Automatic stack tracebacks in warnings?

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

Between "omniscient debugging" for Perl and some really wierd "uninitialised" warnings in a largish script, I'm thinking that one possible solution to my problems is to find a way to get tracebacks to warnings, even those generated by the compiler. As I'm writing this up, it dawns on me that this is exactly what $SIG{__WARN__} is for. Conveniently, perlvar has pretty much exactly this example. So, I try it out. First, the module, 'T.pm':

package T;

use strict;

my $oldwarn;
sub import
{
    $oldwarn = $SIG{__WARN__};
    $SIG{__WARN__} = \&callstack;
}

sub callstack
{
    &$oldwarn if $oldwarn;
    require Carp;
    Carp::confess(@_);
}

1;
[download]

And then the calling code:

#! perl -w

use strict;
use diagnostics;

use T;

sub foo
{
    my $x;
    print $x;
}

foo();

print "\n\ndone.\n\n";
[download]

What I expect is:

$ perl a.pl
Use of uninitialized value in print at a.pl line 11 (#1)
    (W uninitialized) An undefined value was used as if it were alread
+y
    defined.  It was interpreted as a "" or a 0, but maybe it was a mi
+stake.
    To suppress this warning assign a defined value to your variables.

    To help you figure out what was undefined, perl tells you what ope
+ration
    you used the undefined value in.  Note, however, that perl optimiz
+es your
    program and the operation displayed in the warning may not necessa
+rily
    appear literally in your program.  For example, "that $foo" is
    usually optimized into "that " . $foo, and the warning will refer 
+to
    the concatenation (.) operator, even though there is no . in your
    program.

Use of uninitialized value in print at a.pl line 11.
 at T.pm line 16
        T::callstack('Use of uninitialized value in print at a.pl line
+ 11.\x{a}') called at a.pl line 11
        main::foo() called at a.pl line 14 (#1)

done.
[download]

(I'd rather get rid of T::callstack here somehow, but I can live with that for the purposes of finding the context in which my warnings are coming up.) What I actually get is this:

$ perl a.pl
Use of uninitialized value in print at a.pl line 11 (#1)
    (W uninitialized) An undefined value was used as if it were alread
+y
    defined.  It was interpreted as a "" or a 0, but maybe it was a mi
+stake.
    To suppress this warning assign a defined value to your variables.

    To help you figure out what was undefined, perl tells you what ope
+ration
    you used the undefined value in.  Note, however, that perl optimiz
+es your
    program and the operation displayed in the warning may not necessa
+rily
    appear literally in your program.  For example, "that $foo" is
    usually optimized into "that " . $foo, and the warning will refer 
+to
    the concatenation (.) operator, even though there is no . in your
    program.

Use of uninitialized value in print at a.pl line 11.
 at T.pm line 16
        T::callstack('Use of uninitialized value in print at a.pl line
+ 11.\x{a}') called at a.pl line 11
        main::foo() called at a.pl line 14 (#1)
Uncaught exception from user code:
        Use of uninitialized value in print at a.pl line 11.
 at T.pm line 16
        T::callstack('Use of uninitialized value in print at a.pl line
+ 11.\x{a}') called at a.pl line 11
        main::foo() called at a.pl line 14
 at T.pm line 16
        T::callstack('Use of uninitialized value in print at a.pl line
+ 11.\x{a}') called at a.pl line 11
        main::foo() called at a.pl line 14
[download]

Note that the "done" line is missing. I'm not sure what is causing the "uncaught exception" printout, which I definitely would like to avoid - I'd like the behaviour of the perl code to be otherwise the same, and if I remove the "use T" line from the original script, the warning is triggered, and then the script continues to completion, so I'd like to allow the script to complete even with T.

(No, these are not the final package/file names ;->)

Any direction on this would be appreciated.

Comment on Automatic stack tracebacks in warnings? Select or Download Code

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Re: Automatic stack tracebacks in warnings? by ikegami (Patriarch) on Jul 05, 2005 at 17:23 UTC
The following will get rid of T:: callstack. `sub callstack { &$oldwarn if $oldwarn; require Carp; goto &Carp::confess; # was: Carp::confess(@_); }` [download] Are you sure you want to `confess` (die) and not just `cluck` (warn)?	[reply] [d/l] [select]
Re^2: Automatic stack tracebacks in warnings? by Tanktalus (Canon) on Jul 05, 2005 at 18:10 UTC
Well, duh, yeah, I want to cluck. Woops. However, it still gives me a wierd behaviour: $ perl a.pl Use of uninitialized value in print at a.pl line 11 (#1) (W uninitialized) An undefined value was used as if it were alread +y defined. It was interpreted as a "" or a 0, but maybe it was a mi +stake. To suppress this warning assign a defined value to your variables. To help you figure out what was undefined, perl tells you what ope +ration you used the undefined value in. Note, however, that perl optimiz +es your program and the operation displayed in the warning may not necessa +rily appear literally in your program. For example, "that $foo" is usually optimized into "that " . $foo, and the warning will refer +to the concatenation (.) operator, even though there is no . in your program. Use of uninitialized value in print at a.pl line 11. at a.pl line 11 main::foo() called at a.pl line 14 (#1) Use of uninitialized value in print at a.pl line 11. at a.pl line 11 main::foo() called at a.pl line 14 at a.pl line 11 main::foo() called at a.pl line 14 done. [download] For some reason, I'm getting that warning twice. Note also that I had to add a few extra lines to the callstack function: `sub callstack { &$oldwarn if $oldwarn; require Carp; delete $SIG{__WARN__}; goto &Carp::cluck; $SIG{__WARN__} = \&callstack; }` [download] For some reason, Carp::cluck calls warn, so disabling the warn handler gets rid of the infinite recursion. A bit more digging changes the above to: `sub callstack { &$oldwarn if $oldwarn; require Carp; print STDERR Carp::longmess(@_); }` [download] which solves the problem - the warning message only shows on the screen once.	[reply] [d/l] [select]
Re: Automatic stack tracebacks in warnings? by brian_d_foy (Abbot) on Jul 05, 2005 at 18:04 UTC
Does the cluck() function in Carp do what you want? It comes with perl. -- brian d foy <brian@stonehenge.com>	[reply]
Re^2: Automatic stack tracebacks in warnings? by ikegami (Patriarch) on Jul 05, 2005 at 18:09 UTC
He wants to "find a way to get tracebacks to warnings, even those generated by the compiler", so replacing `warn`s with `cluck`s won't do.	[reply] [d/l] [select]
Re^3: Automatic stack tracebacks in warnings? by brian_d_foy (Abbot) on Jul 05, 2005 at 19:40 UTC
You don't have to replace every instance of warn() with cluck(). You just have to make perl call cluck() when it calls warn(). The implementation of cluck() is simply `package Carp; sub cluck { warn longmess @_ }` [download] Make your __WARN__ handler do that and you're all set. -- brian d foy <brian@stonehenge.com>	[reply] [d/l]
Re^2: Automatic stack tracebacks in warnings? by Tanktalus (Canon) on Jul 05, 2005 at 18:13 UTC
No. What Carp::cluck does is warns with trace only when you call it. I'm trying to catch the ones that happen from inside the perl VM - such as "use of uninitialized" warnings. What I'd like to do is basically completely mask the warn built-in function with Carp::cluck (assuming that the perl VM calls warn, which is likely to be the case), but the best I can do now is fake it with the __WARN__ handler. That said, ikegami pointed me along the right path, and I now got this working.	[reply]