I am trying to write some XS code that exposes pieces a library to
Perl code as a stream interface that can be written to. The
get_stream
function below is supposed to be a constructor that
prepares and returns a PerlIO object. I figured that I only need the
Write
and Close
methods, so I left all other function slots blank.
typedef struct {
struct _PerlIO base;
mylib_context* ctx;
} PerlIOmylib;
/* [...] */
PERLIO_FUNCS_DECL(PerlIO_mylib_funcs) = {
.fsize = sizeof(PerlIO_funcs),
.name = "mylib",
.size = sizeof(PerlIOmylib,
.Write = mylib_write,
.Close = mylib_close,
};
/* XS below */
PerlIO*
get_stream (SV* context_obj)
CODE:
mylib_context* ctx = (mylib_context*) SvIV (SvRV (context_obj));
PerlIO* f = PerlIO_allocate (aTHX);
f = PerlIO_push (aTHX, f, PERLIO_FUNCS_CAST(&PerlIO_mylib_funcs), "a", NULL);
PerlIOSelf(f, PerlIOmylib)->ctx = ctx;
PerlIOBase(f)->flags |= PERLIO_F_OPEN;
RETVAL = f;
OUTPUT:
RETVAL
When I use the provided interface like this...
{
my $fh = MyLib::get_stream($lib_ctx);
print $fh "x" x 300;
}
... the mylib_write
function gets called, so I haven't totally
screwed up so far. (I verified this by inserting debug printf
statements.) However, I would like the PerlIO object to be closed when
$fh
goes out of scope, just the way things work with regular
filehandles created by open
. But at the moment, the mylib_close
function is only called during interpreter shutdown.
Directly calling close
works fine, setting $fh
to undef
does
not.
UPDATE: Following ikegami's advice, I used Devel::Peek::Dump
and sv_dump
and found out that the handle returned get_stream
function is a "RV"
which points to a SV = PVGV(...)
. The glob (PVGV
) has its
reference counter set to 3 which doesn't seem right.
I added
CLEANUP:
SvREFCNT_dec (SvRV (ST(0)));
SvREFCNT_dec (SvRV (ST(0)));
which cures the symptom: The close
function is called when $fh
goes out of scope at the end of the block. But I still don't quite
understand the underlying problem.
This is the C code generated for the OUTPUT
section:
ST(0) = sv_newmortal();
{
GV *gv = newGVgen("MyLib");
if (do_open(gv, "+<&", 3, FALSE, 0, 0, RETVAL) )
sv_setsv(ST(0), sv_bless(newRV((SV*)gv), gv_stashpv("MyLib",1)));
else
ST(0) = &PL_sv_undef;
}
XSRETURN(1);
How does the GV's reference count end up at 3?
If close
is called at global destruction, that means your handle still exists at global destruction. You're leaking!
In C/XS code, you can use sv_dump(sv)
to dump a scalar to stderr. In Perl code, you can use Devel::Peek's Dump
to get the same functionality. This will show you reference counts.
In answer to your new question,
You have three allocation, but only one deallocation (the delayed one from sv_2mortal).
gv
: Pointer always discarded. Memory leak!
You could either decrement the refcnt of gv
on error, or you decrement the refcnt on unconditionally after using newRV_inc
to "transfer ownership" to the RV when open succeeds.
SV from newRV
: Pointer always discarded. Memory leak!
Why not just return it instead of copying it? Just mark it as mortal to cause Perl to decrement its refcnt after the caller gets it.
Fixed:
{
GV *gv = newGVgen("MyLib");
if (!do_open(gv, "+<&", 3, FALSE, 0, 0, RETVAL) ) {
SvREFCNT_dec(gv);
XSRETURN_UNDEF;
}
ST(0) = sv_2mortal(sv_bless(newRV_noinc((SV*)gv), gv_stashpv("MyLib",1))));
XSRETURN(1);
}