Hi monks, I've been fighting with this problem for a while, and despite my best efforts, I'm still stuck. Maybe very near to the solution, but still stuck. I believe that the following simple case is the root of the problem. Feel free to correct/direct me.

The code

GV *to_string = gv_fetchmethod (SvSTASH (sv), "toString");
if (to_string) {
    dSP;
    SV *res_sv; /* result scalar */

    ENTER;
    SAVETMPS;

    /* Declare function arguments */
    PUSHMARK (SP);
    XPUSHs (sv_bless (sv_2mortal (newRV_inc (sv)), SvSTASH (sv)));
    PUTBACK;

    /* Call in scalar context (G_SCALAR) */
    call_sv ((SV *)GvCV (to_string), G_SCALAR);
    SPAGAIN;

    /* Fetch returned SV from the stack */
    res_sv = POPs;
    PUTBACK;

    /* Append returned SvPVX to our SV */
    append_sv(res_sv, result);

    FREETMPS;
    LEAVE;
}

[download]
This code is partly taken from JSON::XS. I'm trying to write an XS function that takes an array or hashref in sv and, with some custom rules, serializes the content into a result scalar as one big string with everything.

When an hashref is really a blessed object, I want its toString() method to be called to dump the hashref content to my result SV. Calling the object's toString() method is the task for the code I shown you.

The problem

I think the problem lies here: what happens when, eventually, an object's toString() method code, for whatever reason, happens to contain a call to my XS function? (that could, in turn, call other toString() methods for other kind of objects...).

Does this break my stack manipulation? I think so, because I see segfaults and other bad things when toString() completes and the the code tries to return the resulting SV to the caller.

I feel I'm missing something about the stack manipulation in this case, but I really don't know how to proceed. Thank you for any help you can provide.

UPDATE: 2008-05-25 14:53 CET

The solution

I believe I have solved the problem. It was a real bitch of a hellish problem, trust me. Seems I've been bitten by old XS conventions about returning values to Perl code.

As it turned out, the problem was not in the snippet I showed you, but rather in this one, that called the function above, and incorrectly returned the resulting scalar to the caller. Real names changed to protect the innocents :)

void
func (SV *scalar)
    PPCODE:
        SV *result = newSV(INIT_SIZE);
        sv_setpv(result, "");
        SvPOK_only(result);
        serialize(result, scalar);
        XPUSHs(result);

[download]

The fixed and working version follows:

SV*
func (SV *scalar)
    CODE:
        SV *result = newSV(INIT_SIZE);
        sv_setpv(result, "");
        SvPOK_only(result);
        serialize(result, scalar);
        RETVAL = result;
    OUTPUT:
        RETVAL

[download]

In reply to Problem on perl callbacks from XS by cosimo

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