I'm using Devel::Gladiator to get a list of all refs in use by Perl, which I want to (among other things) calculate the allocated size of with Devel::Size. This is the only way I could find to profile memory usage of unknown variables with reasonable accuracy without requiring a custom recompile of Perl to enable Perl malloc.

It almost works. Unfortunately, Devel::Size is segfaulting trying to calculate the size() of a handful of SCALAR refs (and so far no other kind of ref). So far the affected deref'd scalars contain variable names used/exported by various core pragmas, specifically, one of (no particular order):

$VERSION $XS_VERSION %DeadBits %Bits %Offsets %EXPORT_TAGS 
$Verbose $ExportLevel $Debug %Cache @EXPORT_OK @ISA

[download]

Here's a short script that demonstrates the issue. I've tried this on 5.14, 5.17.x, and 5.18.1, and crashes every time I run it.

use strict;
use warnings;
use Devel::Gladiator qw/walk_arena/;
use Devel::Size qw/size/;

my $arena = walk_arena();

my $i = 0;
$| = 1;
for (@$arena) {
    next unless ref eq 'SCALAR';
    next if not defined $$_;
    printf '%4d : %16s : "%s" :', $i++, $_, $$_;
    print size($_);
    print "\n";
}
@$arena = ();

[download]

I get output like:

   0 : SCALAR(0xf8a4f8) : "$" :56
   1 : SCALAR(0xf8a510) : "::" :128
.
.
  21 : SCALAR(0xf3bbb0) : "1.03" :56
  22 : SCALAR(0xf3bc10) : "Devel::Size" :85
  23 : SCALAR(0xf3bc28) : "$VERSION" :Segmentation fault (core dumped)

[download]

The crash occurs on size($_) (or total_size($arena)), not when printing $$_. I'm not surprised to have issues working with every SV in existence. My best workaround so far has been to use length() to put a lower bound on the size of scalars, instead of size(). This obviously can be wrong by an arbitrarily large factor, as in $ooo = 'o' x 2**20; $ooo = '';, which leaves $ooo with a length of 0, and a size over 1 MB. I could also skip scalars whose contents match any of the above strings, but that's thousands of string comparisons, and without knowing why these specific refs cause a core dump, this seems like the kind of hack that is likely to blow up in my face if another ref with the same problem comes along later.

So, I ask, can anyone shed any light on what's going on, here? I'm more than happy to file a bug if I'm not doing something stupid, but I'd be even happier to learn a stable way to iterate all refs in existence and calculate their sizes, or at least safely skip the few that are going to be problematic. Hopefully this all makes sense.

In reply to Segfault with Devel::Size by wanna_code_perl

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.