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die/warn bug in elsif

by tlm (Prior)
on Jan 12, 2006 at 17:06 UTC ( #522753=perlmeditation: print w/ replies, xml ) Need Help??

This bug is just a minor annoyance, but it can slow down debugging if one is not aware of it, so I thought I'd share. The following snippets illustrates it:

if ( 0 ) { ; } elsif ( die __LINE__ ) { ; }
If you run it as written, the output is:
4 at buggy.pl line 1.
Note that the line reported by die is incorrect. The same bug exists with warn, and with warnings in general. E.g.
use warnings; if ( 0 ) { ; } elsif ( "$_" ) { ; } __END__ Use of uninitialized value in string at buggy.pl line 2.
Curiously, fatal Compile-time errors don't have this problem:
if ( 0 ) { ; } elsif ( 0/0 ) { ; } __END__ Illegal division by zero at buggy.pl line 4
Stuff happens.

Update: the above examples were run with perl 5.8.6. I haven't looked at other versions. YMMV.

Update 2: thanks to merlyn for the clarification on compile-time vs. run-time behavior, which I originally mis-interpreted as a difference in response between non-fatal and fatal and non-fatal errors.

the lowliest monk

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Re: die/warn bug in elsif
by merlyn (Sage) on Jan 12, 2006 at 17:29 UTC
    die and warn are happening at runtime, and from the statement that begins on line 1, so it makes sense. The 0/0 is happening at compile time, where you're compiling line 4, so it also makes sense.

    Well, "makes sense" because I know what's going on behind the scenes. :)

    -- Randal L. Schwartz, Perl hacker
    Be sure to read my standard disclaimer if this is a reply.

      As a way of illustrating this point, here's the OP's first code block after being run through perl -MO=Deparse buggy.pl:

      if (die '4') { (); }

      The B::Deparse module (called by the above command) shows you the Perl equivalent of what the interpreter sees at runtime. This is extremely useful for finding this "compile-time vs. run-time" class of behaviors.

      <-radiant.matrix->
      A collection of thoughts and links from the minds of geeks
      The Code that can be seen is not the true Code
      "In any sufficiently large group of people, most are idiots" - Kaa's Law
Re: die/warn bug in elsif
by Steve_p (Priest) on Jan 19, 2006 at 01:11 UTC

    Congratulations on finding one of the oldest recorded Perl bug!

    The cause is related to how perl interprets Perl code. The easiest explanation comes from Perlbug ticket #1034 by Ilya Zakharevich.

    Diagnostic messages read the line-number-of-currently-executed-chunk-of-code from some memory location. This memory location should be reset when flow of execution winds around. It is reset at the beginning of each statement. Resetting it more often would slow things down, keeping the line-number at each opcode would take a lot of memory.

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