Segmentation fault

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

Hello Monks,

I'm wondering about the following code snippet that triggers a segmentation fault if the script exit.

my $i = 30000;
my %h = (new => {});
my $r = \%h;
for (0..$i) {
   print $_, " ";
   $r = $r->{new};
   $r->{new} = {};
}
print "end\n";
[download]

Output:

...29998 29999 30000 end
Segmentation fault (core dumped)
[download]

I test it with different iterations and from 24.000 the segfault is produced definitely. If I try to undef %h the segfault is produced immediate.

my $i = 30000;
my %h = (new => {});
my $r = \%h;
for (0..$i) {
   print $_, " ";
   $r = $r->{new};
   $r->{new} = {};
}
undef %h;
print "end\n";
[download]

Output: 29837 29838 29839 29840 Segmentation fault (core dumped)
My question is now why that happend? Do you have an answer?

Regards,
Jonny

Edit: g0n - changed pre tags to code tags

Comment on Segmentation fault Select or Download Code

Replies are listed 'Best First'.

Re: Segmentation fault
by Joost (Canon) on Jun 25, 2007 at 20:11 UTC

Increase your stack size to whatever is necessary.

On unix w/ bash:

ulimit -s 30000
perl test.pl
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"What should it profit a man, if he should win a flame war, yet lose his cool?"

[reply]
[d/l]

Re^2: Segmentation fault

by jettero (Monsignor) on Jun 25, 2007 at 20:18 UTC

$SIG{__ATROOSS__} = sub {
   die "about to run out of stack space!!"
};
[download]

This and the regexp one have always bugged the crap out of me.

-Paul

[reply]
[d/l]

Re^3: Segmentation fault

by Joost (Canon) on Jun 25, 2007 at 20:21 UTC

your link

"What should it profit a man, if he should win a flame war, yet lose his cool?"

[reply]

Re^4: Segmentation fault

by dave_the_m (Monsignor) on Jun 25, 2007 at 20:40 UTC

Re^3: Segmentation fault

by jasonk (Parson) on Jun 26, 2007 at 12:56 UTC

The problem is there is no portable way to determine that you are about to run out of stack space...

We're not surrounded, we're in a target-rich environment!

[reply]

Re^4: Segmentation fault

by jettero (Monsignor) on Jun 26, 2007 at 14:09 UTC

Re^2: Segmentation fault

by bloonix (Monk) on Jun 25, 2007 at 20:30 UTC

Hello Joost, big thanks for your fast answer. Yes, if I increase the ulimit the script runs very fine without a stack overflow. Do you know also why the segfault is produced first if the script die and not immediate?

[reply]

Re^3: Segmentation fault

by Joost (Canon) on Jun 25, 2007 at 20:47 UTC

Do you know also why the segfault is produced first if the script die and not immediate?

{
    my $i = 60000;
    my %h = (new => {});
    my $r = \%h;
    for (0..$i) {
       print $_, " ";
       $r = $r->{new};
       $r->{new} = {};
    }
    print "end 1\n";
} # <-- hash goes out of scope; segfault if stacksize is too low
print "end 2\n";
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If you're really careful, you can clean up the linked list before it goes out of scope and prevent this error:

{
    my $i = 60000;
    my %h = (new => {});
    my $r = \%h;
    my $end = $r;   # end of the list
    for (0..$i) {
       print $_, " ";
       $r = $r->{new};
       $r->{new} = {};
       $r->{new}->{old} = $r;   # keep back-references
       $end = $r->{new};
    }
    print "end 1\n";
    while ($end) {  # delete the list from the end
        delete $end->{new};
        $end = delete $end->{old};
    }
}
print "end 2\n";
[download]

"What should it profit a man, if he should win a flame war, yet lose his cool?"

[reply]
[d/l]
[select]

Re^4: Segmentation fault

by martin (Friar) on Jun 25, 2007 at 22:11 UTC

Re^5: Segmentation fault

by Joost (Canon) on Jun 25, 2007 at 22:29 UTC


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