syphilis has asked for the wisdom of the Perl Monks concerning the following question:

Hi,
Hidden below my sig (by readmore tags) is a full Inline::C script that demos the problem. It uses overload, so that the subroutine 'overload_add_eq' is invoked whenever a '+=' is applied to the blessed object. At one stage, 'overload_add_eq' looked like this: 
SV * overload_add_eq(SV * obj, SV * addon, SV * third) {
     Card* c = (Card *)SvIV(SvRV(obj));
     SvREFCNT_inc(obj);
     c->value += SvIV(addon);
     return obj;
}

[download]
And that works fine - but it occurred to me that I should be able to rewrite the function as in 'in place' operation (ie one that did not return anything) so I rewrote it as: 
void overload_add_eq(SV * obj, SV * addon, SV * third) {
     Card* c = (Card *)SvIV(SvRV(obj));
     c->value += SvIV(addon);
}

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Now, when I do $obj += $addon; windows intervenes to tell me that perl.exe has stopped working (but fails to come up with any other information). On linux I get a segfault. Interestingly, I find that this modified 'overload_add_eq' works fine (on both linux and windows) when invoked explicitly in perl code as overload_add_eq($obj, $addon, 0);. It's only when it's invoked by the overload mechanism that the problem arises. Anyone know why that is, and what to do about it ?

Cheers,
Rob 
use warnings;
package My_struct;

use Inline (C => Config =>
            BUILD_NOISY => 1,
            );

use Inline C => <<'EOC';

     typedef struct {
       char * suit;
       int value;
       } Card;

SV * create_struct(SV * s, SV * v) {
     Card * cptr;

     New(123, cptr, sizeof(Card), Card);
     if(cptr == NULL) croak("Failed to allocate memory in create_struc
+t function");
     
     cptr->suit = SvPV_nolen(s);
     cptr->value = SvIV(v);
     return sv_setref_pv(newSViv(0), "My_struct", cptr);
}

void deref_ref(SV * obj) {
     Inline_Stack_Vars;

     Inline_Stack_Reset;
     Inline_Stack_Push(newSVpv(((Card *)SvIV(SvRV(obj)))->suit, 0));
     Inline_Stack_Push(newSViv(((Card *)SvIV(SvRV(obj)))->value));
     Inline_Stack_Done;
     Inline_Stack_Return(2);

}

void DESTROY(SV * obj) {
      printf("Destroying ...");
      Card* c = (Card *)SvIV(SvRV(obj));
      Safefree(c);
      printf("...destroyed\n");
}

void overload_add_eq(SV * obj, SV * addon, SV * third) {
     Card* c = (Card *)SvIV(SvRV(obj));
//     SvREFCNT_inc(obj);
     c->value += SvIV(addon);
//     return obj;
}


EOC

use overload '+='   => \&overload_add_eq;

$suit = ('Hearts', 'Spades', 'Clubs', 'Diamonds')[rand(4)];
$value = int(rand(13)) + 1;

$obj = create_struct($suit, $value);
$addon = 5;

@members = deref_ref($obj);
print "@members\n";

#$obj += $addon; # Crashes
overload_add_eq($obj, $addon, 0);

@members = deref_ref($obj);
print "@members\n";

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Replies are listed 'Best First'.
Re: Overload '+=' with XSub
by Anno (Deacon) on Mar 29, 2007 at 09:34 UTC
    Normal mutators return the SV after they have changed it. My guess is that overload relies on that.

    Anno

[reply]
[d/l]
      Anno, that seems to me to be a reasonable guess .... but I'm looking at a '+=' overload XSub that neither increments the refcount nor (afaict) returns anything ... and I'm wondering "how the hell does that work ?". It's from the GMP module - which is not to be found on CPAN. (It ships with the gmp source - in the demos/perl folder.) I don't know if it's going to help the discussion, but here's that XSub: 
      void
overload_addeq (x, y, o)
    mpz_assume   x
    mpz_coerce   y
    order_noswap o
ALIAS:
    GMP::Mpz::overload_subeq = 1
    GMP::Mpz::overload_muleq = 2
    GMP::Mpz::overload_diveq = 3
    GMP::Mpz::overload_remeq = 4
    GMP::Mpz::overload_andeq = 5
    GMP::Mpz::overload_ioreq = 6
    GMP::Mpz::overload_xoreq = 7
PREINIT:
    static_functable const struct {
      void (*op) (mpz_ptr, mpz_srcptr, mpz_srcptr);
    } table[] = {
      { mpz_add    }, /* 0 */
      { mpz_sub    }, /* 1 */
      { mpz_mul    }, /* 2 */
      { mpz_tdiv_q }, /* 3 */
      { mpz_tdiv_r }, /* 4 */
      { mpz_and    }, /* 5 */
      { mpz_ior    }, /* 6 */
      { mpz_xor    }, /* 7 */
    };
PPCODE:
    assert_table (ix);
    (*table[ix].op) (x->m, x->m, y);
    XPUSHs (ST(0));

      [download]
      First up - I don't really understand this XSub at all. I gather it's also handling -=, *=, /=, %=, &=, |= and ^= via some sort of lookup. The XPUSHs() also puzzles me. What does that achieve ?

      Anyway, it seems to me that this particular module has overloaded '+=' and friends without any need to return anything, and without any need to increase the refcount. (I've searched the entire XS file and there's not a single SvREFCNT_inc() to be found.)

      Cheers,
      Rob
      PS: I haven't actually built that GMP module for a while. I assume it works fine with perl-5.8.8, but that's something that's unverified. Last time I built this module, I also benchmarked the overloading and found it to be very efficient - not surprising, given that the author is a smart and talented programmer.
[reply]
[d/l]
        I'm looking at a '+=' overload XSub that neither increments the refcount nor (afaict) returns anything

        Ah, but it does return a value.

        The XPUSHs() also puzzles me. What does that achieve?

        It pushes a value on the return stack. See perlapi.

        Anno

[reply]

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