Things obvious to others, but not to you

Table of Contents:

• Coverage
• Where do the warnings/errors go?
• Setting environment variables for scripts called from CGI.

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This document describes ``special'' traps you may encounter when running your plain CGIs under Apache::Registry and Apache::PerlRun.

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Your CGI does not work and you want to see what the problem is. The best idea is to check out any errors that the server may be reporting. Where I can find these errors?

Generally all errors are logged into an error_log file. The exact file location and name are defined in the http.conf file. Look for the ErrorLog parameter. My httpd.conf says:

  ErrorLog var/logs/error_log

Hey, where is the beginning of the path? There is another Apache parameter called ServerRoot. Every time apache sees a value of the parameter with no absolute path (e.g /tmp/my.txt) but with relative path (e.g my.txt) it prepends the value of the ServerRoot to this value. I have:

  ServerRoot /usr/local/apache

So I will look for error_log file at /usr/local/apache/var/logs/error_log. Of course you can also use an absolute path to define the file's location at the file system.

<META>: is this 100% correct?

But there are cases when errors don't go to the error_log file. For example some errors are being printed to the console (tty) you have executed the httpd from (unless you redirected the httpd's stderr flow). This happens when the server didn't open the error_log file for writing yet.

For example, if you have mistakenly entered a non-existent directory path in your ErrorLog directive, the error message will be printed on the controlling tty. Or, if the error happens when server executes PerlRequire or PerlModule directive you might see the errors here also.

You are probably wonder where all the errors go when you are running the server in single mode (httpd -X). They go to the console. That is because when running in the single mode there is no parent httpd process to perform all the logging. It includes all the status messages that generally show up in the error_log file.

</META>

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Perl uses sh() for its iteractions for system() and open() calls. So when you want to set a temporary variable when you call a script from your CGI you do:

 open UTIL, "USER=stas ; script.pl | " or die "...: $!\n";

or

  system "USER=stas ; script.pl";

This is useful for example if you need to invoke a script that uses CGI.pm from within a mod_perl script. We are tricking the perl script to think it's a simple CGI, which is not running under mod_perl.

  open(PUBLISH, "GATEWAY_INTERFACE=CGI/1.1 ; script.cgi
       \"param1=value1&param2=value2\" |") or die "...: $!\n";

Make sure, that the parameters you pass are shell safe (All ``unsafe'' characters like single-tick should be properly escaped).

However you are fork-ing to run a Perl script, so you have thrown the so hardly gained performance out the window. Whatever script.cgi is now, it should be moved to a module with a subroutine you can call directly from your script, to avoid the fork.

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Written by Stas Bekman. Last Modified at 12/13/1999		Use of the Camel for Perl is a trademark of O'Reilly & Associates, and is used by permission.