How can I get a call stack listing in Perl? - perl

Question

Is there a way I can access (for printout) a list of sub + module to arbitrary depth of sub-calls preceding a current position in a Perl script?
I need to make changes to some Perl modules (.pm's). The workflow is initiated from a web-page thru a cgi-script, passing input through several modules/objects ending in the module where I need to use the data. Somewhere along the line the data got changed and I need to find out where.

Answer 1

You can use Devel::StackTrace.
use Devel::StackTrace;
my $trace = Devel::StackTrace->new;
print $trace->as_string; # like carp
It behaves like Carp's trace, but you can get more control over the frames.
The one problem is that references are stringified and if a referenced value changes, you won't see it. However, you could whip up some stuff with PadWalker to print out the full data (it would be huge, though).

Answer 2

This code works without any additional modules.
Just include it where needed.
my $i = 1;
print STDERR "Stack Trace:\n";
while ( (my #call_details = (caller($i++))) ){
print STDERR $call_details[1].":".$call_details[2]." in function ".$call_details[3]."\n";
}

Answer 3

Carp::longmess will do what you want, and it's standard.
use Carp qw<longmess>;
use Data::Dumper;
sub A { &B; }
sub B { &C; }
sub C { &D; }
sub D { &E; }
sub E {
# Uncomment below if you want to see the place in E
# local $Carp::CarpLevel = -1;
my $mess = longmess();
print Dumper( $mess );
}
A();
__END__
$VAR1 = ' at - line 14
main::D called at - line 12
main::C called at - line 10
main::B called at - line 8
main::A() called at - line 23
';
I came up with this sub (Now with optional blessin' action!)
my $stack_frame_re = qr{
^ # Beginning of line
\s* # Any number of spaces
( [\w:]+ ) # Package + sub
(?: [(] ( .*? ) [)] )? # Anything between two parens
\s+ # At least one space
called [ ] at # "called" followed by a single space
\s+ ( \S+ ) \s+ # Spaces surrounding at least one non-space character
line [ ] (\d+) # line designation
}x;
sub get_stack {
my #lines = split /\s*\n\s*/, longmess;
shift #lines;
my #frames
= map {
my ( $sub_name, $arg_str, $file, $line ) = /$stack_frame_re/;
my $ref = { sub_name => $sub_name
, args => [ map { s/^'//; s/'$//; $_ }
split /\s*,\s*/, $arg_str
]
, file => $file
, line => $line
};
bless $ref, $_[0] if #_;
$ref
}
#lines
;
return wantarray ? #frames : \#frames;
}

Answer 4

caller can do that, though you may want even more information than that.

Answer 5

There's also Carp::confess and Carp::cluck.

Answer 6

In case you can't use (or would like to avoid) non-core modules, here's a simple subroutine I came up with:
#!/usr/bin/perl
use strict;
use warnings;
sub printstack {
my ($package, $filename, $line, $subroutine, $hasargs, $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash);
my $i = 1;
my #r;
while (#r = caller($i)) {
($package, $filename, $line, $subroutine, $hasargs, $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash) = #r;
print "$filename:$line $subroutine\n";
$i++;
}
}
sub i {
printstack();
}
sub h {
i;
}
sub g {
h;
}
g;
It produces output like as follows:
/root/_/1.pl:21 main::i
/root/_/1.pl:25 main::h
/root/_/1.pl:28 main::g
Or a oneliner:
for (my $i = 0; my #r = caller($i); $i++) { print "$r[1]:$r[2] $r[3]\n"; }
You can find documentation on caller here.

Answer 7

One that is more pretty: Devel::PrettyTrace
use Devel::PrettyTrace;
bt;

Answer 8

Moving my comment to an answer:
Install Devel::Confess the right way
cpanm Devel::Confess
Run with
perl -d:Confess myscript.pl
On errors, this will show the whole call stack list.