Apache: The Definitive Guide, 3rd Edition

Global Variables

The biggest single “gotcha” for scripts running under Apache::Registry is caused by global variables. The mod_cgi environment is rather kind to the slack programmer. Your scripts, which tend to be short and simple, get loaded, run, and then thrown away. Perl rather considerately initializes all variables to undef at startup, so one tends to forget about the dangers they represent.

Unhappily, under mod_perl and Apache::Registry, scripts effectively run as subroutines. Global variables get initialized at startup as usual, but not again, so if you don’t explicitly initialize them at each call, they will carry forward whatever value they had after the last call. What makes these bugs more puzzling is that as the Apache child processes start, each one of them has its variables set to 0. The errant behavior will not begin to show until a child process is used a second time — and maybe not even then.

There are several lines of attack:

• Do away with every global variable that isn’t absolutely necessary

• Make sure that every global variable that survives is initialized

• Put your code into modules as subroutines and call it from the main script — for some reason global variables in the module will be initialized

To illustrate this tiresome behavior we created a new directory /usr/www/APACHE3/APACHE3/site.mod_perl/mod_perl and copied everything across into it from.../mod_cgi. The startup file go was now:

httpd.perl -d /usr/www/APACHE3/APACHE3/site.mod_perl/mod_perl

The Config ...