Re: Tying events to perl warnings and errors

You can trap any warn (or die) yourself like this:

use strict;

$SIG{__WARN__} = \&mywarn;
sub mywarn { print "here!\n @_\n"; }

{ test ('arf', undef, 'dog'); }

sub test {
   @_ == 3 or die "Incorrect number of arguments";
   my ($a, $b, $c) = @_;
   my %hash = ('arf' => {'spot' => {'dog' => 3}});
   print $hash {$a}->{$b}->{$c};
}
[download]

...which prints this output:

here!
 Use of uninitialized value in hash element at test.txt line 23.

here!
 Use of uninitialized value in hash element at test.txt line 23.

here!
 Use of uninitialized value in print at test.txt line 23.
[download]

Hope that helps,
Shendal

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RE: Re: Tying events to perl warnings and errors by Adam (Vicar) on Sep 13, 2000 at 23:37 UTC
Playing with $SIG{__WARN__} and $SIG{__DIE__} is fun, but remember that you can't out-wit death. If die gets called, the script will end, but first it will evaluate/run the expression/subroutine that is $SIG{__DIE__}. It will pass die's arguments in as @_ and will disable the signal (avoiding recursive calls to $SIG{__DIE__}). Good stuff really. Great if you want to say... print a time stamp whenever your script dies... or log warnings to an extra log file. (you trigger __WARN__ when you call warn too.)	[reply]
RE: RE: Re: Tying events to perl warnings and errors by merlyn (Sage) on Sep 13, 2000 at 23:45 UTC
Good idea. See the snippet "Timestamping your warns and dies" which I just added. -- Randal L. Schwartz, Perl hacker	[reply]
RE: RE: RE: Re: Tying events to perl warnings and errors by Adam (Vicar) on Sep 13, 2000 at 23:53 UTC
Thanks.	[reply]
(jcwren) Re: (2) Tying events to perl warnings and errors by jcwren (Prior) on Sep 14, 2000 at 00:17 UTC
The only problem with the $SIG{__WARN__} method is that I don't get the data I want. I'm not a perlguts guy, so maybe someone can point the way for what I mentioned I wanted in the article, which is access to the variable list for the subroutine. What I want to do is upon receipt of the error, dump the variable state for the subroutine out to disk. Timestamping is all well and fine, but it doesn't get me the information that I'm really needing. Any ideas? --Chris e-mail jcwren	[reply]
RE: (jcwren) Re: (2) Tying events to perl warnings and errors by reptile (Monk) on Sep 14, 2000 at 00:23 UTC
Localize the handler with a closure, like this: `sub foo { my ($a, $b, $c) = @_; my %hash = (a => {b => {c => 0}}); # whatever other variables you want to initialize local $SIG{__WARN__} = sub { # dump those variables here }; print $hash{$a}->{$b}->{$c}; # ... }` [download] The variables you want to dump will be in scope of the __WARN__ handler and you can do whatever you like with them. `local $_ = "0A72656B636148206C72655020726568746F6E41207473754A"; while(s/..$//) { print chr(hex($&)) }` [download]	[reply] [d/l] [select]
RE: Re: Tying events to perl warnings and errors by geektron (Curate) on Sep 13, 2000 at 23:37 UTC
this method should work out. i'd also suggest logging to syslog with Sys::Syslog rather than writing out logfiles. that way, any permissions issues are avoided, and no one can remove/replace the log files.	[reply]