Using persistent private variables in xs

[Jeremy White]

Given this code:

{
my $variable;
sub mysub {
# ... accessing $variable
}
}

from:

http://www.unix.org.ua/orelly/perl/cookbook/ch10_04.htm

Is there a way to get hold of the SV for $variable and manipulate it via XS?

I want to be able to write the contents of $variable in XS, then call mysub
via XS and then read the contents of $variable afterwards ($variable needs
to remain a closure as it is shared with other subs in the same block - the
block is evaled into existence during runtime).

All my attempts so far have failed, so I'm either being stupid, or can't it
be done?

Cheers,

jez.

[Steven N. Hirsch]

Post by Jeremy White
{
my $variable;
sub mysub {
# ... accessing $variable
}
}
http://www.unix.org.ua/orelly/perl/cookbook/ch10_04.htm
Is there a way to get hold of the SV for $variable and manipulate it via XS?
I want to be able to write the contents of $variable in XS, then call mysub
via XS and then read the contents of $variable afterwards ($variable needs to
remain a closure as it is shared with other subs in the same block - the block
is evaled into existence during runtime).

Since lexical variables don't appear in the symbol table, the only way I
know of to access it from XS would be to stash a reference somewhere and
pass that to the XS routine. Also, I may have simply been up too long,
but I don't believe your example represents a closure. Only (1) instance
of 'mysub' will exist and it's present in the package's symbol table.

Steve

[Paul Johnson]

Post by Steven N. Hirsch

Post by Jeremy White
{
my $variable;
sub mysub {
# ... accessing $variable
}
}
http://www.unix.org.ua/orelly/perl/cookbook/ch10_04.htm

Careful. Some people won't talk very nicely of you if you go bandying
URLs like that around.

Post by Steven N. Hirsch

Post by Jeremy White
Is there a way to get hold of the SV for $variable and manipulate it via XS?
I want to be able to write the contents of $variable in XS, then call mysub
via XS and then read the contents of $variable afterwards ($variable needs to
remain a closure as it is shared with other subs in the same block - the block
is evaled into existence during runtime).

Since lexical variables don't appear in the symbol table, the only way I
know of to access it from XS would be to stash a reference somewhere and
pass that to the XS routine.

You could take a look at the code in PadWalker on CPAN to get some
ideas, I think.

Post by Steven N. Hirsch
Also, I may have simply been up too long,
but I don't believe your example represents a closure. Only (1) instance
of 'mysub' will exist and it's present in the package's symbol table.

The docs (used to?) talk about closures being anonymous subroutines, but
I think most people nowadays would says that mysub closes over $variable
and hence it is a closure.

[Steven N. Hirsch]

Post by Paul Johnson

Post by Steven N. Hirsch
Since lexical variables don't appear in the symbol table, the only way I
know of to access it from XS would be to stash a reference somewhere and
pass that to the XS routine.

You could take a look at the code in PadWalker on CPAN to get some
ideas, I think.

Oooooh. Cute. Thanks for the pointer to that!

Post by Paul Johnson

Post by Steven N. Hirsch
Also, I may have simply been up too long,
but I don't believe your example represents a closure. Only (1) instance
of 'mysub' will exist and it's present in the package's symbol table.

The docs (used to?) talk about closures being anonymous subroutines, but
I think most people nowadays would says that mysub closes over $variable
and hence it is a closure.

Yup, too long a day... Agreed.

Steve

[Muppet]

Post by Jeremy White
Is there a way to get hold of the SV for $variable and manipulate
it via XS?

Since it's a lexical, no. You can probably get away with an xsub
that returns a reference to an SV allocated and stored in your XS
file, like this:

Foo.xs -=-=-=-=-=-=-=-=-=-

static SV * the_foo;

MODULE = Foo PACKAGE = FOO

SV * get (class)
CODE:
if (!the_foo)
the_foo = newSV ();
RETVAL = newRV (the_foo);
OUTPUT
RETVAL

# more functions that use the_foo

Foo.pm -=-=-=-=-=-=-=-=-=-=-

{
sub mysub {
my $fooref = Foo->get;
$$fooref = "foo!";
}
}

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