Devel::FindRef - where is that reference to my variable hiding?
use Devel::FindRef; print Devel::FindRef::track \$some_variable;
Tracking down reference problems (e.g. you expect some object to be destroyed, but there are still references to it that keep it alive) can be very hard. Fortunately, perl keeps track of all its values, so tracking references "backwards" is usually possible.
The track function can help track down some of those references back to the variables containing them.
track
For example, for this fragment:
package Test; use Devel::FindRef; use Scalar::Util; our $var = "hi\n"; my $global_my = \$var; our %global_hash = (ukukey => \$var); our $global_hashref = { ukukey2 => \$var }; sub testsub { my $testsub_local = $global_hashref; print Devel::FindRef::track \$var; } my $closure = sub { my $closure_var = \$_[0]; Scalar::Util::weaken (my $weak_ref = \$var); testsub; }; $closure->($var);
The output is as follows (or similar to this, in case I forget to update the manpage after some changes):
SCALAR(0x7cc888) [refcount 6] is +- referenced by REF(0x8abcc8) [refcount 1], which is | the lexical '$closure_var' in CODE(0x8abc50) [refcount 4], which is | +- the closure created at tst:18. | +- referenced by REF(0x7d3c58) [refcount 1], which is | | the lexical '$closure' in CODE(0x7ae530) [refcount 2], which is | | +- the containing scope for CODE(0x8ab430) [refcount 3], which is | | | the global &Test::testsub. | | +- the main body of the program. | +- the lexical '&' in CODE(0x7ae530) [refcount 2], which was seen before. +- referenced by REF(0x7cc7c8) [refcount 1], which is | the lexical '$global_my' in CODE(0x7ae530) [refcount 2], which was seen before. +- the global $Test::var. +- referenced by REF(0x7cc558) [refcount 1], which is | the member 'ukukey2' of HASH(0x7ae140) [refcount 2], which is | +- referenced by REF(0x8abad0) [refcount 1], which is | | the lexical '$testsub_local' in CODE(0x8ab430) [refcount 3], which was seen before. | +- referenced by REF(0x8ab4f0) [refcount 1], which is | the global $Test::global_hashref. +- referenced by REF(0x7ae518) [refcount 1], which is | the member 'ukukey' of HASH(0x7d3bb0) [refcount 1], which is | the global %Test::global_hash. +- referenced by REF(0x7ae2f0) [refcount 1], which is a temporary on the stack.
It is a bit convoluted to read, but basically it says that the value stored in $var is referenced by:
$var
$closure_var
$global_my
$Test::var
ukukey2
$testsub_local
Test::testsub
$referenced by Test::hash2
ukukey
%Test::hash
\$var
And all these account for six reference counts.
None.
Track the perl value pointed to by $ref up to a depth of $depth and return a descriptive string. $ref can point at any perl value, be it anonymous sub, hash, array, scalar etc.
$ref
$depth
This is the function you most likely want to use when tracking down references.
Return arrayrefs that contain [$message, $ref] pairs. The message describes what kind of reference was found and the $ref is the reference itself, which can be omitted if find decided to end the search. The returned references are all weak references.
find
The track function uses this to find references to the value you are interested in and recurses on the returned references.
Sometimes you know (from debugging output) the address of a perl value you are interested in (e.g. HASH(0x176ff70)). This function can be used to turn the address into a reference to that value. It is quite safe to call on valid addresses, but extremely dangerous to call on invalid ones. No checks whatsoever will be done, so don't use this unless you really know the value is the address of a valid perl value.
HASH(0x176ff70)
# we know that HASH(0x176ff70) exists, so turn it into a hashref: my $ref_to_hash = Devel::FindRef::ptr2ref 0x176ff70;
The opposite of ptr2ref, above: returns the internal address of the value pointed to by the passed reference. This function is safe to call on anything, and returns the same value that a normal reference would if used in a numeric context.
ptr2ref
You can set the environment variable PERL_DEVEL_FINDREF_DEPTH to an integer to override the default depth in track. If a call explicitly specifies a depth, it is not overridden.
PERL_DEVEL_FINDREF_DEPTH
Marc Lehmann <pcg@goof.com>.
Copyright (C) 2007, 2008, 2009, 2013 by Marc Lehmann.
This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself, either Perl version 5.8.8 or, at your option, any later version of Perl 5 you may have available.
To install Devel::FindRef, copy and paste the appropriate command in to your terminal.
cpanm
cpanm Devel::FindRef
CPAN shell
perl -MCPAN -e shell install Devel::FindRef
For more information on module installation, please visit the detailed CPAN module installation guide.