RE: PerlHack - Adding new flag into a SV
Title: RE: PerlHack - Adding new flag into a SV
From: Orton, Yves [EMAIL PROTECTED]
I thought you are just automagicaly weaken()ing the referenced
stored in the tied hash??? Something like package
Hash::NoRef; use
Scalar::Util qw(weaken); require Tie::Hash; @ISA = (Tie::StdHash);
sub STORE { $_[0]{$_[1]} = $_[2];
weaken($_[0]{$_[1]}) if (ref
$_[2]);
}
'And that\'s it!';
Am I missing something?
Itll segfault under many circumstances.
yves
Then you should report an error in Scalar::Util.
Do you have an example script that segfaults?
No but I can describe one that would. Imagine a binary tree. We store each node using weakrefs in the structure you describe. Now I search and find an internal node of the tree. The root of the tree falls out of scope, essentially stripping the tree of all of its relatives. Now the hash contains a zillion refs to non existant objects. Access one of those and POOF!
Maybe you were thinking I meant a problem with your code. I meant a problem with the entire approach. IMO weak refs are not the greatest solution to the problems that they are usually used for. Often there are much better approaches that don't involve risking segfaults.
Anyway, :-)
Yves
Re: PerlHack - Adding new flag into a SV
On 27 Apr 2004, at 13:01, Orton, Yves wrote: [snip] No but I can describe one that would. Imagine a binary tree. We store each node using weakrefs in the structure you describe. Now I search and find an internal node of the tree. The root of the tree falls out of scope, essentially stripping the tree of all of its relatives. Now the hash contains a zillion refs to non existant objects. Access one of those and POOF! [snip] Erm. I don't think so. If the thing a weakref points to is collected you get a pointer to undef. To quote from the docs: REF will be turned into a weak reference. This means that it not hold a reference count on the object it references. Also the reference count on that object reaches zero, REF will be set undef. If you're getting something that can cause a segfault then something's wrong somewhere surely. Adrian
Re: PerlHack - Adding new flag into a SV
No, weakrefs cast backref magic onto their target to avoid this
problem. When the reference count drops to zero the magic ensures that
the weakref is set to undef.
See Perl_sv_rvweaken() in sv.c for more.
Sam.
Orton, Yves wrote:
From: Orton, Yves [EMAIL PROTECTED]
I thought you are just automagicaly weaken()ing the referenced
stored in the tied hash??? Something like package
Hash::NoRef; use
Scalar::Util qw(weaken); require Tie::Hash; @ISA = (Tie::StdHash);
sub STORE { $_[0]{$_[1]} = $_[2];
weaken($_[0]{$_[1]}) if (ref
$_[2]);
}
'And that\'s it!';
Am I missing something?
Itll segfault under many circumstances.
yves
Then you should report an error in Scalar::Util.
Do you have an example script that segfaults?
Maybe you were thinking I meant a problem with your code. I meant a
problem with the entire approach. IMO weak refs are not the greatest
solution to the problems that they are usually used for. Often there
are much better approaches that don't involve risking segfaults.
Anyway, :-)
Yves
--
Sam Vilain, sam /\T vilain |T net, PGP key ID: 0x05B52F13
(include my PGP key ID in personal replies to avoid spam filtering)
