>>By the way what is the reason you do a PREINIT which does no init rather
>>than
>>
>>CODE:
>> {
>> char *chartext;
>> ...
>> }
>>
>>?
>>
>I once fell foul of the problem described in the PREINIT entry in the
>perlxs manpage, namely: "If a variable is declared inside a CODE:
>section it will follow any typemap code that is emitted for the input
>parameters. This may result in the declaration ending up after C code,
>which is C syntax error."
>
>I was very new to XS at the time (even newer than I am now ;), and was
>confused for ages by it. I've used PREINIT to declare all my variables
>ever since then just to play it safe.
>
>Is there a problem with using PREINIT all the time in this way?
Don't know - but I tend to use {} after CODE: - which creates
a new C block which can declare things.
>At this point I have:
>
> void
> hello(...)
> PREINIT:
> char *chartext;
>
> CODE:
> Newz(0, chartext, 13, char);
> strcpy(chartext, "Hello, world");
> ST(0) = newSV(0);
ST(0) = sv_newmortal(); // like above but does sv_2mortal for you
> sv_usepvn(ST(0), chartext, 13);
> XSRETURN(1);
>
> void
> callhello()
> PREINIT:
> SV *val;
>
> CODE:
> dSP;
> PUSHMARK(SP);
> PUTBACK;
> call_pv("Foo::hello", G_SCALAR);
> SPAGAIN;
> val = POPs;
> PUTBACK;
> ST(0) = val;
> PUTBACK; // this one is not necessary
> XSRETURN(1);
>
>So it seems that you can, after all, assign to ST(0) even when an xsub
>is called with no arguments from another xsub. I'm puzzled by that: I
>thought that before assigning to ST(0), ..., ST(n) you had to call
>EXTEND(SP, n + 1). Calling EXTEND(SP, n) would suffice when the xsub is
>called from Perl, but not when called from another xsub. In this case
>we are calling from another xsub; n is 0, so I thought I would have
>needed to call EXTEND(SP, 1) first.
I think there is one free slot - but I may be wrong.
>
>There's one other problem too: The above code leaks if I call
>callhello() a million times over in a for loop. Where is the leak
>coming from?
You haven't made its return value mortal.
>>
>The SAVETMPS/FREETMPS was to have the (mortal) argument that I'm passing
>to Foo::hello free()'d; I'd forgotten that it free()'s any mortal return
>values as well.
>
>Here's the example with hello() returning a mortal SV * that has its own
>char * space:
>
> void
> hello(...)
> PREINIT:
> char *chartext;
>
> CODE:
> Newz(0, chartext, 13, char);
> strcpy(chartext, "Hello, world");
> ST(0) = sv_2mortal(newSVpv(chartext, 13));
> XSRETURN(1);
>
> void
> callhello()
> PREINIT:
> SV *val;
>
> CODE:
> dSP;
> ENTER;
> SAVETMPS;
> PUSHMARK(SP);
> XPUSHs(sv_2mortal(newSViv(1)));
> PUTBACK;
> call_pv("Foo::hello", G_SCALAR);
> SPAGAIN;
> val = POPs;
> PUTBACK;
> ST(0) = val;
> PUTBACK;
> FREETMPS;
> LEAVE;
> XSRETURN(1);
>
>It does indeed result in callhello()'s caller getting undef.
>
>So how would I avoid coming a cropper like that?
>I want the arguments
>that callhello() passes to hello() to be freed before callhello()
>returns, but I don't want hello()'s return value to be free()'d then as
>well.
>
>This seems to work:
>
> ...
> call_pv("Foo::hello", G_SCALAR);
> SPAGAIN;
> val = newSVsv(POPs);
> PUTBACK;
> FREETMPS;
> LEAVE;
> ST(0) = sv_2mortal(val);
> PUTBACK;
> XSRETURN(1);
>
>Is that OK
That moral equivalent of what perl does ...
>or is there a more efficient way to do this?
Slightly.
val = POPs;
PUTBACK;
SvREFCNT_inc(val); // save it from destruction
FREETMPS;
LEAVE;
// now back in outer "scope".
ST(0) = sv_2mortal(val);
XSRETURN(1);
>
>- Steve
