shall be an array of unknown size or an object type
that is not a variable length array type.
4 All the expressions in an initializer for an object that has static storage duration shall be
constant expressions or string literals.
5 If the declaration of an identifier has block scope, and the identifier has external or
internal linkage, the declaration shall have no initializer for the identifier.
6 If a designator has the form
[ constant-expression ]
then the current object (defined below) shall have array type and the expression shall be
an integer constant expression. If the array is of unknown size, any nonnegative value is
valid.
7 If a designator has the form
. identifier
then the current object (defined below) shall have structure or union type and the
identifier shall be the name of a member of that type.
Semantics
8 An initializer specifies the initial value stored in an object.
9 Except where explicitly stated otherwise, for the purposes of this subclause unnamed
members of objects of structure and union type do not participate in initialization.
Unnamed members of structure objects have indeterminate value even after initialization.
10 If an object that has automatic storage duration is not initialized explicitly, its value is
indeterminate. If an object that has static storage duration is not initialized explicitly,
then:
if it has pointer type, it is initialized to a null pointer;
if it has arithmetic type, it is initialized to (positive or unsigned) zero;
if it is an aggregate, every member is initialized (recursively) according to these rules;
if it is a union, the first named member is initialized (recursively) according to these
rules.
11 The initializer for a scalar shall be a single expression, optionally enclosed in braces. The
initial value of the object is that of the expression (after conversion); the same type
constraints and conversions as for simple assignment apply, taking the type of the scalar
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to be the unqualified version of its declared type.
12 The rest of this subclause deals with initializers for objects that have aggregate or union
type.
13 The initializer for a structure or union object that has automatic storage duration shall be
either an initializer list as described below, or a single expression that has compatible
structure or union type. In the latter case, the initial value of the object, including
unnamed members, is that of the expression.
14 An array of character type may be initialized by a character string literal, optionally
enclosed in braces. Successive characters of the character string literal (including the
terminating null character if there is room or if the array is of unknown size) initialize the
elements of the array.
15 An array with element type compatible with wchar_ t may be initialized by a wide
string literal, optionally enclosed in braces. Successive wide characters of the wide string
literal (including the terminating null wide character if there is room or if the array is of
unknown size) initialize the elements of the array.
16 Otherwise, the initializer for an object that has aggregate or union type shall be a brace-enclosed
list of initializers for the elements or named members.
17 Each brace-enclosed initializer list has an associated current object. When no
designations are present, subobjects of the current object are initialized in order according
to the type of the current object: array elements in increasing subscript order, structure
members in declaration order, and the first named member of a union. 118) In contrast, a
designation causes the following initializer to begin initialization of the subobject
described by the designator. Initialization then continues forward in order, beginning
with the next subobject after that described by the designator. 119)
18 Each designator list begins its description with the current object associated with the
closest surrounding brace pair. Each item in the designator list (in order) specifies a
particular member of its current object and changes the current object for the next
designator (if any) to be that member. 120) The current object that results at the end of the
designator list is the subobject to be initialized by the following initializer.
118) If the initializer list for a subaggregate or contained union does not begin with a left brace, its
subobjects are initialized as usual, but the subaggregate or contained union does not become the
current object: current objects are associated only with brace-enclosed initializer lists.
119) After a union member is initialized, the next object is not the next member of the union; instead, it is
the next subobject of an object containing the union.
120) Thus, a designator can only specify a strict subobject of the aggregate or union that is associated with
the surrounding brace pair. Note, too, that each separate designator list is independent.
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19 The initialization shall occur in initializer list order, each initializer provided for a
particular subobject overriding any previously listed initializer for the same subobject; all
subobjects that are not initialized explicitly shall be initialized implicitly the same as
objects that have static storage duration.
20 If the aggregate or union contains elements or members that are aggregates or unions,
these rules apply recursively to the subaggregates or contained unions. If the initializer of
a subaggregate or contained union begins with a left brace, the initializers enclosed by
that brace and its matching right brace initialize the elements or members of the
subaggregate or the contained union. Otherwise, only enough initializers from the list are
taken to account for the elements or members of the subaggregate or the first member of
the contained union; any remaining initializers are left to initialize the next element or
member of the aggregate of which the current subaggregate or contained union is a part.
21 If there are fewer initializers in a brace-enclosed list than there are elements or members
of an aggregate, or fewer characters in a string literal used to initialize an array of known
size than there are elements in the array, the remainder of the aggregate shall be
initialized implicitly the same as objects that have static storage duration.
22 If an array of unknown size is initialized, its size is determined by the largest indexed
element with an explicit initializer. At the end of its initializer list, the array no longer
has incomplete type.
23 The order in which any side effects occur among the initialization list expressions is
unspecified. 121)
24 EXAMPLE 1 Provided that <complex. h> has been #included, the declarations
int i = 3.5;
complex c = 5 + 3 * I;
define and initialize i with the value 3 and c with the value 5. 0 + 3. 0i.
25 EXAMPLE 2 The declaration
int x[] = { 1, 3, 5 };
defines and initializes x as a one-dimensional array object that has three elements, as no size was specified
and there are three initializers.
26 EXAMPLE 3 The declaration
int y[ 4][ 3] = {
{1, 3,5 },
{2, 4,6 },
{3, 5,7 },
};
is a definition with a fully bracketed initialization: 1, 3, and 5 initialize the first row of y (the array object
y[ 0]), namely y[ 0][ 0], y[ 0][ 1], and y[ 0][ 2]. Likewise the next two lines initialize y[ 1] and
121) In particular, the evaluation order need not be the same as the order of subobject initialization.
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y[ 2]. The initializer ends early, so y[ 3] is initialized with zeros. Precisely the same effect could have
been achieved by
int y[ 4][ 3] = {
1, 3, 5, 2, 4, 6, 3, 5, 7
};
The initializer for y[ 0] does not begin with a left brace, so three items from the list are used. Likewise the
next three are taken successively for y[ 1] and y[ 2].
27 EXAMPLE 4 The declaration
int z[ 4][ 3] = {
{1 },{2 },{3 },{4 }
};
initializes the first column of z as specified and initializes the rest with zeros.
28 EXAMPLE 5 The declaration
struct { int a[ 3], b; } w[] = { { 1 }, 2 };
is a definition with an inconsistently bracketed initialization. It defines an array with two element
structures: w[ 0]. a[ 0] is 1 and w[ 1]. a[ 0] is 2; all the other elements are zero.
29 EXAMPLE 6 The declaration
short q[ 4][ 3][ 2] = {
{1 },
{2,3 },
{4, 5,6 }
};
contains an incompletely but consistently bracketed initialization. It defines a three-dimensional array
object: q[ 0][ 0][ 0] is 1, q[ 1][ 0][ 0] is 2, q[ 1][ 0][ 1] is 3, and 4, 5, and 6 initialize
q[ 2][ 0][ 0], q[ 2][ 0][ 1], and q[ 2][ 1][ 0], respectively; all the rest are zero. The initializer for
q[ 0][ 0] does not begin with a left brace, so up to six items from the current list may be used. There is
only one, so the values for the remaining five elements are initialized with zero. Likewise, the initializers
for q[ 1][ 0] and q[ 2][ 0] do not begin with a left brace, so each uses up to six items, initializing their
respective two-dimensional subaggregates. If there had been more than six items in any of the lists, a
diagnostic message would have been issued. The same initialization result could have been achieved by:
short q[ 4][ 3][ 2] = {
1, 0, 0, 0, 0, 0,
2, 3, 0, 0, 0, 0,
4, 5, 6
};
or by:
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short q[ 4][ 3][ 2] = {
{
{1 },
},
{
{2,3 },
},
{
{4,5 },
{6 },
}
};
in a fully bracketed form.
30 Note that the fully bracketed and minimally bracketed forms of initialization are, in general, less likely to cause confusion.
31 EXAMPLE 7 One form of initialization that completes array types involves typedef names. Given the declaration
typedef int A[]; // OK -declared with block scope
the declaration
A a ={ 1,2 },b ={ 3, 4,5 };
is identical to
int a[] = { 1, 2 }, b[] = { 3, 4, 5 };
due to the rules for incomplete types.
32 EXAMPLE 8 The declaration
char s[] = "abc", t[ 3] = "abc";
defines '' plain'' char array objects s and t whose elements are initialized with character string literals.
This declaration is identical to
char s[] = { 'a', 'b', 'c', '\ 0' },
t[] = { 'a', 'b', 'c' };
The contents of the arrays are modifiable. On the other hand, the declaration
char *p = "abc";
defines p with type '' pointer to char'' and initializes it to point to an object with type '' array of char''
with length 4 whose elements are initialized with a character string literal. If an attempt is made to use p to
modify the contents of the array, the behavior is undefined.
33 EXAMPLE 9 Arrays can be initialized to correspond to the elements of an enumeration by using designators:
enum { member_ one, member_ two };
const char *nm[] = {
[member_ two] = "member two",
[member_ one] = "member one",
};
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34 EXAMPLE 10 Structure members can be initialized to nonzero values without depending on their order:
div_ t answer = { .quot = 2, .rem = -1 };
35 EXAMPLE 11 Designators can be used to provide explicit initialization when unadorned initializer lists might be misunderstood:
struct { int a[ 3], b; } w[] =
{ [0]. a = {1}, [1]. a[ 0] = 2 };
36 EXAMPLE 12 Space can be '' allocated'' from both ends of an array by using a single designator:
int a[ MAX] = {
1, 3, 5, 7, 9, [MAX-5] = 8, 6, 4, 2, 0
};
37 In the above, if MAX is greater than ten, there will be some zero-valued elements in the middle; if it is less than ten, some of the values provided by the first five initializers will be overridden by the second five.
38 EXAMPLE 13 Any member of a union can be initialized:
union { /* ... */ } u = { .any_ member = 42 };
Forward references: common definitions <stddef. h> (7.17).
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6.8 Statements
Syntax
1 statement:
labeled-statement
compound-statement
expression-statement
selection-statement
iteration-statement
jump-statement
Semantics
2 Astatement specifies an action to be performed. Except as indicated, statements are
executed in sequence.
3 Ablock allows a set of declarations and statements to be grouped into one syntactic unit.
The initializers of objects that have automatic storage duration, and the variable length
array declarators of ordinary identifiers with block scope, are evaluated and the values are
stored in the objects (including storing an indeterminate value in objects without an
initializer) each time the declaration is reached in the order of execution, as if it were a
statement, and within each declaration in the order that declarators appear.
4 Afull expression is an expression that is not part of another expression or declarator.
Each of the following is a full expression: an initializer; the expression in an expression
statement; the controlling expression of a selection statement (if or switch); the
controlling expression of a while or do statement; each of the (optional) expressions of
a for statement; the (optional) expression in a return statement. The end of a full
expression is a sequence point.
Forward references: expression and null statements (6.8.3), selection statements
(6.8.4), iteration statements (6.8.5), the return statement (6.8.6.4).
6.8.1 Labeled statements
Syntax
1 labeled-statement:
identifier : statement
case constant-expression : statement
default : statement
Constraints
2 Acase or default label shall appear only in a switch statement. Further
constraints on such labels are discussed under the switch statement.
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3 Label names shall be unique within a function.
Semantics
4 Any statement may be preceded by a prefix that declares an identifier as a label name.
Labels in themselves do not alter the flow of control, which continues unimpeded across
them.
Forward references: the goto statement (6.8.6.1), the switch statement (6.8.4.2).
6.8.2 Compound statement, or block
Syntax
1 compound-statement:
{ block-item-list opt }
block-item-list:
block-item
block-item-list block-item
block-item:
declaration
statement
Semantics
2 Acompound statement is a block.
6.8.3 Expression and null statements
Syntax
1 expression-statement:
expression opt ;
Semantics
2 The expression in an expression statement is evaluated as a void expression for its side
effects. 122)
3 Anull statement (consisting of just a semicolon) performs no operations.
4 EXAMPLE 1 If a function call is evaluated as an expression statement for its side effects only, the discarding of its value may be made explicit by converting the expression to a void expression by means of
a cast:
int p( int);
/* ... */
(void) p( 0);
122) Such as assignments, and function calls which have side effects.
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5 EXAMPLE 2 In the program fragment
char *s;
/* ... */
while (* s++ != '\ 0')
;
a null statement is used to supply an empty loop body to the iteration statement.
6 EXAMPLE 3 A null statement may also be used to carry a label just before the closing } of a compound statement.
while (loop1) {
/* ... */
while (loop2) {
/* ... */
if (want_ out)
goto end_ loop1;
/* ... */
}
/* ... */
end_ loop1: ;
}
Forward references: iteration statements (6.8.5).
6.8.4 Selection statements
Syntax
1 selection-statement:
if ( expression ) statement
if ( expression ) statement else statement
switch ( expression ) statement
Semantics
2 A selection statement selects among a set of statements depending on the value of a
controlling expression.
3 Aselection statement is a block whose scope is a strict subset of the scope of its
enclosing block. Each associated substatement is also a block whose scope is a strict
subset of the scope of the selection statement.
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6.8.4.1 The if statement
Constraints
1 The controlling expression of an if statement shall have scalar type.
Semantics
2 In both forms, the first substatement is executed if the expression compares unequal to 0.
In the else form, the second substatement is executed if the expression compares equal
to 0. If the first substatement is reached via a label, the second substatement is not
executed.
3 Anelse is associated with the lexically nearest preceding if that is allowed by the
syntax.
6.8.4.2 The switch statement
Constraints
1 The controlling expression of a switch statement shall have integer type.
2 If a switch statement has an accessible case or default label within the scope of an
identifier with a variably modified type, the entire switch statement shall be within the
scope of that identifier. 123)
3 The expression of each case label shall be an integer constant expression and no two of
the case constant expressions in the same switch statement shall have the same value
after conversion. There may be at most one default label in a switch statement.
(Any enclosed switch statement may have a default label or case constant
expressions with values that duplicate case constant expressions in the enclosing
switch statement.)
Semantics
4 Aswitch statement causes control to jump to, into, or past the statement that is the
switch body, depending on the value of a controlling expression, and on the presence of a
default label and the values of any case labels on or in the switch body. A case or
default label is accessible only within the closest enclosing switch statement.
5 The integer promotions are performed on the controlling expression. The constant
expression in each case label is converted to the promoted type of the controlling
expression. If a converted value matches that of the promoted controlling expression,
control jumps to the statement following the matched case label. Otherwise, if there is
a default label, control jumps to the labeled statement. If no converted case constant
expression matches and there is no default label, no part of the switch body is
123) That is, the declaration either precedes the switch statement, or it follows the last case or
default label associated with the switch that is in the block containing the declaration.
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executed.
Implementation limits
6 As discussed in 5.2.4.1, the implementation may limit the number of case values in a
switch statement.
7 EXAMPLE In the artificial program fragment
switch (expr)
{
int i = 4;
f( i);
case 0:
i = 17;
/* falls through into default code */
default:
printf("% d\ n", i);
}
the object whose identifier is i exists with automatic storage duration (within the block) but is never
initialized, and thus if the controlling expression has a nonzero value, the call to the printf function will
access an indeterminate value. Similarly, the call to the function f cannot be reached.
6.8.5 Iteration statements
Syntax
1 iteration-statement:
while ( expression ) statement
do statement while ( expression );
for ( expression opt ; expression opt ; expression opt ) statement
for ( declaration expression opt ; expression opt ) statement
Constraints
2 The controlling expression of an iteration statement shall have scalar type.
3 The declaration part of a for statement shall only declare identifiers for objects having
storage class auto or register.
Semantics
4 An iteration statement causes a statement called the loop body to be executed repeatedly
until the controlling expression compares equal to 0.
5 An iteration statement is a block whose scope is a strict subset of the scope of its
enclosing block. The loop body is also a block whose scope is a strict subset of the scope
of the iteration statement.
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6.8.5.1 The while statement
1 The evaluation of the controlling expression takes place before each execution of the loop
body.
6.8.5.2 The do statement
1 The evaluation of the controlling expression takes place after each execution of the loop
body.
6.8.5.3 The for statement
1 The statement
for ( clause-1 ; expression-2 ; expression-3 ) statement
behaves as follows: The expression expression-2 is the controlling expression that is
evaluated before each execution of the loop body. The expression expression-3 is
evaluated as a void expression after each execution of the loop body. If clause-1 is an
expression, it is evaluated as a void expression before the first evaluation of the
controlling expression. 124) *
2 Both clause-1 and expression-3 can be omitted. An omitted expression-2 is replaced by a *
nonzero constant. *
6.8.6 Jump statements
Syntax
1 jump-statement:
goto identifier ;
continue ;
break ;
return expression opt ;
Semantics
2 A jump statement causes an unconditional jump to another place.
124) Thus, clause-1 specifies initialization for the loop, possibly declaring one or more variables for use in
the loop; the controlling expression, expression-2, specifies an evaluation made before each iteration,
such that execution of the loop continues until the expression compares equal to 0; and expression-3
specifies an operation (such as incrementing) that is performed after each iteration. If clause-1 is a
declaration, then the scope of any variable it declares is the remainder of the declaration and the entire
loop, including the other two expressions.
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6.8.6.1 The goto statement
Constraints
1 The identifier in a goto statement shall name a label located somewhere in the enclosing
function. A goto statement shall not jump from outside the scope of an identifier having
a variably modified type to inside the scope of that identifier.
Semantics
2 Agoto statement causes an unconditional jump to the statement prefixed by the named
label in the enclosing function.
3 EXAMPLE 1 It is sometimes convenient to jump into the middle of a complicated set of statements. The following outline presents one possible approach to a problem based on these three assumptions:
1. The general initialization code accesses objects only visible to the current function.
2. The general initialization code is too large to warrant duplication.
3. The code to determine the next operation is at the head of the loop. (To allow it to be reached by
continue statements, for example.)
/* ... */
goto first_ time;
for (;;) {
// determine next operation
/* ... */
if (need to reinitialize){
// reinitialize-only code
/* ... */
first_ time:
// general initialization code
/* ... */
continue;
}
// handle other operations
/* ... */
}
4 EXAMPLE 2 A goto statement is not allowed to jump past any declarations of objects with variably modified types. A jump within the scope, however, is permitted.
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goto lab3; // Error: going INTO scope of VLA.
{
double a[ n];
a[ j] = 4.4;
lab3:
a[ j] = 3.3;
goto lab4; // OK, going WITHIN scope of VLA.
a[ j] = 5.5;
lab4:
a[ j] = 6.6;
}
goto lab4; // Error: going INTO scope of VLA.
6.8.6.2 The continue statement
Constraints
1 Acontinue statement shall appear only in or as a loop body.
Semantics
2 Acontinue statement causes a jump to the loop-continuation portion of the smallest
enclosing iteration statement; that is, to the end of the loop body. More precisely, in each
of the statements
while (/* ... */) {
/* ... */
continue;
/* ... */
contin: ;
}
do {
/* ... */
continue;
/* ... */
contin: ;
} while (/* ... */);
for (/* ... */) {
/* ... */
continue;
/* ... */
contin: ;
}
unless the continue statement shown is in an enclosed iteration statement (in which
case it is interpreted within that statement), it is equivalent to goto contin;. 125)
125) Following the contin: label is a null statement.
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6.8.6.3 The break statement
Constraints
1 Abreak statement shall appear only in or as a switch body or loop body.
Semantics
2 Abreak statement terminates execution of the smallest enclosing switch or iteration
statement.
6.8.6.4 The return statement
Constraints
1 Areturn statement with an expression shall not appear in a function whose return type
is void. Areturn statement without an expression shall only appear in a function
whose return type is void.
Semantics
2 Areturn statement terminates execution of the current function and returns control to
its caller. A function may have any number of return statements.
3 If a return statement with an expression is executed, the value of the expression is
returned to the caller as the value of the function call expression. If the expression has a
type different from the return type of the function in which it appears, the value is
converted as if by assignment to an object having the return type of the function. 126)
4 EXAMPLE In:
126) The return statement is not an assignment. The overlap restriction of subclause 6.5.16.1 does not
apply to the case of function return.
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struct s { double i; } f( void);
union {
struct {
int f1;
struct s f2;
} u1;
struct {
struct s f3;
int f4;
} u2;
}g;
struct s f( void)
{
return g. u1. f2;
}
/* ... */
g. u2. f3 = f();
there is no undefined behavior, although there would be if the assignment were done directly (without using
a function call to fetch the value).
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6.9 External definitions
Syntax
1 translation-unit:
external-declaration
translation-unit external-declaration
external-declaration:
function-definition
declaration
Constraints
2 The storage-class specifiers auto and register shall not appear in the declaration
specifiers in an external declaration.
3 There shall be no more than one external definition for each identifier declared with
internal linkage in a translation unit. Moreover, if an identifier declared with internal
linkage is used in an expression (other than as a part of the operand of a sizeof
operator), there shall be exactly one external definition for the identifier in the translation
unit.
Semantics
4 As discussed in 5.1.1.1, the unit of program text after preprocessing is a translation unit,
which consists of a sequence of external declarations. These are described as '' external''
because they appear outside any function (and hence have file scope). As discussed in
6.7, a declaration that also causes storage to be reserved for an object or a function named
by the identifier is a definition.
5 Anexternal definition is an external declaration that is also a definition of a function or an
object. If an identifier declared with external linkage is used in an expression (other than
as part of the operand of a sizeof operator), somewhere in the entire program there
shall be exactly one external definition for the identifier; otherwise, there shall be no more
than one. 127)
127) Thus, if an identifier declared with external linkage is not used in an expression, there need be no
external definition for it.
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6.9.1 Function definitions
Syntax
1 function-definition:
declaration-specifiers declarator declaration-list opt compound-statement
declaration-list:
declaration
declaration-list declaration
Constraints
2 The identifier declared in a function definition (which is the name of the function) shall
have a function type, as specified by the declarator portion of the function definition. 128)
3 The return type of a function shall be void or an object type other than array type.
4 The storage-class specifier, if any, in the declaration specifiers shall be either extern or
static.
5 If the declarator includes a parameter type list, the declaration of each parameter shall
include an identifier, except for the special case of a parameter list consisting of a single
parameter of type void, in which case there shall not be an identifier. No declaration list
shall follow.
6 If the declarator includes an identifier list, each declaration in the declaration list shall
have at least one declarator, those declarators shall declare only identifiers from the
identifier list, and every identifier in the identifier list shall be declared. An identifier
declared as a typedef name shall not be redeclared as a parameter. The declarations in the
declaration list shall contain no storage-class specifier other than register and no
initializations.
128) The intent is that the type category in a function definition cannot be inherited from a typedef:
typedef int F( void); /* type F is '' function of no arguments returning int'' */
F f, g; /* fand g both have type compatible with F */
F f {/*... */ } /* WRONG: syntax/ constraint error */
F g() { /* ... */ } /* WRONG: declares that g returns a function */
int f( void) { /* ... */ } /* RIGHT: f has type compatible with F */
int g() { /* ... */ } /* RIGHT: g has type compatible with F */
F *e( void) { /* ... */ } /* e returns a pointer to a function */
F *(( e))( void) { /* ... */ } /* same: parentheses irrelevant */
int (* fp)( void); /* fp points to a function that has type F */
F *Fp; /* Fp points to a function that has type F */
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Semantics
7 The declarator in a function definition specifies the name of the function being defined
and the identifiers of its parameters. If the declarator includes a parameter type list, the
list also specifies the types of all the parameters; such a declarator also serves as a
function prototype for later calls to the same function in the same translation unit. If the
declarator includes an identifier list, 129) the types of the parameters shall be declared in a
following declaration list. In either case, the type of each parameter is adjusted as
described in 6.7.5.3 for a parameter type list; the resulting type shall be an object type.
8 If a function that accepts a variable number of arguments is defined without a parameter
type list that ends with the ellipsis notation, the behavior is undefined.
9 Each parameter has automatic storage duration. Its identifier is an lvalue, which is in
effect declared at the head of the compound statement that constitutes the function body
(and therefore cannot be redeclared in the function body except in an enclosed block).
The layout of the storage for parameters is unspecified.
10 On entry to the function, the size expressions of each variably modified parameter are
evaluated and the value of each argument expression is converted to the type of the
corresponding parameter as if by assignment. (Array expressions and function
designators as arguments were converted to pointers before the call.)
11 After all parameters have been assigned, the compound statement that constitutes the
body of the function definition is executed.
12 If the } that terminates a function is reached, and the value of the function call is used by
the caller, the behavior is undefined.
13 EXAMPLE 1 In the following:
extern int max( int a, int b)
{
return a > b ? a : b;
}
extern is the storage-class specifier and int is the type specifier; max( int a, int b) is the
function declarator; and
{ return a > b ? a : b; }
is the function body. The following similar definition uses the identifier-list form for the parameter
declarations:
129) See '' future language directions'' (6.11.5).
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extern int max( a, b)
int a, b;
{
return a > b ? a : b;
}
Here int a, b; is the declaration list for the parameters. The difference between these two definitions is
that the first form acts as a prototype declaration that forces conversion of the arguments of subsequent calls
to the function, whereas the second form does not.
14 EXAMPLE 2 To pass one function to another, one might say
int f( void);
/* ... */
g( f);
Then the definition of g might read
void g( int (* funcp)( void))
{
/* ... */ (* funcp)() /* or funcp() ... */
}
or, equivalently,
void g( int func( void))
{
/* ... */ func() /* or (* func)() ... */
}
6.9.2 External object definitions
Semantics
1 If the declaration of an identifier for an object has file scope and an initializer, the
declaration is an external definition for the identifier.
2 A declaration of an identifier for an object that has file scope without an initializer, and
without a storage-class specifier or with the storage-class specifier static, constitutes a
tentative definition. If a translation unit contains one or more tentative definitions for an
identifier, and the translation unit contains no external definition for that identifier, then
the behavior is exactly as if the translation unit contains a file scope declaration of that
identifier, with the composite type as of the end of the translation unit, with an initializer
equal to 0.
3 If the declaration of an identifier for an object is a tentative definition and has internal
linkage, the declared type shall not be an incomplete type.
4 EXAMPLE 1
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int i1 = 1; // definition, external linkage
static int i2 = 2; // definition, internal linkage
extern int i3 = 3; // definition, external linkage
int i4; // tentative definition, external linkage
static int i5; // tentative definition, internal linkage
int i1; // valid tentative definition, refers to pre vious
int i2; // 6.2.2 renders undefined, linkage disagreement
int i3; // valid tentative definition, refers to pre vious
int i4; // valid tentative definition, refers to pre vious
int i5; // 6.2.2 renders undefined, linkage disagreement
extern int i1; // refers to pre vious, whose linkage is external
extern int i2; // refers to pre vious, whose linkage is internal
extern int i3; // refers to pre vious, whose linkage is external
extern int i4; // refers to pre vious, whose linkage is external
extern int i5; // refers to pre vious, whose linkage is internal
5 EXAMPLE 2 If at the end of the translation unit containing
int i[];
the array i still has incomplete type, the implicit initializer causes it to have one element, which is set to
zero on program startup.
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6.10 Preprocessing directives
Syntax
1 preprocessing-file:
group opt
group:
group-part
group group-part
group-part:
pp-tokens opt new-line
if-section
control-line
if-section:
if-group elif-groups opt else-group opt endif-line
if-group:
#if constant-expression new-line group opt
# ifdef identifier new-line group opt
# ifndef identifier new-line group opt
elif-groups:
elif-group
elif-groups elif-group
elif-group:
# elif constant-expression new-line group opt
else-group:
# else new-line group opt
endif-line:
# endif new-line
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control-line:
# include pp-tokens new-line
# define identifier replacement-list new-line
# define identifier lparen identifier-list opt )
replacement-list new-line
# define identifier lparen ... ) replacement-list new-line
# define identifier lparen identifier-list , ... )
replacement-list new-line
# undef identifier new-line
# line pp-tokens new-line
# error pp-tokens opt new-line
# pragma pp-tokens opt new-line
# new-line
lparen:
a ( character not immediately preceded by white-space
replacement-list:
pp-tokens opt
pp-tokens:
preprocessing-token
pp-tokens preprocessing-token
new-line:
the new-line character
Description
2 A preprocessing directive consists of a sequence of preprocessing tokens that begins with
a # preprocessing token that (at the start of translation phase 4) is either the first character
in the source file (optionally after white space containing no new-line characters) or that
follows white space containing at least one new-line character, and is ended by the next
new-line character. 130) A new-line character ends the preprocessing directive even if it
occurs within what would otherwise be an invocation of a function-like macro.
Constraints
3 The only white-space characters that shall appear between preprocessing tokens within a
preprocessing directive (from just after the introducing # preprocessing token through
just before the terminating new-line character) are space and horizontal-tab (including
spaces that have replaced comments or possibly other white-space characters in
130) Thus, preprocessing directives are commonly called '' lines''. These '' lines'' hav e no other syntactic
significance, as all white space is equivalent except in certain situations during preprocessing (see the
# character string literal creation operator in 6.10.3.2, for example).
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translation phase 3).
Semantics
4 The implementation can process and skip sections of source files conditionally, include
other source files, and replace macros. These capabilities are called preprocessing,
because conceptually they occur before translation of the resulting translation unit.
5 The preprocessing tokens within a preprocessing directive are not subject to macro
expansion unless otherwise stated.
6 EXAMPLE In:
#define EMPTY
EMPTY # include <file. h>
the sequence of preprocessing tokens on the second line is not a preprocessing directive, because it does not
begin with a # at the start of translation phase 4, even though it will do so after the macro EMPTY has been
replaced.
6.10.1 Conditional inclusion
Constraints
1 The expression that controls conditional inclusion shall be an integer constant expression
except that: it shall not contain a cast; identifiers (including those lexically identical to
keywords) are interpreted as described below; 131) and it may contain unary operator
expressions of the form
defined identifier
or
defined ( identifier )
which evaluate to 1 if the identifier is currently defined as a macro name (that is, if it is
predefined or if it has been the subject of a #define preprocessing directive without an
intervening #undef directive with the same subject identifier), 0 if it is not.
Semantics
2 Preprocessing directives of the forms
#if constant-expression new-line group opt
# elif constant-expression new-line group opt
check whether the controlling constant expression evaluates to nonzero.
3 Prior to evaluation, macro invocations in the list of preprocessing tokens that will become
the controlling constant expression are replaced (except for those macro names modified
by the defined unary operator), just as in normal text. If the token defined is
131) Because the controlling constant expression is evaluated during translation phase 4, all identifiers
either are or are not macro names there simply are no keywords, enumeration constants, etc.
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generated as a result of this replacement process or use of the defined unary operator
does not match one of the two specified forms prior to macro replacement, the behavior is
undefined. After all replacements due to macro expansion and the defined unary
operator have been performed, all remaining identifiers are replaced with the pp-number
0, and then each preprocessing token is converted into a token. The resulting tokens
compose the controlling constant expression which is evaluated according to the rules of
6.6, except that all signed integer types and all unsigned integer types act as if they hav e
the same representation as, respectively, the types intmax_ t and uintmax_ t defined
in the header <stdint. h>. This includes interpreting character constants, which may
involve converting escape sequences into execution character set members. Whether the
numeric value for these character constants matches the value obtained when an identical
character constant occurs in an expression (other than within a #if or #elif directive)
is implementation-defined. 132) Also, whether a single-character character constant may
have a neg ative value is implementation-defined.
4 Preprocessing directives of the forms
# ifdef identifier new-line group opt
# ifndef identifier new-line group opt
check whether the identifier is or is not currently defined as a macro name. Their
conditions are equivalent to #if defined identifier and #if !defined identifier
respectively.
5 Each directive's condition is checked in order. If it evaluates to false (zero), the group
that it controls is skipped: directives are processed only through the name that determines
the directive in order to keep track of the level of nested conditionals; the rest of the
directives' preprocessing tokens are ignored, as are the other preprocessing tokens in the
group. Only the first group whose control condition evaluates to true (nonzero) is
processed. If none of the conditions evaluates to true, and there is a #else directive, the
group controlled by the #else is processed; lacking a #else directive, all the groups
until the #endif are skipped. 133)
132) Thus, the constant expression in the following #if directive and if statement is not guaranteed to
evaluate to the same value in these two contexts.
#if 'z' -'a' == 25
if ('z' -'a' == 25)
133) As indicated by the syntax, a preprocessing token shall not follow a #else or #endif directive
before the terminating new-line character. Howev er, comments may appear anywhere in a source file,
including within a preprocessing directive.
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Forward references: macro replacement (6.10.3), source file inclusion (6.10.2), largest
integer types (7.18.1.5).
6.10.2 Source file inclusion
Constraints
1 A# include directive shall identify a header or source file that can be processed by the
implementation.
Semantics
2 A preprocessing directive of the form
# include <h-char-sequence> new-line
searches a sequence of implementation-defined places for a header identified uniquely by
the specified sequence between the < and > delimiters, and causes the replacement of that
directive by the entire contents of the header. How the places are specified or the header
identified is implementation-defined.
3 A preprocessing directive of the form
# include "q-char-sequence" new-line
causes the replacement of that directive by the entire contents of the source file identified
by the specified sequence between the " delimiters. The named source file is searched
for in an implementation-defined manner. If this search is not supported, or if the search
fails, the directive is reprocessed as if it read
# include <h-char-sequence> new-line
with the identical contained sequence (including > characters, if any) from the original
directive.
4 A preprocessing directive of the form
# include pp-tokens new-line
(that does not match one of the two previous forms) is permitted. The preprocessing
tokens after include in the directive are processed just as in normal text. (Each
identifier currently defined as a macro name is replaced by its replacement list of
preprocessing tokens.) The directive resulting after all replacements shall match one of
the two previous forms. 134) The method by which a sequence of preprocessing tokens
between a < and a > preprocessing token pair or a pair of " characters is combined into a
single header name preprocessing token is implementation-defined.
134) Note that adjacent string literals are not concatenated into a single string literal (see the translation
phases in 5.1.1.2); thus, an expansion that results in two string literals is an invalid directive.
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5 The implementation shall provide unique mappings for sequences consisting of one or
more letters or digits (as defined in 5.2.1) followed by a period (.) and a single letter.
The first character shall be a letter. The implementation may ignore the distinctions of
alphabetical case and restrict the mapping to eight significant characters before the
period.
6 A# include preprocessing directive may appear in a source file that has been read
because of a #include directive in another file, up to an implementation-defined
nesting limit (see 5.2.4.1).
7 EXAMPLE 1 The most common uses of #include preprocessing directives are as in the following:
#include <stdio. h>
#include "myprog. h"
8 EXAMPLE 2 This illustrates macro-replaced #include directives:
#if VERSION == 1
#define INCFILE "vers1. h"
#elif VERSION == 2
#define INCFILE "vers2. h" // and so on
#else
#define INCFILE "versN. h"
#endif
#include INCFILE
Forward references: macro replacement (6.10.3).
6.10.3 Macro replacement
Constraints
1 Tw o replacement lists are identical if and only if the preprocessing tokens in both have
the same number, ordering, spelling, and white-space separation, where all white-space
separations are considered identical.
2 An identifier currently defined as a macro without use of lparen (an object-like macro)
shall not be redefined by another #define preprocessing directive unless the second
definition is an object-like macro definition and the two replacement lists are identical.
3 An identifier currently defined as a macro using lparen (a function-like macro) shall not
be redefined by another #define preprocessing directive unless the second definition is
a function-like macro definition that has the same number and spelling of parameters, and
the two replacement lists are identical.
4 If the identifier-list in the macro definition does not end with an ellipsis, the number of
arguments, including those arguments consisting of no preprocessing tokens, in an
invocation of a function-like macro shall agree with the number of parameters in the
macro definition. Otherwise, there shall be more arguments in the invocation than there
are parameters in the macro definition (excluding the ...). There shall exist a )
preprocessing token that terminates the invocation.
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5 The identifier _ _VA_ ARGS_ _ shall only occur in the replacement-list of a #define
preprocessing directive using the ellipsis notation in the arguments.
6 A parameter identifier in a function-like macro shall be uniquely declared within its
scope.
Semantics
7 The identifier immediately following the define is called the macro name. There is one
name space for macro names. Any white-space characters preceding or following the
replacement list of preprocessing tokens are not considered part of the replacement list
for either form of macro.
8 If a # preprocessing token, followed by an identifier, occurs lexically at the point at which
a preprocessing directive could begin, the identifier is not subject to macro replacement.
9 A preprocessing directive of the form
# define identifier replacement-list new-line
defines an object-like macro that causes each subsequent instance of the macro name 135)
to be replaced by the replacement list of preprocessing tokens that constitute the
remainder of the directive.
10 A preprocessing directive of the form
# define identifier lparen identifier-list opt ) replacement-list new-line
# define identifier lparen ... ) replacement-list new-line
# define identifier lparen identifier-list , ... ) replacement-list new-line
defines a function-like macro with arguments, similar syntactically to a function call. The
parameters are specified by the optional list of identifiers, whose scope extends from their
declaration in the identifier list until the new-line character that terminates the #define
preprocessing directive. Each subsequent instance of the function-like macro name
followed by a ( as the next preprocessing token introduces the sequence of preprocessing
tokens that is replaced by the replacement list in the definition (an invocation of the
macro). The replaced sequence of preprocessing tokens is terminated by the matching )
preprocessing token, skipping intervening matched pairs of left and right parenthesis
preprocessing tokens. Within the sequence of preprocessing tokens making up an
invocation of a function-like macro, new-line is considered a normal white-space
character.
135) Since, by macro-replacement time, all character constants and string literals are preprocessing tokens,
not sequences possibly containing identifier-like subsequences (see 5.1.1.2, translation phases), they
are never scanned for macro names or parameters.
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11 The sequence of preprocessing tokens bounded by the outside-most matching parentheses
forms the list of arguments for the function-like macro. The individual arguments within
the list are separated by comma preprocessing tokens, but comma preprocessing tokens
between matching inner parentheses do not separate arguments. If there are sequences of
preprocessing tokens within the list of arguments that would otherwise act as
preprocessing directives, the behavior is undefined.
12 If there is a ... in the identifier-list in the macro definition, then the trailing arguments,
including any separating comma preprocessing tokens, are merged to form a single item:
the variable arguments. The number of arguments so combined is such that, following
merger, the number of arguments is one more than the number of parameters in the macro
definition (excluding the ...).
6.10.3.1 Argument substitution
1 After the arguments for the invocation of a function-like macro have been identified,
argument substitution takes place. A parameter in the replacement list, unless preceded
by a # or ## preprocessing token or followed by a ## preprocessing token (see below), is
replaced by the corresponding argument after all macros contained therein have been
expanded. Before being substituted, each argument's preprocessing tokens are
completely macro replaced as if they formed the rest of the preprocessing file; no other
preprocessing tokens are available.
2 An identifier _ _VA_ ARGS_ _ that occurs in the replacement list shall be treated as if it
were a parameter, and the variable arguments shall form the preprocessing tokens used to
replace it.
6.10.3.2 The # operator
Constraints
1 Each # preprocessing token in the replacement list for a function-like macro shall be
followed by a parameter as the next preprocessing token in the replacement list.
Semantics
2 If, in the replacement list, a parameter is immediately preceded by a # preprocessing
token, both are replaced by a single character string literal preprocessing token that
contains the spelling of the preprocessing token sequence for the corresponding
argument. Each occurrence of white space between the argument's preprocessing tokens
becomes a single space character in the character string literal. White space before the
first preprocessing token and after the last preprocessing token composing the argument
is deleted. Otherwise, the original spelling of each preprocessing token in the argument
is retained in the character string literal, except for special handling for producing the
spelling of string literals and character constants: a \ character is inserted before each "
and \ character of a character constant or string literal (including the delimiting "
characters), except that it is unspecified whether a \ character is inserted before the \
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character beginning a universal character name. If the replacement that results is not a
valid character string literal, the behavior is undefined. The character string literal
corresponding to an empty argument is "". The order of evaluation of # and ## operators
is unspecified.
6.10.3.3 The ## operator
Constraints
1 A## preprocessing token shall not occur at the beginning or at the end of a replacement
list for either form of macro definition.
Semantics
2 If, in the replacement list of a function-like macro, a parameter is immediately preceded
or followed by a ## preprocessing token, the parameter is replaced by the corresponding
argument's preprocessing token sequence; however, if an argument consists of no
preprocessing tokens, the parameter is replaced by a placemarker preprocessing token
instead.
3 For both object-like and function-like macro invocations, before the replacement list is
reexamined for more macro names to replace, each instance of a ## preprocessing token
in the replacement list (not from an argument) is deleted and the preceding preprocessing
token is concatenated with the following preprocessing token. Placemarker
preprocessing tokens are handled specially: concatenation of two placemarkers results in
a single placemarker preprocessing token, and concatenation of a placemarker with a
nonplacemarker preprocessing token results in the nonplacemarker preprocessing
token. If the result is not a valid preprocessing token, the behavior is undefined. The
resulting token is available for further macro replacement. The order of evaluation of ##
operators is unspecified.
4 EXAMPLE In the following fragment:
#define hash_ hash # ## #
#define mkstr( a) # a
#define in_ between( a) mkstr( a)
#define join( c, d) in_ between( c hash_ hash d)
char p[] = join( x, y); // equivalent to
// char p[] = "x ## y";
The expansion produces, at various stages:
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join( x, y)
in_ between( x hash_ hash y)
in_ between( x ## y)
mkstr( x ## y)
"x ## y"
In other words, expanding hash_ hash produces a new token, consisting of two adjacent sharp signs, but
this new token is not the ## operator.
6.10.3.4 Rescanning and further replacement
1 After all parameters in the replacement list have been substituted and # and ##
processing has taken place, all placemarker preprocessing tokens are removed. Then, the
resulting preprocessing token sequence is rescanned, along with all subsequent
preprocessing tokens of the source file, for more macro names to replace.
2 If the name of the macro being replaced is found during this scan of the replacement list
(not including the rest of the source file's preprocessing tokens), it is not replaced.
Further, if any nested replacements encounter the name of the macro being replaced, it is
not replaced. These nonreplaced macro name preprocessing tokens are no longer
available for further replacement even if they are later (re) examined in contexts in which
that macro name preprocessing token would otherwise have been replaced.
3 The resulting completely macro-replaced preprocessing token sequence is not processed
as a preprocessing directive even if it resembles one, but all pragma unary operator
expressions within it are then processed as specified in 6.10.9 below.
6.10.3.5 Scope of macro definitions
1 A macro definition lasts (independent of block structure) until a corresponding #undef
directive is encountered or (if none is encountered) until the end of the preprocessing
translation unit. Macro definitions have no significance after translation phase 4.
2 A preprocessing directive of the form
# undef identifier new-line
causes the specified identifier no longer to be defined as a macro name. It is ignored if
the specified identifier is not currently defined as a macro name.
3 EXAMPLE 1 The simplest use of this facility is to define a '' manifest constant'', as in
#define TABSIZE 100
int table[ TABSIZE];
4 EXAMPLE 2 The following defines a function-like macro whose value is the maximum of its arguments. It has the advantages of working for any compatible types of the arguments and of generating in-line code
without the overhead of function calling. It has the disadvantages of evaluating one or the other of its
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arguments a second time (including side effects) and generating more code than a function if invoked
several times. It also cannot have its address taken, as it has none.
#define max( a, b) (( a) > (b) ? (a) : (b))
The parentheses ensure that the arguments and the resulting expression are bound properly.
5 EXAMPLE 3 To illustrate the rules for redefinition and reexamination, the sequence
#define x 3
#define f( a) f( x * (a))
#undef x
#define x 2
#define g f
#define z z[ 0]
#define h g(~
#define m( a) a( w)
#define w 0,1
#define t( a) a
#define p() int
#define q( x) x
#define r( x, y) x ## y
#define str( x) # x
f( y+ 1) + f( f( z)) % t( t( g)( 0) + t)( 1);
g( x+( 3,4)-w) | h 5) & m
(f)^ m( m);
p() i[ q()] = { q( 1), r( 2,3), r( 4,), r(, 5), r(,) };
char c[ 2][ 6] = { str( hello), str() };
results in
f( 2 * (y+ 1)) + f( 2 * (f( 2 * (z[ 0])))) % f( 2 * (0)) + t( 1);
f( 2 * (2+( 3,4)-0,1)) | f( 2 * (~ 5)) & f( 2 * (0,1))^ m( 0,1);
int i[] = { 1, 23, 4, 5, };
char c[ 2][ 6] = { "hello", "" };
6 EXAMPLE 4 To illustrate the rules for creating character string literals and concatenating tokens, the sequence
#define str( s) # s
#define xstr( s) str( s)
#define debug( s, t) printf(" x" # s "= %d, x" # t "= %s", \
x ## s,x ## t)
#define INCFILE( n) vers ## n // from previous #include example
#define glue( a, b) a ## b
#define xglue( a, b) glue( a, b)
#define HIGHLOW "hello"
#define LOW LOW ", world"
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debug( 1, 2);
fputs( str( strncmp(" abc\ 0d", "abc", '\ 4') // this goes away
== 0) str(: @\ n), s);
#include xstr( INCFILE( 2). h)
glue( HIGH, LOW);
xglue( HIGH, LOW)
results in
printf(" x" "1" "= %d, x" "2" "= %s", x1, x2);
fputs(
"strncmp(\" abc\\ 0d\", \" abc\", '\\ 4') == 0" ": @\ n",
s);
#include "vers2. h" (after macro replacement, before file access)
"hello";
"hello" ", world"
or, after concatenation of the character string literals,
printf(" x1= %d, x2= %s", x1, x2);
fputs(
"strncmp(\" abc\\ 0d\", \" abc\", '\\ 4') == 0: @\ n",
s);
#include "vers2. h" (after macro replacement, before file access)
"hello";
"hello, world"
Space around the # and ## tokens in the macro definition is optional.
7 EXAMPLE 5 To illustrate the rules for
placemarker ## placemarker
the sequence
#define t( x, y, z) x ## y ## z
int j[] = { t( 1,2,3), t(, 4,5), t( 6,, 7), t( 8,9,),
t( 10,,), t(, 11,), t(,, 12), t(,,) };
results in
int j[] = { 123, 45, 67, 89,
10, 11, 12, };
8 EXAMPLE 6 To demonstrate the redefinition rules, the following sequence is valid.
#define OBJ_ LIKE (1-1)
#define OBJ_ LIKE /* white space */ (1-1) /* other */
#define FUNC_ LIKE( a) ( a )
#define FUNC_ LIKE( a )( /* note the white space */ \
a /* other stuff on this line
*/ )
But the following redefinitions are invalid:
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#define OBJ_ LIKE (0) /* different token sequence */
#define OBJ_ LIKE (1 -1) /* different white space */
#define FUNC_ LIKE( b) ( a ) /* different parameter usage */
#define FUNC_ LIKE( b) ( b ) /* different parameter spelling */
9 EXAMPLE 7 Finally, to show the variable argument list macro facilities:
#define debug(...) fprintf( stderr, _ _VA_ ARGS_ _)
#define showlist(...) puts(#_ _VA_ ARGS_ _)
#define report( test, ...) (( test)? puts(# test):\
printf(_ _VA_ ARGS_ _))
debug(" Flag");
debug(" X = %d\ n", x);
showlist( The first, second, and third items.);
report( x> y, "x is %d but y is %d", x, y);
results in
fprintf( stderr, "Flag" );
fprintf( stderr, "X = %d\ n", x );
puts( "The first, second, and third items." );
(( x> y)? puts(" x> y"):
printf(" x is %d but y is %d", x, y));
6.10.4 Line control
Constraints
1 The string literal of a #line directive, if present, shall be a character string literal.
Semantics
2 The line number of the current source line is one greater than the number of new-line
characters read or introduced in translation phase 1 (5.1.1.2) while processing the source
file to the current token.
3 A preprocessing directive of the form
# line digit-sequence new-line
causes the implementation to behave as if the following sequence of source lines begins
with a source line that has a line number as specified by the digit sequence (interpreted as
a decimal integer). The digit sequence shall not specify zero, nor a number greater than
2147483647.
4 A preprocessing directive of the form
# line digit-sequence "s-char-sequence opt " new-line
sets the presumed line number similarly and changes the presumed name of the source
file to be the contents of the character string literal.
5 A preprocessing directive of the form
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# line pp-tokens new-line
(that does not match one of the two previous forms) is permitted. The preprocessing
tokens after line on the directive are processed just as in normal text (each identifier
currently defined as a macro name is replaced by its replacement list of preprocessing
tokens). The directive resulting after all replacements shall match one of the two
previous forms and is then processed as appropriate.
6.10.5 Error directive
Semantics
1 A preprocessing directive of the form
# error pp-tokens opt new-line
causes the implementation to produce a diagnostic message that includes the specified
sequence of preprocessing tokens.
6.10.6 Pragma directive
Semantics
1 A preprocessing directive of the form
# pragma pp-tokens opt new-line
where the preprocessing token STDC does not immediately follow pragma in the
directive (prior to any macro replacement) 136) causes the implementation to behave in an
implementation-defined manner. The behavior might cause translation to fail or cause the
translator or the resulting program to behave in a non-conforming manner. Any such
pragma that is not recognized by the implementation is ignored.
2 If the preprocessing token STDC does immediately follow pragma in the directive (prior
to any macro replacement), then no macro replacement is performed on the directive, and
the directive shall have one of the following forms whose meanings are described
elsewhere:
136) An implementation is not required to perform macro replacement in pragmas, but it is permitted
except for in standard pragmas (where STDC immediately follows pragma). If the result of macro
replacement in a non-standard pragma has the same form as a standard pragma, the behavior is still
implementation-defined; an implementation is permitted to behave as if it were the standard pragma,
but is not required to.
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#pragma STDC FP_ CONTRACT on-off-switch
#pragma STDC FENV_ ACCESS on-off-switch
#pragma STDC CX_ LIMITED_ RANGE on-off-switch
on-off-switch: one of
ON OFF DEFAULT
Forward references: the FP_ CONTRACT pragma (7.12.2), the FENV_ ACCESS pragma
(7.6.1), the CX_ LIMITED_ RANGE pragma (7.3.4).
6.10.7 Null directive
Semantics
1 A preprocessing directive of the form
# new-line
has no effect.
6.10.8 Predefined macro names
1 The following macro names shall be defined by the implementation:
_ _LINE_ _ The presumed line number (within the current source file) of the current
source line (a decimal constant). 137)
_ _FILE_ _ The presumed name of the current source file (a character string literal). 137)
_ _DATE_ _ The date of translation of the source file: a character string literal of the
form "Mmm dd yyyy", where the names of the months are the same as
those generated by the asctime function, and the first character of dd is
a space character if the value is less than 10. If the date of translation is not
available, an implementation-defined valid date shall be supplied.
_ _TIME_ _ The time of translation of the source file: a character string literal of the
form "hh: mm: ss" as in the time generated by the asctime function. If
the time of translation is not available, an implementation-defined valid
time shall be supplied.
_ _STDC_ _ The decimal constant 1, intended to indicate a conforming implementation.
_ _STDC_ VERSION_ _ The decimal constant 199901L. 138)
137) The presumed line number and source file name can be changed by the #line directive.
138) This macro was not specified in ISO/ IEC 9899: 1990 and was specified as 199409L in ISO/ IEC
9899/ AMD1: 1995
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2 The following macro names are conditionally defined by the implementation:
_ _STDC_ ISO_ 10646_ _ A decimal constant of the form yyyymmL (for example,
199712L), intended to indicate that values of type
wchar_ t are the coded representations of the characters
defined by ISO/ IEC 10646, along with all amendments and
technical corrigenda as of the specified year and month.
_ _STDC_ IEC_ 559_ _ The decimal constant 1, intended to indicate conformance to
the specifications in annex F (IEC 60559 floating-point
arithmetic).
_ _STDC_ IEC_ 559_ COMPLEX_ _ The decimal constant 1, intended to indicate
adherence to the specifications in informative annex G (IEC
60559 compatible complex arithmetic).
3 The values of the predefined macros (except for _ _LINE_ _ and _ _FILE_ _) remain
constant throughout the translation unit.
4 None of these macro names, nor the identifier defined, shall be the subject of a
#define or a #undef preprocessing directive. Any other predefined macro names
shall begin with a leading underscore followed by an uppercase letter or a second
underscore.
Forward references: the asctime function (7.23.3.1).
6.10.9 Pragma operator
Semantics
1 A unary operator expression of the form:
_Pragma ( string-literal )
is processed as follows: The string literal is destringized by deleting the L prefix, if
present, deleting the leading and trailing double-quotes, replacing each escape sequence
\" by a double-quote, and replacing each escape sequence \\ by a single backslash. The
resulting sequence of characters is processed through translation phase 3 to produce
preprocessing tokens that are executed as if they were the pp-tokens in a pragma
directive. The original four preprocessing tokens in the unary operator expression are
removed.
2 EXAMPLE A directive of the form:
#pragma listing on "..\ listing. dir"
can also be expressed as:
_Pragma ( "listing on \"..\\ listing. dir\"" )
The latter form is processed in the same way whether it appears literally as shown, or results from macro
replacement, as in:
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#define LISTING( x) PRAGMA( listing on #x)
#define PRAGMA( x) _Pragma(# x)
LISTING ( ..\ listing. dir )
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6.11 Future language directions
6.11.1 Floating Types
1 Future standardization may include additional floating-point types, including those with
greater range, precision, or both than long double.
6.11.2 Character escape sequences
1 Lowercase letters as escape sequences are reserved for future standardization. Other
characters may be used in extensions.
6.11.3 Storage-class specifiers
1 The placement of a storage-class specifier other than at the beginning of the declaration
specifiers in a declaration is an obsolescent feature.
6.11.4 Function declarators
1 The use of function declarators with empty parentheses (not prototype-format parameter
type declarators) is an obsolescent feature.
6.11.5 Function definitions
1 The use of function definitions with separate parameter identifier and declaration lists
(not prototype-format parameter type and identifier declarators) is an obsolescent feature.
6.11.6 Pragma directives
1 Pragmas whose first pp-token is STDC are reserved for future standardization.
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7. Library
7.1 Introduction
7.1.1 Definitions of terms
1 Astring is a contiguous sequence of characters terminated by and including the first null
character. The term multibyte string is sometimes used instead to emphasize special
processing given to multibyte characters contained in the string or to avoid confusion
with a wide string. A pointer to a string is a pointer to its initial (lowest addressed)
character. The length of a string is the number of characters preceding the null character
and the value of a string is the sequence of the values of the contained characters, in
order.
2 Aletter is a printing character in the execution character set corresponding to any of the
52 required lowercase and uppercase letters in the source character set, listed in 5.2.1.
3 The decimal-point character is the character used by functions that convert floating-point
numbers to or from character sequences to denote the beginning of the fractional part of
such character sequences. 139) It is represented in the text and examples by a period, but
may be changed by the setlocale function.
4 Awide character is a code value (a binary encoded integer) of an object of type
wchar_ t that corresponds to a member of the extended character set. 140)
5 Anull wide character is a wide character with code value zero.
6 Awide string is a contiguous sequence of wide characters terminated by and including
the first null wide character. A pointer to a wide string is a pointer to its initial (lowest
addressed) wide character. The length of a wide string is the number of wide characters
preceding the null wide character and the value of a wide string is the sequence of code
values of the contained wide characters, in order.
7 Ashift sequence is a contiguous sequence of bytes within a multibyte string that
(potentially) causes a change in shift state (see 5.2.1.2). A shift sequence shall not have a
corresponding wide character; it is instead taken to be an adjunct to an adjacent multibyte
character. 141)
139) The functions that make use of the decimal-point character are the string conversion functions
(7.20.1), the wide-string numeric conversion functions (7.24.4.1), the formatted input/ output functions
(7.19.6), and the formatted wide-character input/ output functions (7.24.2).
140) An equivalent definition can be found in 6.4.4.4.
7 Library 7.1.1
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Forward references: character handling (7.4), the setlocale function (7.11.1.1).
7.1.2 Standard headers
1 Each library function is declared, with a type that includes a prototype, in a header, 142)
whose contents are made available by the #include preprocessing directive. The
header declares a set of related functions, plus any necessary types and additional macros
needed to facilitate their use. Declarations of types described in this clause shall not
include type qualifiers, unless explicitly stated otherwise.
2 The standard headers are
<assert. h>
<complex. h>
<ctype. h>
<errno. h>
<fenv. h>
<float. h>
<inttypes. h>
<iso646. h>
<limits. h>
<locale. h>
<math. h>
<setjmp. h>
<signal. h>
<stdarg. h>
<stdbool. h>
<stddef. h>
<stdint. h>
<stdio. h>
<stdlib. h>
<string. h>
<tgmath. h>
<time. h>
<wchar. h>
<wctype. h>
3 If a file with the same name as one of the above < and > delimited sequences, not
provided as part of the implementation, is placed in any of the standard places that are
searched for included source files, the behavior is undefined.
4 Standard headers may be included in any order; each may be included more than once in
a giv en scope, with no effect different from being included only once, except that the
effect of including <assert. h> depends on the definition of NDEBUG (see 7.2). If
used, a header shall be included outside of any external declaration or definition, and it
shall first be included before the first reference to any of the functions or objects it
declares, or to any of the types or macros it defines. However, if an identifier is declared
or defined in more than one header, the second and subsequent associated headers may be
included after the initial reference to the identifier. The program shall not have any
macros with names lexically identical to keywords currently defined prior to the
inclusion.
5 Any definition of an object-like macro described in this clause shall expand to code that is
fully protected by parentheses where necessary, so that it groups in an arbitrary
expression as if it were a single identifier.
141) For state-dependent encodings, the values for MB_ CUR_ MAX and MB_ LEN_ MAX shall thus be large
enough to count all the bytes in any complete multibyte character plus at least one adjacent shift
sequence of maximum length. Whether these counts provide for more than one shift sequence is the
implementation's choice.
142) A header is not necessarily a source file, nor are the < and > delimited sequences in header names
necessarily valid source file names.
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6 Any declaration of a library function shall have external linkage.
7 A summary of the contents of the standard headers is given in annex B.
Forward references: diagnostics (7.2).
7.1.3 Reserved identifiers
1 Each header declares or defines all identifiers listed in its associated subclause, and
optionally declares or defines identifiers listed in its associated future library directions
subclause and identifiers which are always reserved either for any use or for use as file
scope identifiers.
All identifiers that begin with an underscore and either an uppercase letter or another
underscore are always reserved for any use.
All identifiers that begin with an underscore are always reserved for use as identifiers
with file scope in both the ordinary and tag name spaces.
Each macro name in any of the following subclauses (including the future library
directions) is reserved for use as specified if any of its associated headers is included;
unless explicitly stated otherwise (see 7.1.4).
All identifiers with external linkage in any of the following subclauses (including the
future library directions) are always reserved for use as identifiers with external
linkage. 143)
Each identifier with file scope listed in any of the following subclauses (including the
future library directions) is reserved for use as macro and as an identifier with file
scope in the same name space if any of its associated headers is included.
2 No other identifiers are reserved. If the program declares or defines an identifier in a
context in which it is reserved (other than as allowed by 7.1.4), or defines a reserved
identifier as a macro name, the behavior is undefined.
3 If the program removes (with #undef) any macro definition of an identifier in the first
group listed above, the behavior is undefined.
143) The list of reserved identifiers with external linkage includes errno, setjmp, and va_ end.
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7.1.4 Use of library functions
1 Each of the following statements applies unless explicitly stated otherwise in the detailed
descriptions that follow: If an argument to a function has an invalid value (such as a value
outside the domain of the function, or a pointer outside the address space of the program,
or a null pointer) or a type (after promotion) not expected by a function with variable
number of arguments, the behavior is undefined. If a function argument is described as
being an array, the pointer actually passed to the function shall have a value such that all
address computations and accesses to objects (that would be valid if the pointer did point
to the first element of such an array) are in fact valid. Any function declared in a header
may be additionally implemented as a function-like macro defined in the header, so if a
library function is declared explicitly when its header is included, one of the techniques
shown below can be used to ensure the declaration is not affected by such a macro. Any
macro definition of a function can be suppressed locally by enclosing the name of the
function in parentheses, because the name is then not followed by the left parenthesis that
indicates expansion of a macro function name. For the same syntactic reason, it is
permitted to take the address of a library function even if it is also defined as a macro. 144)
The use of #undef to remove any macro definition will also ensure that an actual
function is referred to. Any inv ocation of a library function that is implemented as a
macro shall expand to code that evaluates each of its arguments exactly once, fully
protected by parentheses where necessary, so it is generally safe to use arbitrary
expressions as arguments. 145) Likewise, those function-like macros described in the
following subclauses may be invoked in an expression anywhere a function with a
compatible return type could be called. 146) All object-like macros listed as expanding to
integer constant expressions shall additionally be suitable for use in #if preprocessing
directives.
2 Provided that a library function can be declared without reference to any type defined in a
header, it is also permissible to declare the function and use it without including its
associated header.
3 There is a sequence point immediately before a library function returns.
4 The functions in the standard library are not guaranteed to be reentrant and may modify
objects with static storage duration. 147)
5 EXAMPLE The function atoi may be used in any of sev eral ways:
by use of its associated header (possibly generating a macro expansion)
144) This means that an implementation shall provide an actual function for each library function, even if it
also provides a macro for that function.
145) Such macros might not contain the sequence points that the corresponding function calls do.
7.1.4 Library 7.1.4
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#include <stdlib. h>
const char *str;
/* ... */
i = atoi( str);
by use of its associated header (assuredly generating a true function reference)
#include <stdlib. h>
#undef atoi
const char *str;
/* ... */
i = atoi( str);
or
#include <stdlib. h>
const char *str;
/* ... */
i = (atoi)( str);
by explicit declaration
extern int atoi( const char *);
const char *str;
/* ... */
i = atoi( str);
146) Because external identifiers and some macro names beginning with an underscore are reserved,
implementations may provide special semantics for such names. For example, the identifier
_BUILTIN_ abs could be used to indicate generation of in-line code for the abs function. Thus, the
appropriate header could specify
#define abs( x) _BUILTIN_ abs( x)
for a compiler whose code generator will accept it.
In this manner, a user desiring to guarantee that a given library function such as abs will be a genuine
function may write
#undef abs
whether the implementation's header provides a macro implementation of abs or a built-in
implementation. The prototype for the function, which precedes and is hidden by any macro
definition, is thereby revealed also.
147) Thus, a signal handler cannot, in general, call standard library functions.
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7.2 Diagnostics <assert. h>
1 The header <assert. h> defines the assert macro and refers to another macro,
NDEBUG
which is not defined by <assert. h>. IfNDEBUG is defined as a macro name at the
point in the source file where <assert. h> is included, the assert macro is defined
simply as
#define assert( ignore) (( void) 0)
The assert macro is redefined according to the current state of NDEBUG each time that
<assert. h> is included.
2 The assert macro shall be implemented as a macro, not as an actual function. If the
macro definition is suppressed in order to access an actual function, the behavior is
undefined.
7.2.1 Program diagnostics
7.2.1.1 The assert macro
Synopsis
1 #include <assert. h>
void assert( scalar expression);
Description
2 The assert macro puts diagnostic tests into programs; it expands to a void expression.
When it is executed, if expression (which shall have a scalar type) is false (that is,
compares equal to 0), the assert macro writes information about the particular call that
failed (including the text of the argument, the name of the source file, the source line
number, and the name of the enclosing function the latter are respectively the values of
the preprocessing macros _ _FILE_ _ and _ _LINE_ _ and of the identifier
_ _func_ _) on the standard error file in an implementation-defined format. 148) It then
calls the abort function.
Returns
3 The assert macro returns no value.
Forward references: the abort function (7.20.4.1).
148) The message written might be of the form:
Assertion failed: expression, function abc, file xyz, line nnn.
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7.3 Complex arithmetic <complex. h>
7.3.1 Introduction
1 The header <complex. h> defines macros and declares functions that support complex
arithmetic. 149) Each synopsis specifies a family of functions consisting of a principal
function with one or more double complex parameters and a double complex or
double return value; and other functions with the same name but with f and l suffixes
which are corresponding functions with float and long double parameters and
return values.
2 The macro
complex
expands to _Complex; the macro
_Complex_ I
expands to a constant expression of type const float _Complex, with the value of
the imaginary unit. 150)
3 The macros
imaginary
and
_Imaginary_ I
are defined if and only if the implementation supports imaginary types; 151) if defined,
they expand to _Imaginary and a constant expression of type const float
_Imaginary with the value of the imaginary unit.
4 The macro
I
expands to either _Imaginary_ I or _Complex_ I. If_ Imaginary_ I is not
defined, I shall expand to _Complex_ I.
5 Notwithstanding the provisions of 7.1.3, a program is permitted to undefine and perhaps
then redefine the macros complex, imaginary, and I.
149) See '' future library directions'' (7.26.1).
150) The imaginary unit is a number i such that i 2 = -1.
151) A specification for imaginary types is in informative annex G.
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Forward references: IEC 60559-compatible complex arithmetic (annex G).
7.3.2 Conventions
1 Values are interpreted as radians, not degrees. An implementation may set errno but is
not required to.
7.3.3 Branch cuts
1 Some of the functions below hav e branch cuts, across which the function is
discontinuous. For implementations with a signed zero (including all IEC 60559
implementations) that follow the specification of annex G, the sign of zero distinguishes
one side of a cut from another so the function is continuous (except for format
limitations) as the cut is approached from either side. For example, for the square root
function, which has a branch cut along the negative real axis, the top of the cut, with
imaginary part +0, maps to the positive imaginary axis, and the bottom of the cut, with
imaginary part 0, maps to the negative imaginary axis.
2 Implementations that do not support a signed zero (see annex F) cannot distinguish the
sides of branch cuts. These implementations shall map a cut so the function is continuous
as the cut is approached coming around the finite endpoint of the cut in a counter
clockwise direction. (Branch cuts for the functions specified here have just one finite
endpoint.) For example, for the square root function, coming counter clockwise around
the finite endpoint of the cut along the negative real axis approaches the cut from above,
so the cut maps to the positive imaginary axis.
7.3.4 The CX_ LIMITED_ RANGE pragma
Synopsis
1 #include <complex. h>
#pragma STDC CX_ LIMITED_ RANGE on-off-switch
Description
2 The usual mathematical formula for complex multiply, divide, and absolute value are
problematic because of their treatment of infinities and because of undue overflow and
underflow. The CX_ LIMITED_ RANGE pragma can be used to inform the
implementation that (where the state is on) the usual mathematical formulas are
acceptable. 152) The pragma can occur either outside external declarations or preceding all
explicit declarations and statements inside a compound statement. When outside external
declarations, the pragma takes effect from its occurrence until another
CX_ LIMITED_ RANGE pragma is encountered, or until the end of the translation unit.
When inside a compound statement, the pragma takes effect from its occurrence until
another CX_ LIMITED_ RANGE pragma is encountered (including within a nested
compound statement), or until the end of the compound statement; at the end of a
compound statement the state for the pragma is restored to its condition just before the
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compound statement. If this pragma is used in any other context, the behavior is
undefined. The default state for the pragma is off .
7.3.5 Trigonometric functions
7.3.5.1 The cacos functions
Synopsis
1 #include <complex. h>
double complex cacos( double complex z);
float complex cacosf( float complex z);
long double complex cacosl( long double complex z);
Description
2 The cacos functions compute the complex arc cosine of z, with branch cuts outside the
interval [1, 1] along the real axis.
Returns
3 The cacos functions return the complex arc cosine value, in the range of a strip
mathematically unbounded along the imaginary axis and in the interval [0, p] along the
real axis.
152) The purpose of the pragma is to allow the implementation to use the formulas:
(x + iy) ΄ (u + iv) = (xu -yv) + i( yu + xv)
(x + iy)/( u+iv) = [( xu + yv) + i( yu -xv)] / (u 2 + v 2 )
| x + iy| = Φ ` ```` x 2 + y 2
where the programmer can determine they are safe.
7.3.4 Library 7.3.5.1
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7.3.5.2 The casin functions
Synopsis
1 #include <complex. h>
double complex casin( double complex z);
float complex casinf( float complex z);
long double complex casinl( long double complex z);
Description
2 The casin functions compute the complex arc sine of z, with branch cuts outside the
interval [1, 1] along the real axis.
Returns
3 The casin functions return the complex arc sine value, in the range of a strip
mathematically unbounded along the imaginary axis and in the interval [ p/ 2, p/ 2] along
the real axis.
7.3.5.3 The catan functions
Synopsis
1 #include <complex. h>
double complex catan( double complex z);
float complex catanf( float complex z);
long double complex catanl( long double complex z);
Description
2 The catan functions compute the complex arc tangent of z, with branch cuts outside the
interval [i, i] along the imaginary axis.
Returns
3 The catan functions return the complex arc tangent value, in the range of a strip
mathematically unbounded along the imaginary axis and in the interval [ p/ 2, p/ 2] along
the real axis.
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7.3.5.4 The ccos functions
Synopsis
1 #include <complex. h>
double complex ccos( double complex z);
float complex ccosf( float complex z);
long double complex ccosl( long double complex z);
Description
2 The ccos functions compute the complex cosine of z.
Returns
3 The ccos functions return the complex cosine value.
7.3.5.5 The csin functions
Synopsis
1 #include <complex. h>
double complex csin( double complex z);
float complex csinf( float complex z);
long double complex csinl( long double complex z);
Description
2 The csin functions compute the complex sine of z.
Returns
3 The csin functions return the complex sine value.
7.3.5.6 The ctan functions
Synopsis
1 #include <complex. h>
double complex ctan( double complex z);
float complex ctanf( float complex z);
long double complex ctanl( long double complex z);
Description
2 The ctan functions compute the complex tangent of z.
Returns
3 The ctan functions return the complex tangent value.
7.3.5.3 Library 7.3.5.6
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7.3.6 Hyperbolic functions
7.3.6.1 The cacosh functions
Synopsis
1 #include <complex. h>
double complex cacosh( double complex z);
float complex cacoshf( float complex z);
long double complex cacoshl( long double complex z);
Description
2 The cacosh functions compute the complex arc hyperbolic cosine of z, with a branch
cut at values less than 1 along the real axis.
Returns
3 The cacosh functions return the complex arc hyperbolic cosine value, in the range of a
half-strip of non-negative values along the real axis and in the interval [i p, i p] along the
imaginary axis.
7.3.6.2 The casinh functions
Synopsis
1 #include <complex. h>
double complex casinh( double complex z);
float complex casinhf( float complex z);
long double complex casinhl( long double complex z);
Description
2 The casinh functions compute the complex arc hyperbolic sine of z, with branch cuts
outside the interval [i, i] along the imaginary axis.
Returns
3 The casinh functions return the complex arc hyperbolic sine value, in the range of a
strip mathematically unbounded along the real axis and in the interval [i p/ 2, i p/ 2] along
the imaginary axis.
7.3.6 Library 7.3.6.2
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7.3.6.3 The catanh functions
Synopsis
1 #include <complex. h>
double complex catanh( double complex z);
float complex catanhf( float complex z);
long double complex catanhl( long double complex z);
Description
2 The catanh functions compute the complex arc hyperbolic tangent of z, with branch
cuts outside the interval [1, 1] along the real axis.
Returns
3 The catanh functions return the complex arc hyperbolic tangent value, in the range of a
strip mathematically unbounded along the real axis and in the interval [i p/ 2, i p/ 2] along
the imaginary axis.
7.3.6.4 The ccosh functions
Synopsis
1 #include <complex. h>
double complex ccosh( double complex z);
float complex ccoshf( float complex z);
long double complex ccoshl( long double complex z);
Description
2 The ccosh functions compute the complex hyperbolic cosine of z.
Returns
3 The ccosh functions return the complex hyperbolic cosine value.
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7.3.6.5 The csinh functions
Synopsis
1 #include <complex. h>
double complex csinh( double complex z);
float complex csinhf( float complex z);
long double complex csinhl( long double complex z);
Description
2 The csinh functions compute the complex hyperbolic sine of z.
Returns
3 The csinh functions return the complex hyperbolic sine value.
7.3.6.6 The ctanh functions
Synopsis
1 #include <complex. h>
double complex ctanh( double complex z);
float complex ctanhf( float complex z);
long double complex ctanhl( long double complex z);
Description
2 The ctanh functions compute the complex hyperbolic tangent of z.
Returns
3 The ctanh functions return the complex hyperbolic tangent value.
7.3.7 Exponential and logarithmic functions
7.3.6.4 Library 7.3.7
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7.3.7.1 The cexp functions
Synopsis
1 #include <complex. h>
double complex cexp( double complex z);
float complex cexpf( float complex z);
long double complex cexpl( long double complex z);
Description
2 The cexp functions compute the complex base-e exponential of z.
Returns
3 The cexp functions return the complex base-e exponential value.
7.3.7.2 The clog functions
Synopsis
1 #include <complex. h>
double complex clog( double complex z);
float complex clogf( float complex z);
long double complex clogl( long double complex z);
Description
2 The clog functions compute the complex natural (base-e) logarithm of z, with a branch
cut along the negative real axis.
Returns
3 The clog functions return the complex natural logarithm value, in the range of a strip
mathematically unbounded along the real axis and in the interval [i p, i p] along the
imaginary axis.
7.3.8 Power and absolute-value functions
7.3.7 Library 7.3.8
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7.3.8.1 The cabs functions
Synopsis
1 #include <complex. h>
double cabs( double complex z);
float cabsf( float complex z);
long double cabsl( long double complex z);
Description
2 The cabs functions compute the complex absolute value (also called norm, modulus, or
magnitude) of z.
Returns
3 The cabs functions return the complex absolute value.
7.3.8.2 The cpow functions
Synopsis
1 #include <complex. h>
double complex cpow( double complex x, double complex y);
float complex cpowf( float complex x, float complex y);
long double complex cpowl( long double complex x,
long double complex y);
Description
2 The cpow functions compute the complex power function x y , with a branch cut for the
first parameter along the negative real axis.
Returns
3 The cpow functions return the complex power function value.
7.3.8 Library 7.3.8.2
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7.3.8.3 The csqrt functions
Synopsis
1 #include <complex. h>
double complex csqrt( double complex z);
float complex csqrtf( float complex z);
long double complex csqrtl( long double complex z);
Description
2 The csqrt functions compute the complex square root of z, with a branch cut along the
negative real axis.
Returns
3 The csqrt functions return the complex square root value, in the range of the right half-plane
(including the imaginary axis).
7.3.9 Manipulation functions
7.3.9.1 The carg functions
Synopsis
1 #include <complex. h>
double carg( double complex z);
float cargf( float complex z);
long double cargl( long double complex z);
Description
2 The carg functions compute the argument (also called phase angle) of z, with a branch
cut along the negative real axis.
Returns
3 The carg functions return the value of the argument in the range [ p, p].
7.3.8.2 Library 7.3.9.1
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7.3.9.2 The cimag functions
Synopsis
1 #include <complex. h>
double cimag( double complex z);
float cimagf( float complex z);
long double cimagl( long double complex z);
Description
2 The cimag functions compute the imaginary part of z. 153)
Returns
3 The cimag functions return the imaginary part value (as a real).
7.3.9.3 The conj functions
Synopsis
1 #include <complex. h>
double complex conj( double complex z);
float complex conjf( float complex z);
long double complex conjl( long double complex z);
Description
2 The conj functions compute the complex conjugate of z, by rev ersing the sign of its
imaginary part.
Returns
3 The conj functions return the complex conjugate value.
153) For a variable z of complex type, z == creal( z) + cimag( z)* I.
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7.3.9.4 The cproj functions
Synopsis
1 #include <complex. h>
double complex cproj( double complex z);
float complex cprojf( float complex z);
long double complex cprojl( long double complex z);
Description
2 The cproj functions compute a projection of z onto the Riemann sphere: z projects to
z except that all complex infinities (even those with one infinite part and one NaN part)
project to positive infinity on the real axis. If z has an infinite part, then cproj( z) is
equivalent to
INFINITY + I * copysign( 0.0, cimag( z))
Returns
3 The cproj functions return the value of the projection onto the Riemann sphere.
7.3.9.5 The creal functions
Synopsis
1 #include <complex. h>
double creal( double complex z);
float crealf( float complex z);
long double creall( long double complex z);
Description
2 The creal functions compute the real part of z. 154)
Returns
3 The creal functions return the real part value.
154) For a variable z of complex type, z == creal( z) + cimag( z)* I.
7.3.9.3 Library 7.3.9.5
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7.4 Character handling <ctype. h>
1 The header <ctype. h> declares several functions useful for testing and mapping
characters. 155) In all cases the argument is an int, the value of which shall be
representable as an unsigned char or shall equal the value of the macro EOF. If the
argument has any other value, the behavior is undefined.
2 The behavior of these functions is affected by the current locale. Those functions that
have locale-specific aspects only when not in the "C" locale are noted below.
3 The term printing character refers to a member of a locale-specific set of characters, each
of which occupies one printing position on a display device; the term control character
refers to a member of a locale-specific set of characters that are not printing
characters. 156)
Forward references: EOF (7.19.1), localization (7.11).
7.4.1 Character testing functions
1 The functions in this subclause return nonzero (true) if and only if the value of the
argument c conforms to that in the description of the function.
7.4.1.1 The isalnum function
Synopsis
1 #include <ctype. h>
int isalnum( int c);
Description
2 The isalnum function tests for any character for which isalpha or isdigit is true.
155) See '' future library directions'' (7.26.2).
156) In an implementation that uses the seven-bit US ASCII character set, the printing characters are those
whose values lie from 0x20 (space) through 0x7E (tilde); the control characters are those whose
values lie from 0 (NUL) through 0x1F (US), and the character 0x7F (DEL).
7.4 Library 7.4.1.1
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7.4.1.2 The isalpha function
Synopsis
1 #include <ctype. h>
int isalpha( int c);
Description
2 The isalpha function tests for any character for which isupper or islower is true,
or any character that is one of a locale-specific set of alphabetic characters for which
none of iscntrl, isdigit, ispunct, orisspace is true. 157) In the "C" locale,
isalpha returns true only for the characters for which isupper or islower is true.
7.4.1.3 The iscntrl function
Synopsis
1 #include <ctype. h>
int iscntrl( int c);
Description
2 The iscntrl function tests for any control character.
7.4.1.4 The isdigit function
Synopsis
1 #include <ctype. h>
int isdigit( int c);
Description
2 The isdigit function tests for any decimal-digit character (as defined in 5.2.1).
157) The functions islower and isupper test true or false separately for each of these additional
characters; all four combinations are possible.
7.4.1.1 Library 7.4.1.4
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7.4.1.5 The isgraph function
Synopsis
1 #include <ctype. h>
int isgraph( int c);
Description
2 The isgraph function tests for any printing character except space (' ').
7.4.1.6 The islower function
Synopsis
1 #include <ctype. h>
int islower( int c);
Description
2 The islower function tests for any character that is a lowercase letter or is one of a
locale-specific set of characters for which none of iscntrl, isdigit, ispunct, or
isspace is true. In the "C" locale, islower returns true only for the characters
defined as lowercase letters (as defined in 5.2.1).
7.4.1.7 The isprint function
Synopsis
1 #include <ctype. h>
int isprint( int c);
Description
2 The isprint function tests for any printing character including space (' ').
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7.4.1.8 The ispunct function
Synopsis
1 #include <ctype. h>
int ispunct( int c);
Description
2 The ispunct function tests for any printing character that is one of a locale-specific set
of punctuation characters for which neither isspace nor isalnum is true. In the "C"
locale, ispunct returns true for every printing character for which neither isspace
nor isalnum is true.
7.4.1.9 The isspace function
Synopsis
1 #include <ctype. h>
int isspace( int c);
Description
2 The isspace function tests for any character that is a standard white-space character or
is one of a locale-specific set of characters for which isalnum is false. The standard
white-space characters are the following: space (' '), form feed ('\ f'), new-line
('\ n'), carriage return ('\ r'), horizontal tab ('\ t'), and vertical tab ('\ v'). In the
"C" locale, isspace returns true only for the standard white-space characters.
7.4.1.10 The isupper function
Synopsis
1 #include <ctype. h>
int isupper( int c);
Description
2 The isupper function tests for any character that is an uppercase letter or is one of a
locale-specific set of characters for which none of iscntrl, isdigit, ispunct, or
isspace is true. In the "C" locale, isupper returns true only for the characters
defined as uppercase letters (as defined in 5.2.1).
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7.4.1.11 The isxdigit function
Synopsis
1 #include <ctype. h>
int isxdigit( int c);
Description
2 The isxdigit function tests for any hexadecimal-digit character (as defined in 6.4.4.2).
7.4.2 Character case mapping functions
7.4.2.1 The tolower function
Synopsis
1 #include <ctype. h>
int tolower( int c);
Description
2 The tolower function converts an uppercase letter to a corresponding lowercase letter.
Returns
3 If the argument is a character for which isupper is true and there are one or more
corresponding characters, as specified by the current locale, for which islower is true,
the tolower function returns one of the corresponding characters (always the same one
for any giv en locale); otherwise, the argument is returned unchanged.
7.4.2.2 The toupper function
Synopsis
1 #include <ctype. h>
int toupper( int c);
Description
2 The toupper function converts a lowercase letter to a corresponding uppercase letter.
Returns
3 If the argument is a character for which islower is true and there are one or more
corresponding characters, as specified by the current locale, for which isupper is true,
the toupper function returns one of the corresponding characters (always the same one
for any giv en locale); otherwise, the argument is returned unchanged.
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7.5 Errors <errno. h>
1 The header <errno. h> defines several macros, all relating to the reporting of error
conditions.
2 The macros are
EDOM
EILSEQ
ERANGE
which expand to integer constant expressions with type int, distinct positive values, and
which are suitable for use in #if preprocessing directives; and
errno
which expands to a modifiable lvalue 158) that has type int, the value of which is set to a
positive error number by several library functions. It is unspecified whether errno is a
macro or an identifier declared with external linkage. If a macro definition is suppressed
in order to access an actual object, or a program defines an identifier with the name
errno, the behavior is undefined.
3 The value of errno is zero at program startup, but is never set to zero by any library
function. 159) The value of errno may be set to nonzero by a library function call
whether or not there is an error, provided the use of errno is not documented in the
description of the function in this International Standard.
4 Additional macro definitions, beginning with E and a digit or E and an uppercase
letter, 160) may also be specified by the implementation.
158) The macro errno need not be the identifier of an object. It might expand to a modifiable lvalue
resulting from a function call (for example, *errno()).
159) Thus, a program that uses errno for error checking should set it to zero before a library function call,
then inspect it before a subsequent library function call. Of course, a library function can save the
value of errno on entry and then set it to zero, as long as the original value is restored if errno's
value is still zero just before the return.
160) See '' future library directions'' (7.26.3).
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7.6 Floating-point environment <fenv. h>
1 The header <fenv. h> declares two types and several macros and functions to provide
access to the floating-point environment. The floating-point environment refers
collectively to any floating-point status flags and control modes supported by the
implementation. 161) A floating-point status flag is a system variable whose value is set
(but never cleared) as a side effect of floating-point arithmetic to provide auxiliary
information. A floating-point control mode is a system variable whose value may be set
by the user to affect the subsequent behavior of floating-point arithmetic.
2 Certain programming conventions support the intended model of use for the floating-point
environment: 162)
a function call does not alter its caller's modes, clear its caller's flags, nor depend on
the state of its caller's flags unless the function is so documented;
a function call is assumed to require default modes, unless its documentation
promises otherwise or unless the function is known not to use floating-point;
a function call is assumed to have the potential for raising floating-point exceptions,
unless its documentation promises otherwise, or unless the function is known not to
use floating-point.
3 The type
fenv_ t
represents the entire floating-point environment.
4 The type
fexcept_ t
represents the floating-point exception flags collectively, including any status the
implementation associates with the flags.
5 Each of the macros
161) This header is designed to support the exception status flags and directed-rounding control modes
required by IEC 60559, and other similar floating-point state information. Also it is designed to
facilitate code portability among all systems.
162) With these conventions, a programmer can safely assume default modes (or be unaware of them). The
responsibilities associated with accessing the floating-point environment fall on the programmer or
program that does so explicitly.
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FE_ DIVBYZERO
FE_ INEXACT
FE_ INVALID
FE_ OVERFLOW
FE_ UNDERFLOW
is defined if and only if the implementation supports the exception by means of the
functions in 7.6.2. Additional floating-point exceptions, with macro definitions beginning
with FE_ and an uppercase letter, may also be specified by the implementation. The
defined macros expand to integer constant expressions with values such that bitwise ORs
of all combinations of the macros result in distinct values.
6 The macro
FE_ ALL_ EXCEPT
is simply the bitwise OR of all exception macros defined by the implementation.
7 Each of the macros
FE_ DOWNWARD
FE_ TONEAREST
FE_ TOWARDZERO
FE_ UPWARD
is defined if and only if the implementation supports getting and setting the represented
rounding direction by means of the fegetround and fesetround functions.
Additional rounding directions, with macro definitions beginning with FE_ and an
uppercase letter, may also be specified by the implementation. The defined macros
expand to integer constant expressions whose values are distinct nonnegative values. 163)
8 The macro
FE_ DFL_ ENV
represents the default floating-point environment the one installed at program startup
and has type pointer to const-qualified fenv_ t. It can be used as an argument to
<fenv. h> functions that manage the floating-point environment.
9 Additional macro definitions, beginning with FE_ and having type pointer to const-qualified
fenv_ t, may also be specified by the implementation.
163) Even though the rounding direction macros may expand to constants corresponding to the values of
FLT_ ROUNDS, they are not required to do so.
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7.6.1 The FENV_ ACCESS pragma
Synopsis
1 #include <fenv. h>
#pragma STDC FENV_ ACCESS on-off-switch
Description
2 The FENV_ ACCESS pragma provides a means to inform the implementation when a
program might access the floating-point environment to test flags or run under non-default
modes. 164) The pragma shall occur either outside external declarations or
preceding all explicit declarations and statements inside a compound statement. When
outside external declarations, the pragma takes effect from its occurrence until another
FENV_ ACCESS pragma is encountered, or until the end of the translation unit. When
inside a compound statement, the pragma takes effect from its occurrence until another
FENV_ ACCESS pragma is encountered (including within a nested compound statement),
or until the end of the compound statement; at the end of a compound statement the state
for the pragma is restored to its condition just before the compound statement. If this
pragma is used in any other context, the behavior is undefined. If part of a program tests
flags or runs under non-default mode settings, but was translated with the state for the
FENV_ ACCESS pragma off , then the behavior is undefined. The default state (on or off )
for the pragma is implementation-defined.
3 EXAMPLE
#include <fenv. h>
void f( double x)
{
#pragma STDC FENV_ ACCESS ON
void g( double);
void h( double);
/* ... */
g( x + 1);
h( x + 1);
/* ... */
}
4 If the function g might depend on status flags set as a side effect of the first x +1, or if the second x +1might depend on control modes set as a side effect of the call to function g, then the program shall
contain an appropriately placed invocation of #pragma STDC FENV_ ACCESS ON. 165)
164) The purpose of the FENV_ ACCESS pragma is to allow certain optimizations, for example global
common subexpression elimination, code motion, and constant folding, that could subvert flag tests
and mode changes. In general, if the state of FENV_ ACCESS is off then the translator can assume that
default modes are in effect and the flags are not tested.
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7.6.2 Exceptions
1 The following functions provide access to the exception flags. 166) The int input
argument for the functions represents a subset of floating-point exceptions, and can be
zero or the bitwise OR of one or more exception macros, for example FE_ OVERFLOW |
FE_ INEXACT. For other argument values the behavior of these functions is undefined.
7.6.2.1 The feclearexcept function
Synopsis
1 #include <fenv. h>
void feclearexcept( int excepts);
Description
2 The feclearexcept function clears the supported exceptions represented by its
argument.
7.6.2.2 The fegetexceptflag function
Synopsis
1 #include <fenv. h>
void fegetexceptflag( fexcept_ t *flagp,
int excepts);
Description
2 The fegetexceptflag function stores an implementation-defined representation of
the exception flags indicated by the argument excepts in the object pointed to by the
argument flagp.
165) The side effects impose a temporal ordering that requires two evaluations of x +1. On the other
hand, without the #pragma STDC FENV_ ACCESS ON pragma, and assuming the default state is
off , just one evaluation of x +1would suffice.
166) The functions fetestexcept, feraiseexcept, and feclearexcept support the basic
abstraction of flags that are either set or clear. An implementation may endow exception flags with
more information for example, the address of the code which first raised the exception; the
functions fegetexceptflag and fesetexceptflag deal with the full content of flags.
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7.6.2.3 The feraiseexcept function
Synopsis
1 #include <fenv. h>
void feraiseexcept( int excepts);
Description
2 The feraiseexcept function raises the supported exceptions represented by its
argument. 167) The order in which these exceptions are raised is unspecified, except as
stated in F. 7.6. Whether the feraiseexcept function additionally raises the inexact
exception whenever it raises the overflow or underflow exception is implementation-defined.
7.6.2.4 The fesetexceptflag function
Synopsis
1 #include <fenv. h>
void fesetexceptflag( const fexcept_ t *flagp,
int excepts);
Description
2 The fesetexceptflag function sets the complete status for those exception flags
indicated by the argument excepts, according to the representation in the object
pointed to by flagp. The value of *flagp shall have been set by a previous call to
fegetexceptflag whose second argument represented at least those exceptions
represented by the argument excepts. This function does not raise exceptions, but only
sets the state of the flags.
167) The effect is intended to be similar to that of exceptions raised by arithmetic operations. Hence,
enabled traps for exceptions raised by this function are taken. The specification in F. 7.6 is in the same
spirit.
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7.6.2.5 The fetestexcept function
Synopsis
1 #include <fenv. h>
int fetestexcept( int excepts);
Description
2 The fetestexcept function determines which of a specified subset of the exception
flags are currently set. The excepts argument specifies the exception flags to be
queried. 168)
Returns
3 The fetestexcept function returns the value of the bitwise OR of the exception
macros corresponding to the currently set exceptions included in excepts.
4 EXAMPLE Call f if invalid is set, then g if overflow is set:
#include <fenv. h>
/* ... */
{
#pragma STDC FENV_ ACCESS ON
int set_ excepts;
feclearexcept( FE_ INVALID | FE_ OVERFLOW);
// maybe raise exceptions
set_ excepts =
fetestexcept( FE_ INVALID | FE_ OVERFLOW);
if (set_ excepts & FE_ INVALID) f();
if (set_ excepts & FE_ OVERFLOW) g();
/* ... */
}
7.6.3 Rounding
1 The fegetround and fesetround functions provide control of rounding direction
modes.
168) This mechanism allows testing several exceptions with just one function call.
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7.6.3.1 The fegetround function
Synopsis
1 #include <fenv. h>
int fegetround( void);
Description
2 The fegetround function gets the current rounding direction.
Returns
3 The fegetround function returns the value of the rounding direction macro
representing the current rounding direction.
7.6.3.2 The fesetround function
Synopsis
1 #include <fenv. h>
int fesetround( int round);
Description
2 The fesetround function establishes the rounding direction represented by its
argument round. If the argument is not equal to the value of a rounding direction macro,
the rounding direction is not changed.
Returns
3 The fesetround function returns a zero value if and only if the argument is equal to a
rounding direction macro (that is, if and only if the requested rounding direction was
established).
4 EXAMPLE 1 Save, set, and restore the rounding direction. Report an error and abort if setting the rounding direction fails.
#include <fenv. h>
#include <assert. h>
/* ... */
{
#pragma STDC FENV_ ACCESS ON
int save_ round;
int setround_ ok;
save_ round = fegetround();
setround_ ok = fesetround( FE_ UPWARD);
assert( setround_ ok == 0);
/* ... */
fesetround( save_ round);
/* ... */
}
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7.6.4 Environment
1 The functions in this section manage the floating-point environment status flags and
control modes as one entity.
7.6.4.1 The fegetenv function
Synopsis
1 #include <fenv. h>
void fegetenv( fenv_ t *envp);
Description
2 The fegetenv function stores the current floating-point environment in the object
pointed to by envp.
7.6.4.2 The feholdexcept function
Synopsis
1 #include <fenv. h>
int feholdexcept( fenv_ t *envp);
Description
2 The feholdexcept function saves the current floating-point environment in the object
pointed to by envp, clears the exception flags, and then installs a non-stop (continue on
exceptions) mode, if available, for all exceptions. 169)
Returns
3 The feholdexcept function returns zero if and only if non-stop exception handling
was successfully installed.
169) IEC 60559 systems have a default non-stop mode, and typically at least one other mode for trap
handling or aborting; if the system provides only the non-stop mode then installing it is trivial. For
such systems, the feholdexcept function can be used in conjunction with the feupdateenv
function to write routines that hide spurious exceptions from their callers.
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7.6.4.3 The fesetenv function
Synopsis
1 #include <fenv. h>
void fesetenv( const fenv_ t *envp);
Description
2 The fesetenv function establishes the floating-point environment represented by the
object pointed to by envp. The argument envp shall point to an object set by a call to
fegetenv or feholdexcept, or equal the macro FE_ DFL_ ENV or an
implementation-defined environment macro. Note that fesetenv merely installs the
state of the exception flags represented through its argument, and does not raise these
exceptions.
7.6.4.4 The feupdateenv function
Synopsis
1 #include <fenv. h>
void feupdateenv( const fenv_ t *envp);
Description
2 The feupdateenv function saves the currently raised exceptions in its automatic
storage, installs the floating-point environment represented by the object pointed to by
envp, and then raises the saved exceptions. The argument envp shall point to an object
set by a call to feholdexcept or fegetenv, or equal the macro FE_ DFL_ ENV or an
implementation-defined environment macro.
3 EXAMPLE 1 Hide spurious underflow exceptions:
#include <fenv. h>
double f( double x)
{
#pragma STDC FENV_ ACCESS ON
double result;
fenv_ t save_ env;
feholdexcept(& save_ env);
// compute result
if (/* test spurious underflow */)
feclearexcept( FE_ UNDERFLOW);
feupdateenv(& save_ env);
return result;
}
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7.7 Characteristics of floating types <float. h>
1 The header <float. h> defines several macros that expand to various limits and
parameters of the standard floating-point types.
2 The macros, their meanings, and the constraints (or restrictions) on their values are listed
in 5.2.4.2.2.
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7.8 Format conversion of integer types <inttypes. h>
1 The header <inttypes. h> includes the header <stdint. h> and extends it with
additional facilities provided by hosted implementations.
2 It declares four functions for converting numeric character strings to greatest-width
integers and, for each type declared in <stdint. h>, it defines corresponding macros
for conversion specifiers for use with the formatted input/ output functions. 170)
Forward references: integer types <stdint. h> (7.18).
7.8.1 Macros for format specifiers
1 Each of the following object-like macros 171) expands to a character string literal
containing a conversion specifier, possibly modified by a length modifier, suitable for use
within the format argument of a formatted input/ output function when converting the
corresponding integer type. These macro names have the general form of PRI (character
string literals for the fprintf family) or SCN (character string literals for the fscanf
family), 172) followed by the conversion specifier, followed by a name corresponding to a
similar type name in 7.18.1. In these names, N represents the width of the type as
described in 7.18.1. For example, PRIdFAST32 can be used in a format string to print
the value of an integer of type int_ fast32_ t.
2 The fprintf macros for signed integers are:
PRIdN PRIdLEASTN PRIdFASTN PRIdMAX PRIdPTR
PRIiN PRIiLEASTN PRIiFASTN PRIiMAX PRIiPTR
3 The fprintf macros for unsigned integers are:
PRIoN PRIoLEASTN PRIoFASTN PRIoMAX PRIoPTR
PRIuN PRIuLEASTN PRIuFASTN PRIuMAX PRIuPTR
PRIxN PRIxLEASTN PRIxFASTN PRIxMAX PRIxPTR
PRIXN PRIXLEASTN PRIXFASTN PRIXMAX PRIXPTR
4 The fscanf macros for signed integers are:
SCNdN SCNdLEASTN SCNdFASTN SCNdMAX SCNdPTR
SCNiN SCNiLEASTN SCNiFASTN SCNiMAX SCNiPTR
170) See '' future library directions'' (7.26.4).
171) C++ implementations should define these macros only when _ _STDC_ FORMAT_ MACROS is defined
before <inttypes. h> is included.
172) Separate macros are given for use with fprintf and fscanf functions because, in the general case,
different format specifiers may be required for fprintf and fscanf, even when the type is the
same.
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5 The fscanf macros for unsigned integers are:
SCNoN SCNoLEASTN SCNoFASTN SCNoMAX SCNoPTR
SCNuN SCNuLEASTN SCNuFASTN SCNuMAX SCNuPTR
SCNxN SCNxLEASTN SCNxFASTN SCNxMAX SCNxPTR
6 For each type that the implementation provides in <stdint. h>, the corresponding
fprintf macros shall be defined and the corresponding fscanf macros shall be
defined unless the implementation does not have a suitable fscanf length modifier for
the type.
7 EXAMPLE
#include <inttypes. h>
#include <wchar. h>
int main( void)
{
uintmax_ t i = UINTMAX_ MAX; // this type always exists
wprintf( L" The largest integer value is %020"
PRIxMAX "\ n", i);
return 0;
}
7.8.2 Conversion functions for greatest-width integer types
7.8.2.1 The strtoimax and strtoumax functions
Synopsis
1 #include <inttypes. h>
intmax_ t strtoimax( const char * restrict nptr,
char ** restrict endptr, int base);
uintmax_ t strtoumax( const char * restrict nptr,
char ** restrict endptr, int base);
Description
2 The strtoimax and strtoumax functions are equivalent to the strtol, strtoll,
strtoul, and strtoull functions, except that the initial portion of the string is
converted to intmax_ t and uintmax_ t representation, respectively.
Returns
3 The strtoimax and strtoumax functions return the converted value, if any. If no
conversion could be performed, zero is returned. If the correct value is outside the range
of representable values, INTMAX_ MAX, INTMAX_ MIN, orUINTMAX_ MAX is returned
(according to the return type and sign of the value, if any), and the value of the macro
ERANGE is stored in errno.
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7.8.2.2 The wcstoimax and wcstoumax functions
Synopsis
1 #include <stddef. h> // for wchar_ t
#include <inttypes. h>
intmax_ t wcstoimax( const wchar_ t * restrict nptr,
wchar_ t ** restrict endptr, int base);
uintmax_ t wcstoumax( const wchar_ t * restrict nptr,
wchar_ t ** restrict endptr, int base);
Description
2 The wcstoimax and wcstoumax functions are equivalent to the wcstol, wcstoll,
wcstoul, and wcstoull functions except that the initial portion of the wide string is
converted to intmax_ t and uintmax_ t representation, respectively.
Returns
3 The wcstoimax function returns the converted value, if any. If no conversion could be
performed, zero is returned. If the correct value is outside the range of representable
values, INTMAX_ MAX, INTMAX_ MIN, orUINTMAX_ MAX is returned (according to the
return type and sign of the value, if any), and the value of the macro ERANGE is stored in
errno.
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7.9 Alternative spellings <iso646. h>
1 The header <iso646. h> defines the following eleven macros (on the left) that expand
to the corresponding tokens (on the right):
and &&
and_ eq &=
bitand &
bitor |
compl ~
not !
not_ eq !=
or ||
or_ eq |=
xor ^
xor_ eq ^=
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7.10 Sizes of integer types <limits. h>
1 The header <limits. h> defines several macros that expand to various limits and
parameters of the standard integer types.
2 The macros, their meanings, and the constraints (or restrictions) on their values are listed
in 5.2.4.2.1.
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7.11 Localization <locale. h>
1 The header <locale. h> declares two functions, one type, and defines several macros.
2 The type is
struct lconv
which contains members related to the formatting of numeric values. The structure shall
contain at least the following members, in any order. The semantics of the members and
their normal ranges are explained in 7.11.2.1. In the "C" locale, the members shall have
the values specified in the comments.
char *decimal_ point; // "."
char *thousands_ sep; // ""
char *grouping; // ""
char *mon_ decimal_ point; // ""
char *mon_ thousands_ sep; // ""
char *mon_ grouping; // ""
char *positive_ sign; // ""
char *negative_ sign; // ""
char *currency_ symbol; // ""
char frac_ digits; // CHAR_ MAX
char p_ cs_ precedes; // CHAR_ MAX
char n_ cs_ precedes; // CHAR_ MAX
char p_ sep_ by_ space; // CHAR_ MAX
char n_ sep_ by_ space; // CHAR_ MAX
char p_ sign_ posn; // CHAR_ MAX
char n_ sign_ posn; // CHAR_ MAX
char *int_ curr_ symbol; // ""
char int_ frac_ digits; // CHAR_ MAX
char int_ p_ cs_ precedes; // CHAR_ MAX
char int_ n_ cs_ precedes; // CHAR_ MAX
char int_ p_ sep_ by_ space; // CHAR_ MAX
char int_ n_ sep_ by_ space; // CHAR_ MAX
char int_ p_ sign_ posn; // CHAR_ MAX
char int_ n_ sign_ posn; // CHAR_ MAX
3 The macros defined are NULL (described in 7.17); and
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LC_ ALL
LC_ COLLATE
LC_ CTYPE
LC_ MONETARY
LC_ NUMERIC
LC_ TIME
which expand to integer constant expressions with distinct values, suitable for use as the
first argument to the setlocale function. 173) Additional macro definitions, beginning
with the characters LC_ and an uppercase letter, 174) may also be specified by the
implementation.
7.11.1 Locale control
7.11.1.1 The setlocale function
Synopsis
1 #include <locale. h>
char *setlocale( int category, const char *locale);
Description
2 The setlocale function selects the appropriate portion of the program's locale as
specified by the category and locale arguments. The setlocale function may be
used to change or query the program's entire current locale or portions thereof. The value
LC_ ALL for category names the program's entire locale; the other values for
category name only a portion of the program's locale. LC_ COLLATE affects the
behavior of the strcoll and strxfrm functions. LC_ CTYPE affects the behavior of
the character handling functions 175) and the multibyte and wide-character functions.
LC_ MONETARY affects the monetary formatting information returned by the
localeconv function. LC_ NUMERIC affects the decimal-point character for the
formatted input/ output functions and the string conversion functions, as well as the
nonmonetary formatting information returned by the localeconv function. LC_ TIME
affects the behavior of the strftime function.
3 A value of "C" for locale specifies the minimal environment for C translation; a value
of "" for locale specifies the locale-specific native environment. Other
implementation-defined strings may be passed as the second argument to setlocale.
173) ISO/ IEC 99452 specifies locale and charmap formats that may be used to specify locales for C.
174) See '' future library directions'' (7.26.5).
175) The only functions in 7.4 whose behavior is not affected by the current locale are isdigit and
isxdigit.
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4 At program startup, the equivalent of
setlocale( LC_ ALL, "C");
is executed.
5 The implementation shall behave as if no library function calls the setlocale function.
Returns
6 If a pointer to a string is given for locale and the selection can be honored, the
setlocale function returns a pointer to the string associated with the specified
category for the new locale. If the selection cannot be honored, the setlocale
function returns a null pointer and the program's locale is not changed.
7 A null pointer for locale causes the setlocale function to return a pointer to the
string associated with the category for the program's current locale; the program's
locale is not changed. 176)
8 The pointer to string returned by the setlocale function is such that a subsequent call
with that string value and its associated category will restore that part of the program's
locale. The string pointed to shall not be modified by the program, but may be
overwritten by a subsequent call to the setlocale function.
Forward references: formatted input/ output functions (7.19.6), the multibyte character
functions (7.20.7), the multibyte string functions (7.20.8), string conversion functions
(7.20.1), the strcoll function (7.21.4.3), the strftime function (7.23.3.5), the *
strxfrm function (7.21.4.5).
7.11.2 Numeric formatting convention inquiry
176) The implementation shall arrange to encode in a string the various categories due to a heterogeneous
locale when category has the value LC_ ALL.
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7.11.2.1 The localeconv function
Synopsis
1 #include <locale. h>
struct lconv *localeconv( void);
Description
2 The localeconv function sets the components of an object with type struct lconv
with values appropriate for the formatting of numeric quantities (monetary and otherwise)
according to the rules of the current locale.
3 The members of the structure with type char * are pointers to strings, any of which
(except decimal_ point) can point to "", to indicate that the value is not available in
the current locale or is of zero length. Apart from grouping and mon_ grouping, the
strings shall start and end in the initial shift state. The members with type char are
nonnegative numbers, any of which can be CHAR_ MAX to indicate that the value is not
available in the current locale. The members include the following:
char *decimal_ point
The decimal-point character used to format nonmonetary quantities.
char *thousands_ sep
The character used to separate groups of digits before the decimal-point
character in formatted nonmonetary quantities.
char *grouping
A string whose elements indicate the size of each group of digits in formatted
nonmonetary quantities.
char *mon_ decimal_ point
The decimal-point used to format monetary quantities.
char *mon_ thousands_ sep
The separator for groups of digits before the decimal-point in formatted
monetary quantities.
char *mon_ grouping
A string whose elements indicate the size of each group of digits in formatted
monetary quantities.
char *positive_ sign
The string used to indicate a nonnegative-valued formatted monetary quantity.
char *negative_ sign
The string used to indicate a negative-valued formatted monetary quantity.
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char *currency_ symbol
The local currency symbol applicable to the current locale.
char frac_ digits
The number of fractional digits (those after the decimal-point) to be displayed
in a locally formatted monetary quantity.
char p_ cs_ precedes
Set to 1 or 0 if the currency_ symbol respectively precedes or succeeds
the value for a nonnegative locally formatted monetary quantity.
char n_ cs_ precedes
Set to 1 or 0 if the currency_ symbol respectively precedes or succeeds
the value for a negative locally formatted monetary quantity.
char p_ sep_ by_ space
Set to a value indicating the separation of the currency_ symbol, the sign
string, and the value for a nonnegative locally formatted monetary quantity.
char n_ sep_ by_ space
Set to a value indicating the separation of the currency_ symbol, the sign
string, and the value for a negative locally formatted monetary quantity.
char p_ sign_ posn
Set to a value indicating the positioning of the positive_ sign for a
nonnegative locally formatted monetary quantity.
char n_ sign_ posn
Set to a value indicating the positioning of the negative_ sign for a
negative locally formatted monetary quantity.
char *int_ curr_ symbol
The international currency symbol applicable to the current locale. The first
three characters contain the alphabetic international currency symbol in
accordance with those specified in ISO 4217: 1995. The fourth character
(immediately preceding the null character) is the character used to separate the
international currency symbol from the monetary quantity.
char int_ frac_ digits
The number of fractional digits (those after the decimal-point) to be displayed
in an internationally formatted monetary quantity.
char int_ p_ cs_ precedes
Set to 1 or 0 if the int_ currency_ symbol respectively precedes or
succeeds the value for a nonnegative internationally formatted monetary
quantity.
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char int_ n_ cs_ precedes
Set to 1 or 0 if the int_ currency_ symbol respectively precedes or
succeeds the value for a negative internationally formatted monetary quantity.
char int_ p_ sep_ by_ space
Set to a value indicating the separation of the int_ currency_ symbol, the
sign string, and the value for a nonnegative internationally formatted monetary
quantity.
char int_ n_ sep_ by_ space
Set to a value indicating the separation of the int_ currency_ symbol, the
sign string, and the value for a negative internationally formatted monetary
quantity.
char int_ p_ sign_ posn
Set to a value indicating the positioning of the positive_ sign for a
nonnegative internationally formatted monetary quantity.
char int_ n_ sign_ posn
Set to a value indicating the positioning of the negative_ sign for a
negative internationally formatted monetary quantity.
4 The elements of grouping and mon_ grouping are interpreted according to the
following:
CHAR_ MAX No further grouping is to be performed.
0 The previous element is to be repeatedly used for the remainder of the
digits.
other The integer value is the number of digits that compose the current group.
The next element is examined to determine the size of the next group of
digits before the current group.
5 The values of p_ sep_ by_ space, n_ sep_ by_ space, int_ p_ sep_ by_ space,
and int_ n_ sep_ by_ space are interpreted according to the following:
0 No space separates the currency symbol and value.
1 If the currency symbol and sign string are adjacent, a space separates them from the
value; otherwise, a space separates the currency symbol from the value.
2 If the currency symbol and sign string are adjacent, a space separates them;
otherwise, a space separates the sign string from the value.
6 The values of p_ sign_ posn, n_ sign_ posn, int_ p_ sign_ posn, and
int_ n_ sign_ posn are interpreted according to the following:
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0 Parentheses surround the quantity and currency symbol.
1 The sign string precedes the quantity and currency symbol.
2 The sign string succeeds the quantity and currency symbol.
3 The sign string immediately precedes the currency symbol.
4 The sign string immediately succeeds the currency symbol.
7 The implementation shall behave as if no library function calls the localeconv
function.
Returns
8 The localeconv function returns a pointer to the filled-in object. The structure
pointed to by the return value shall not be modified by the program, but may be
overwritten by a subsequent call to the localeconv function. In addition, calls to the
setlocale function with categories LC_ ALL, LC_ MONETARY, orLC_ NUMERIC may
overwrite the contents of the structure.
9 EXAMPLE 1 The following table illustrates the rules which may well be used by four countries to format monetary quantities.
Local format International format
Country Positive Neg ative Positive Neg ative
Finland 1.234,56 mk -1.234,56 mk FIM 1.234,56 FIM -1.234,56
Italy L. 1.234 -L. 1.234 ITL 1.234 -ITL 1.234
Netherlands 1.234,56 -1.234,56 NLG 1.234,56 NLG -1.234,56
Switzerland SFrs. 1,234.56 SFrs. 1,234.56C CHF 1,234.56 CHF 1,234.56C
10 For these four countries, the respective values for the monetary members of the structure returned by localeconv are:
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Finland Italy Netherlands Switzerland
mon_ decimal_ point "," "" "," "."
mon_ thousands_ sep "." "." "." ","
mon_ grouping "\ 3" "\ 3" "\ 3" "\ 3"
positive_ sign "" "" "" ""
negative_ sign "-" "-" "-" "C"
currency_ symbol "mk" "L." "\ u0192" "SFrs."
frac_ digits 2 0 2 2
p_ cs_ precedes 0 1 1 1
n_ cs_ precedes 0 1 1 1
p_ sep_ by_ space 1 0 1 0
n_ sep_ by_ space 1 0 1 0
p_ sign_ posn 1 1 1 1
n_ sign_ posn 1 1 4 2
int_ curr_ symbol "FIM " "ITL " "NLG " "CHF "
int_ frac_ digits 2 0 2 2
int_ p_ cs_ precedes 1 1 1 1
int_ n_ cs_ precedes 1 1 1 1
int_ p_ sep_ by_ space 0 0 0 0
int_ n_ sep_ by_ space 0 0 0 0
int_ p_ sign_ posn 1 1 1 1
int_ n_ sign_ posn 4 1 4 2
11 EXAMPLE 2 The following table illustrates how the cs_ precedes, sep_ by_ space, and sign_ posn members affect the formatted value.
p_ sep_ by_ space
p_ cs_ precedes p_ sign_ posn 0 1 2
0 0 (1.25$) (1.25 $) (1.25$)
1 +1.25$ +1.25 $ + 1.25$
2 1.25$+ 1.25 $+ 1.25$ +
3 1.25+$ 1.25 +$ 1.25+ $
4 1.25$+ 1.25 $+ 1.25$ +
1 0 ($ 1.25) ($ 1.25) ($ 1.25)
1 +$ 1.25 +$ 1.25 + $1.25
2 $1.25+ $ 1.25+ $1.25 +
3 +$ 1.25 +$ 1.25 + $1.25
4 $+ 1.25 $+ 1.25 $ +1.25
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7.12 Mathematics <math. h>
1 The header <math. h> declares two types and several mathematical functions and
defines several macros. Most synopses specify a family of functions consisting of a
principal function with one or more double parameters, a double return value, or
both; and other functions with the same name but with f and l suffixes which are
corresponding functions with float and long double parameters, return values, or
both. 177) Integer arithmetic functions and conversion functions are discussed later.
2 The types
float_ t
double_ t
are floating types at least as wide as float and double, respectively, and such that
double_ t is at least as wide as float_ t. IfFLT_ EVAL_ METHOD equals 0,
float_ t and double_ t are float and double, respectively; if
FLT_ EVA