Learning to use the httpd error log to debug CGI programs

This month, we will examine the error log to see what it contains, look through several errors and how they appear in the error log, and finally consider some ways in which we can use the error log to make debugging easier.
Sending MIME Headers First

In all of the above examples, we call die only after having already sent the MIME header to the browser (with the built-in CGI.pm “header” method). Failing to do so can create all sorts of odd problems, and results in the dreaded message:

Server Error. The server encountered an internal
error or misconfiguration and was unable to complete your request.

For example, running the following CGI program:

#!/usr/local/bin/perl5 -w
  use strict;      # Check our syntax strictly
  use diagnostics; # Tell us how to fix mistakes
  use CGI;         # Import the CGI module
  use CGI::Carp;
  # Create an instance of CGI
  my $query = new CGI;
  # Don't do very much
  die "This program crashed on purpose ";
The above program dies upon reaching the last line, and inserts an appropriate message into the httpd error log. What will the user see on his or her browser? In the above example, the user will get the dreaded:
The server encountered an internal error or
misconfiguration and was unable to complete your request.
You can alleviate this problem by sending a MIME header before performing any serious computations:
#!/usr/local/bin/perl5 -w
  use strict;      # Check our syntax strictly
  use diagnostics; # Tell us how to fix mistakes
  use CGI;         # Import the CGI module
  use CGI::Carp;
  # Create an instance of CGI
  my $query = new CGI;
  print $query->header("text/html");
  # Don't do very much
  die "This program crashed on purpose ";
Note: while the documentation for CGI::Carp claims to alleviate this problem, experiments with the version installed on my system (from early September 1996) didn't seem to work quite this way. I thus suggest that you output MIME headers as close to the beginning of your programs as possible, simply to alleviate such problems.

Despite the colorful language that might occur to you when your program produces an error message, good warnings and logs are extremely helpful when writing and debugging code. This is especially true for CGI programs, where much of the work takes place behind the scenes and the only output to the user comes via a web browser. Learn to use the httpd error log effectively, and while your programming problems certainly won't disappear, your mistakes should be easier to identify and correct.

Reuven M. Lerner has been playing with the Web since early 1993, when it seemed more like a fun toy than the world's Next Great Medium. He currently works from his apartment in Haifa, Israel as an independent Internet and Web consultant. When not working on the Web or informally volunteering with school-age children, he enjoys reading (on just about any subject, but especially computers, politics, and philosophy—separately and together), cooking, solving crossword puzzles, and hiking. You can reach him at reuven@the-tech.mit.edu or reuven@netvision.net.il.

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