Premature end of script headers

First take the popular step to make sure that the first 20 characters you send from your CGI are exactly

     Content-type: text/html

followed by two (nb two) line feed characters -- 0xA 0xA, not carriage return characters (0xD 0xD). Or, in code:

     printf("Content-type: text/html%c%c",10,10);

This next putative fix is obscure. If you're running under *nix, it seems that your executable in ~/public_html/cgi-bin cannot have write privileges enabled for group or for all. It seems this executable will always fail:

  -rwxrwxr-x 1 pwilson pwilson 6218 Apr 15 18:38 this-fails*

while this one will work:

  -rwxr-xr-x 1 pwilson pwilson 6218 Apr 16 06:53 this-works*

It seems that Apache will refuse to run a writeable object.

And from a Usenet group: "I recently saw a similar problem, where the server was refusing to run a script because the DIRECTORY it was in was world-writeable (how the directory became world writable is another question....)." [go there]

DHTML and CGI links

Apache CGI how-to, an outstanding doc.

CGI newsgroup on Google -- active, responsive, and helpful.

Newsgroup FAQ tells only how to post.

CGI working group mailing list home page. Don't even bother, it's comatose.

Harvey Mudd College CS Dept. Cookbookish overview of how to get a CGI script to actually run.

Summary: CGI parameters

1. GET and POST pass parameter strings in three possible ways
2. Different browsers deliver different parameter strings
3. IE6 follows its own rules

We examine parameter passing and demonstrate by example.

The examples call the C-language CGI script cgi-inp, a C script that deals direct at a low level and sidesteps any interpreter like PHP or Perl.

Goals

1. Show in fine detail the parameter strings the CGI script sees and how it sees them
2. Invoke the C-language script from GET and POST in many ordinary, vanilla ways
3. Reveal HTML-CGI interaction by complete, correct description and practical example

Non-goals

1. Subordinate appearance and presentation to organization and example
2. Favor completeness over common use, practice, and sense
3. Ignore methods other than GET and POST
4. Forget scripts written in other than the C language
5. Disdain client-side scripts

Organization

Every example link in this section invokes a CGI script called "cgi-inp." That code obsesses to show every single microscopic detail of every single parameter string it receives in each invocation, on this wise:

1. HTTP_USER_AGENT
2. argc and all *argv[]
3. REQUEST_METHOD
4. pointer to QUERY_STRING and its length
5. QUERY_STRING and STDIN in four forms:
   1. as delivered char-for-char
   2. as a legible string
   3. as extracted tokens
   4. as the browser renders it
6. CONTENT_LENGTH and total characters on STDIN

So the display's a kind of formatted dump that reveals how individual browsers interpret and realize the specs.

The anchor and link elements

Text links plain and decorated

In this test, I want to see if a link with a background image works differently from a plain-text link. In particular, I want to know if anything about the image shows up on the script side of the interface. Answer: it does not.

Text-link conclusions:

1. REQUEST_METHOD == GET on entry always
2. a parameter needs to be encoded in the link if the CGI script wants to discriminate among links, thus: <a href="cgi-bin/cgi-inp?query string"> link text </a>
3. different from the "button" links on this page, text links deliver argv[] and argc on the interface
4. a parameter string encoded in the link is delivered to the CGI script both in QUERY_STRING and in argv[]

The button element

[ To come, maybe: button description or reference pointer; attributes; traps and trip-ups; styling ]

   char * qs;
   int qlen;
   qs = getenv("QUERY_STRING");
   if (NULL != qs)   /* NULL means there is/was no query string */
   {
     qlen = strlen(qs);
   }

   int ch, clen, n = 0;
   char stdin_line[128];
   char * qs;
   qs = getenv("QUERY_STRING");
   if (NULL != qs)
   {
     clen = atoi(getenv("CONTENT_LENGTH"));
     while ((ch = getchar()) != EOF) 
     {
       if (n < sizeof(stdin_line)-1) {
         stdin_line[n]   = (char)ch;
         stdin_line[n+1] = '\0';
     }
     ++n;
   }   /* clen must equal n here, else error */

<form method="get" 
 action="/cgi-bin/cgi-inp">
 <input name="txt-in:" 
  size="5" type="text" 
  value="get this" />
 <button 
  name="go!" 
  value=
   "get button val" 
  type="submit">
  label<br /> with 
  deprecated
  <font color="red">
   markup</font>
 </button>
</form>
    

<form method="post" 
 action="/cgi-bin/cgi-inp">
 <input name="txt-in:" 
  size="5" type="text" 
  value="post this" />
 <button 
  name="b-name" 
  value=
   "some <i>italic</i> val" 
  type="submit">
  your 
   <b>post-button
   </b> 
   label
 </button>
</form>
    

<form method="get" 
 action="/cgi-bin/cgi-inp">
  <button 
   name="Weeza!" 
   value="Weeza pic" 
   type="submit">
   <img src="broglie.jpg" 
    title="title: Ingres"
    alt="alt: Louise de Broglie" />
  </button>
</form>
    

<form method="post" 
 action="/cgi-bin/cgi-inp">
  <button 
   name="Vermeer" 
   value="button-value" 
   type="submit">
   <img src="pearl-girl.jpg" 
    title="title: VermeerGwP"
    alt="alt: Girl with Turban" />
  </button>
</form>
    

The image attribute

1. W3C, Forms in HTML documents: 17.4 The input element
2. Web Design Group, INPUT - Form Input
3. Alan Flavell, INPUT TYPE=IMAGE for text users?
4. Jukka Korpela, About image buttons in HTML forms and multi-line texts in normal submit buttons

<form 
 method="get" 
 action="/cgi-bin/cgi-inp">
 <input 
  type="image" 
  src="show-me.gif" 
  value="left" 
  name="push-me" />
</form>
    

<form method="get" 
 action=
  "/cgi-bin/cgi-inp?q=GET query;">
 <input 
  type="image" 
  src="show-me.gif" 
  value="left" 
  name="the show-me image" />
</form>
    

<form 
 method="post" 
 action="/cgi-bin/cgi-inp">
 <input name="txt-in:" 
  size="5" type="text" 
  value="post me" />
 <input 
  type="image" 
  src="show-me.gif" 
  value="left" 
  name="image-button" />
</form>
    

<form 
 method="post" 
 action=
  "/cgi-bin/cgi-inp?qs=POST-query;">
 <input name="txt-in:" 
  size="5" type="text" 
  value="post me" />
 <input 
  type="image" 
  src="show-me.gif" 
  value="left" 
  name="show-me-image" />
</form>
    

The submit element

The reset element