Strange warn behavior in loop

deibyz has asked for the wisdom of the Perl Monks concerning the following question:

Hi monks,

I've noticed a weird behavior of a very simple loop with a warn statement.

#!/usr/bin/perl
use strict;
use warnings;

for my $f (qw(a b)){
        print $f;
        warn ".";
}
[download]

This code runs as expected an produces thew following output:

. at test.pl line 7.
. at test.pl line 7.
ab
[download]

However, if I don't put an argument to warn:

#!/usr/bin/perl
use strict;
use warnings;

for my $f (qw(a b)){
        print $f;
        warn ;
}
[download]

I get the following output:

Warning: something's wrong at test.pl line 7.
Warning: something's wrong at test.pl line 7.
a1
[download]

Notice the "1" instead of "b".

Any ideas about this behavior?

My perl version:

$ perl -v

This is perl, v5.8.8 built for i486-linux-gnu-thread-multi
[download]

Comment on Strange warn behavior in loop Select or Download Code

Replies are listed 'Best First'.

Re: Strange warn behavior in loop
by wazoox (Prior) on Jul 16, 2007 at 12:09 UTC

I've tried on some of the systems I have at hand, and it looks like a v5.8.8 bug :

System	Perl version	correct
Linux Slackware 11.0	v5.8.8 i486-linux	No
Linux Debian Sarge	v5.8.4 i386-linux-thread-multi	Yes
Linux Debian Etch	v5.8.8 i486-linux-thread-multi	No
Mac OS X 10.4	v5.8.6 darwin-thread-multi-2level	Yes
IRIX 6.5.22	v5.6.1 irix-n32	Yes

[reply]

Re^2: Strange warn behavior in loop

by blazar (Canon) on Jul 17, 2007 at 10:21 UTC

I've tried on some of the systems I have at hand, and it looks like a v5.8.8 bug :

Confirmed under the Redmondish side of the world, too:

C:\temp>perl -e "for (2,2) {print;warn}"
Warning: something's wrong at -e line 1.
Warning: something's wrong at -e line 1.
21
[download]

Note that I wanted to further simplify the test code, but the bug doesn't seem to show up with a c<for> statement modifier:

C:\temp>perl -e "print,warn for 2,2"
Warning: something's wrong at -e line 1.
Warning: something's wrong at -e line 1.
22
[download]

BTW: what if $@ is set?

C:\temp>perl -e "$@='all your base are belong to us';for (2,2) {print;
+warn}"
all your base are belong to us  ...caught at -e line 1.
all your base are belong to us  ...caught       ...caught at -e line 1
+.
21
[download]

Nope, the bug is still there...

[reply]
[d/l]
[select]

Re: Strange warn behavior in loop
by moritz (Cardinal) on Jul 16, 2007 at 11:21 UTC

a1

$ perl -v
This is perl, v5.8.8 built for i486-linux-gnu-thread-multi
[download]

But it seems to be fixed in 5.9.5 (correctly produces ab):

This is perl, v5.9.5 DEVEL31566 built for i686-linux

Perl 6 in German

[reply]
[d/l]
[select]

Re: Strange warn behavior in loop
by rpanman (Scribe) on Jul 16, 2007 at 11:20 UTC

Warning: something's wrong at test.pl line 7.
Warning: something's wrong at test.pl line 7.
ab
[download]

There has been a fair amount of refactoring of the C source code, partly to make it tidier and more maintainable. The resulting object code and the perl binary may well be smaller than 5.8.7, in particular due to a change contributed by Dave Mitchell which reworked the warnings code to be significantly smaller. Apart from being smaller and possibly faster, there should be no user-detectable changes.

[reply]
[d/l]

Re: Strange warn behavior in loop
by johngg (Canon) on Jul 16, 2007 at 11:06 UTC

Warning: something's wrong at spw626811 line 7.
Warning: something's wrong at spw626811 line 7.
ab
[download]

I can't think what could be causing that.

Cheers,

JohnGG

[reply]
[d/l]

Re: Strange warn behavior in loop
by betterworld (Curate) on Jul 16, 2007 at 13:40 UTC

tinita

this bug

In both cases the loop variable is set to "1" by some mysterious force, and in both cases warn must be called without arguments to trigger the bug.

Update: I've added the code from that bug report under the readmore tags.