Section 5: Troubleshooting a CGI application

Since this subject is quite well covered by other documents, this FAQ has
relatively little to say. 

Eric Wienke has a page "Debugging CGI Scripts 101" at

Tom Christiansen's "Idiot's guide to solving Perl/CGI problems" is a
slightly tongue-in-cheek list of common problems, and how to track
them down.  Much of what Tom covers is not specifically Perl, but
applies equally to CGI programming in other languages. 

Marc Hedlund's CGI FAQ and Thomas Boutell's WWW FAQ also
deal with this subject. 

See "Further Reading" below (if you don't already know where to find these

5.1: Are there some interactive debugging tools and services available?

(1) Several CGI programming libraries offer powerful interactive
    debugging facilities.   These include:

	- for Perl, Lincoln Stein's

	- for Tcl, Don Libes' cgi.tcl

	- for C++, Nick Kew's CGI++

(2) Nathan Neulinger's cgiwrap is another package with debugging aids.

(3) The "mod_cgi" Apache module (new with Apache 1.2) enables you to
capture script output and errors for diagnosis.

See also the next question.

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5.2: I'm having trouble with my headers. What can I do?

For simple cases, examining your response headers "by hand" may suffice:
(1) telnet to the host and port where the server is running - e.g.
        telnet 80
(2) Enter HTTP request.   The most useful for this purpose is usually HEAD; eg
        HEAD /index.html HTTP/1.0
        (optionally other headers)
        (followed by a blank line)
Now you'll get a full HTTP response header back.

For complex cases, such as sending a request with several headers
(as a browser does) or POSTing a form, there several free online
diagnosis services available:
  1. cg-eye
	This combines an offline cgi "linter" with two online services:
	(a) Interactive mode permits you to formulate an HTTP request,
	which is then sent to your server.
	(b) Live mode submits your form, exactly as it gets it from your
	In both cases, it will print a detailed report of the transaction,
	and optionally (if the CGI is producing an HTML page) validate it.
  2. httpeek
	(somewhat like cg-eye interactive mode)
  3. The Header Dumper
	(somewhat like cg-eye live mode, which supersedes it)

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5.3: Why do I get Error 500 ("the script misbehaved", or "Internal Server Error")

Your script must follow the CGI interface, which requires it to print:
(1) One or more Header lines.
(2) A blank line
(3) (optional, but strongly advised) a document body.

This error means it didn't.

The Header lines can include anything that's valid under HTTP, but must
normally include at least one of the three special CGI headers:

Example (a very minimal HTML page via CGI)
Content-Type: text/html			<= Header
					<= Blank Line
<title>HelloWorld</title>Hello World	<= Document Body

A common reason for a script to fail is that it crashed before printing
the header and blank line (or while these are buffered).  Or that it
didn't run at all: you _did_ try it from the commandline as well as
check the file permissions and server configuration, didn't you?

Another possible reason is that it printed something else - like an
error message - in the Headers.   Check error logs, put a dummy header
right at the top (for debugging only), check the "Idiot's Guide",
and use the debug mode of your CGI library.

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5.4: I tried to use (Content-Type|Location|whatever), but it appears in my Browser?

That means you put the line in the wrong place.  It must appear in the
CGI Header, not the document body.  See previous question.

It's also possible that you didn't print a header at all, or had a blank
line or other noise before or in the header, but that the HTTPD has
corrected this error for you (servers which correct your errors may give
rise to the "works on A not on B" phenomenon).   See previous question.

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5.5: How can I run my CGI program 'live' in a debugger?

David S. Jackson offers the following tip:

> I have a very good trick for debugging CGIs written in C/C++ running on
> UNIX. You might want to add it to the debugging section of your CGI faq.
> First, in your CGI code, at it's start, add "sleep(30);". This will cause
> the CGI to do nothing for thiry seconds (you may need to adjust this
> time). Compile the CGI with debuging info ("-g" in gcc) and install the
> CGI as normal. Next, using your web browser, activate the CGI. It will of
> course just sit there doing nothing. While it is 'sleeping', find it's PID
> (ps -a | grep <cgi name>). Load your debugger and attach to that PID
> ("attach <pid>" in gdb). You will also need to tell it where to find the
> symbol definitions ("symbol-file <cgi>" in gdb). Then set a break point
> after the invocation of the sleep function and you are ready to debug. Do
> be aware that your browser will eventually timeout if it doesn't recieve
> anything.

(Anyone know similar tricks for scripting languages)?

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