Thanks Tassilo! I did not know about savepv, very convenient, like
strdup. Just curious, is the the SV memory being re-used, or is it
not going out of scope? I didn't think it would be static.
On Mon, 2004-04-19 at 12:54, Tassilo von Parseval wrote:
> On Mon, Apr 19, 2004 at 12:08:03PM -0500 Scott T. Hildreth wrote:
>
> > I have a xsub that has a pointer to a c-structure, and a sv as the
> > parameters,
> >
> > IV
> > set_fld_name(fld, name)
> >
> > FLD * fld
> > SV * name
> > PREINIT:
> > STRLEN len;
> > CODE:
> > if (name == (SV *)NULL)
> > RETVAL = 0;
> > else {
> > fld->name = SvPV(name, len);
> > RETVAL = 1;
> > }
> > OUTPUT:
> > RETVAL
> >
> >
> > ..which changes the name of a 'fld->name'. Which works fine if I only
> > change one name. If the sub is called more than once the 'fld->name'
> > pointers all point to the same string. The name is a my variable passed
> > in via a method, so I thought there would be an SV created, but the fld
> > names changed all point to the same memory, and have the same string. I
> > used Devel::Peek to confirm that the address is same. Here is the
> > method,
> >
> > sub set_name {
> > my ($self, $fldnm) = @_;
> > my $rc;
> > my $newself = {};
> >
> > $fldnm = lc $fldnm; #== Fmt is case insensitive.
> > $rc = fmt_set_fld_name($self->{_ptr}, $fldnm);
> >
> > .
> > .
> > .
> > return $newself
> > }
> >
> > .. so the address of $fldnm is always the same (static). Therefore the
> > fld->names that are sent in all point to the same address. How do I get
> > a new SV so that the memory area is different.
>
> By making a copy:
>
> fld->name = savepv( SvPV(name, len) );
>
> If you don't do that, you'll sooner or later get corrupted memory. You
> pass a mortal scalar to the XSUB and use the pointer to its PV slot
> elsewhere. Perl doesn't know that and once it frees this scalar, the
> pointer is garbage as well.
>
> Another thing you could probably do is not making a copy but
> incrementing the scalar's refcount:
>
> fld->name = SvPV(name, len);
> SvREFCNT_inc(name);
>
> and decrementing it accordingly once you no longer need to the pointer.
>
> Tassilo
