Beefy Boxes and Bandwidth Generously Provided by pair Networks
The stupid question is the question not asked
 
PerlMonks  

Is adding print lines the only way to go to find computational error in a scrpt?

( #46846=categorized question: print w/ replies, xml ) Need Help??
Contributed by Elias on Dec 15, 2000 at 20:45 UTC
Q&A  > debugging


Answer: Is adding print lines the only way to go to find computational error in a scrpt?
contributed by chipmunk

The only way? Absolutely not. I know that adding print lines is the preferred method of debugging for some people, but in my mind an actual debugger is by far the best tool.

Advantages for print statements:

  1. Simple - don't need to learn anything new.
Advantages for a debugger:

  1. Powerful - stepping, (conditional) breakpoints, actions, watchpoints, etc.
  2. Versatile - examine any value -- or the result of any expression -- at any point.
  3. Informative - dump complicated data structures, view stack traces, etc.
When perl5.6.1 is released, it will include a tutorial on using the debugger, written by Richard Foley.

Editor's note: thanks to davorg and neophyte for the following information (combined into one answer for better reference):
If you have trouble using the built-in debugger (see perldoc perldebug), check out the tutorial Using the Perl Debugger on this site.

Answer: Is adding print lines the only way to go to find computational error in a scrpt?
contributed by hexcoder

Lets assume, you want to use the debugger. Then the problem might be to get the debugger to break after a warning condition happened.

The warnings contain a line number which is fine, but when the line is part of a loop, this is not enough information. We would want a stop with the current context. Only then can we examine the state of the program in the context that produced the problem.

So what to do?
I replace the signal handler for SIGWARN with my own handler that checks for the typical format of a Perl warning. I do that because I am interested only in warnings from the Perl interpreter. If this format has been detected, the code branches into a special path where we can setup the debugger. We want the debugger to stop and to return to the caller, where there warning was caused. So we set a variable that causes the debugger to stop. This will take effect when the signal handler has returned. After that setup stage, the warning message is printed as before the modification.

The signal handler code should go into the debugger initialization file .perldb. Then I do not have to modify the original source code.

This is the content of file .perldb (place it in the current or in the home directory):

sub afterinit { $::SIG{'__WARN__'} = sub { my $warning = shift; if ( $warning =~ m{\s at \s \S+ \s line \s \d+ \. $}xms ) { $DB::single = 1; # debugger stops automatically after # the line that caused the warning. } warn $warning; }; print "sigwarn handler installed!\n"; return; }
Note: This is mostly stolen from my writeup Re: Debugging a program and Re^3: Debugging a program, but I think it fits better here.
Answer: Is adding print lines the only way to go to find computational error in a scrpt?
contributed by cLive ;-)

If your code doesn't crash at shell, but causes a 500 error in browser, use this at the top of your script:

use CGI::Carp qw(fatalsToBrowser);
It doesn't catch everything though.

Please (register and) log in if you wish to add an answer



  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • Outside of code tags, you may need to use entities for some characters:
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.
  • Log In?
    Username:
    Password:

    What's my password?
    Create A New User
    Chatterbox?
    and the web crawler heard nothing...

    How do I use this? | Other CB clients
    Other Users?
    Others drinking their drinks and smoking their pipes about the Monastery: (5)
    As of 2014-08-29 03:24 GMT
    Sections?
    Information?
    Find Nodes?
    Leftovers?
      Voting Booth?

      The best computer themed movie is:











      Results (275 votes), past polls