Debugging Perl

 How to Debug Perl Library and Scripts?

 

1. Changing @INC - where Perl loads its modules (http://www.wellho.net/mouth/588_Changing-INC-where-Perl-loads-its-modules.html) 

The @INC array is a list of directories Perl searches when attempting to load modules. 

Where does Perl load modules from in its use and require statements? It loads them from directorys in a special list called @INC,from files with a .pm extension in those directorys. 
 
When Perl's installed,@INC is set to a list of directorys that includes generic locations for its standard modules,some release specific directories,and "." the current directory,which are checked in order each time you do a  or . 
 
Some ways to modify @INC 

• ** You can add to the list in @INC by using the -I command line option:

perl -I /Users/grahamellis/jan06 i2 
says "run the perl program i2,additionally checking the jan06 directory for modules" 

• ** You can add to the list within your program by doing so in a BEGIN block prior to the  statements:

BEGIN { 
push @INC,"/Users/grahamellis/jan06"; 
} 
use demo; 
print "hello world"; 

Rather curIoUsly,  calls are run at compile time not at run time ... but then so are BEGIN blocks ... so you put your manipulation of @INC into one of those to get it to happen early enough. 

• ** You can add to the beginning of the list by setting the PERL5LIB environment variable prior to running your program:

export PERL5LIB=/Users/grahamellis/jan06 

and you can use a colon separated list for that if you want to pre-pend more than one directory.

• **There is also the "use lib" pragma - this is probably the most common approach. And $PERL5LIB can also be called just $PERLLIB.

2. Debugging Perl http://ralphie.perlmonk.org/mosix/debug.html

To check the Syntax of your scripts,type "perl -c scriptname.pl " . It is also worthwhile to add the -w switch ("perl -cw ..."),to turn on warning mode. If you were wise enough to "use strict" in the body of the code,the Syntax check will also specify inappropriate scalar scoping,which goes a little beyond the normal syntactical problems that are identified. Once your script will pass through the Syntax check with a "Syntax ok" message,you will have a legal perl script from the standpoint that all of the statements are legal perl statements. That doesn't mean that the manner in which any given statement is expressed will return the result that you are looking for,that the functions that are used in the code are the right ones for the job,or that the overall structure of the script does anything like what you think it supposed to do. It just means that all of the perl i's are dotted and t's are crossed. (On the other hand,this is not such a minor thing,either.)

3. perldebug (http://perldoc.perl.org/perldebug.html)

Calling the debugger

There are several ways to call the debugger:

• perl -d program_name

On the given program identified by program_name .

• perl -d -e 0

Interactively supply an arbitrary expression using -e .

• perl -d:Ptkdb program_name

Debug a given program via the Devel::Ptkdb GUI.

• perl -dt threaded_program_name

Debug a given program using threads (experimental).

4. Summary of Subprocess Operations

Table 14.1: Summary of Subprocess Operations
Operation	Standard Input	Standard Output	Standard Error	Waited for?
system()	Inherited from program	Inherited from program	Inherited from program	Yes
Backquoted string	Inherited from program	Captured as string value	Inherited from program	Yes
open() command as filehandle for output	Connected to filehandle	Inherited from program	Inherited from program	Only at time of close()
open() command as filehandle for input	Inherited from program	Connected to filehandle	Inherited from program	Only at time of close()
fork, exec, wait, waitpid	User selected	User selected	User selected	User selected

 

Above table is from the book Learning Perl.