p5pRT
commented
20 years ago From @jlokier
Created by @jlokier
The following code causes the error message "Attempt to free unreferenced scalar at test.pl line 1.":
{ local $handle = new (); undef; } sub TIEHANDLE { $_[1] } sub DESTROY { close $handle; } sub new { my $iosym = \*{"FOO"}; delete ${"main::"}{"FOO"}; tie *$iosym\, __PACKAGE__\, (bless []\, __PACKAGE__); *$iosym{IO}; }
Although that test looks contrived and silly\, it is the result of pruning a real (and far too large to submit) program down to the essential circumstances which trigger this bug.
The general problem appears to be that it the destructor\, DESTROY\, calls anything which would re-enter the tied handle object\, that causes the error. That can be problematic when something like STDOUT is tied\, and DESTROY calls functions which try to print. In theory\, the methods of the tied handle (in the more complex program) will detect reentrancy\, but they don't get the chance.
The obvious workaround is to not call anything much from DESTROY. But it shouldn't crash Perl or trash its heap.
When the real program runs\, there are several messages like that\, plus some more:
Attempt to free unreferenced scalar at httpd.pl line 516. Attempt to free non-existent shared string 'content-length' during global destruction. Unbalanced string table refcount: (1) for "content-length" during global destruction.
A short while after comes "Segmentation fault".
Here is a test program which immediately triggers a segmentation fault:
{ local $handle = new (); undef; } sub TIEHANDLE { $_[1] } sub CLOSE { close $handle; } sub DESTROY { close $handle; } sub new { my $iosym = \*{"FOO"}; delete ${"main::"}{"FOO"}; tie *$iosym\, __PACKAGE__\, (bless []\, __PACKAGE__); return *$iosym{IO}; }
Thanks\, -- Jamie
Migrated from rt.perl.org#27638 (status was 'resolved')
Searchable as RT27638$