Should I manually unref these SVs in XS code?

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

In a XS function, I want to return a series of AVs. Each of the AV store one object and three integers. Currently, I suppose the code looks like this:

  PPCODE:
    // the number of return value is known
    EXTEND(SP, things.size());
    // cycle a list of things
    for (size_t i=0; i<things.size(); i++)
    {
        AV* curr_slot = newAV();
        SV* sv_obj = newSV(NULL);
        sv_setref_pv(sv_obj, "My::Class", things[i].obj);
        SV* sv_num1 = newSViv(things[i].foo);
        SV* sv_num2 = newSViv(things[i].bar);
        SV* sv_num3 = newSViv(things[i].baz);
        av_push(curr_slot, sv_obj);
        av_push(curr_slot, sv_num1);
        av_push(curr_slot, sv_num2);
        av_push(curr_slot, sv_num3);
        mPUSHs((SV*) curr_slot);
    }
[download]

Now, I have two questions:

1. There are four SVs created on each for cycle. Should I manually unref them, or they are automatically destroyed on XS function return?

2. Does the use of mPUSHs(AV) correct?

Thanks!!!

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Re: Should I manually unref these SVs in XS code? by ikegami (Patriarch) on Sep 04, 2014 at 16:38 UTC
You increment the reference count when you take hold of an SV/AV/etc. (This is already done for you by the `new` functions.) You decrement the reference count (or mortalize) when you relinquish your hold of an SV/AV/etc. Should I manually unref them* `av_push` says it "takes ownership of one reference count". This means it assumes you are relinquishing your hold on the scalar, and it will adjust the reference count accordingly. There's nothing else to be done (unless you store a copy of the SV locally, in which case you'd want to increment the reference count). or they are automatically destroyed on XS function return? That would be bad, since the array still references them. They are destroyed when their reference count becomes zero. Does the use of `mPUSHs(AV)` correct? Subs can only return scalars, so no. You need to place a reference to the array on the stack. `mPUSHs(newRV_noinc(MUTABLE_SV(av)));` [download] Like with `av_push`, `newRV_noinc` takes ownership of a reference count.	[reply] [d/l] [select]

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Re: Should I manually unref these SVs in XS code?
by ikegami (Patriarch) on Sep 04, 2014 at 16:38 UTC

You increment the reference count when you take hold of an SV/AV/etc. (This is already done for you by the new* functions.)

You decrement the reference count (or mortalize) when you relinquish your hold of an SV/AV/etc.

Should I manually unref them

av_push says it "takes ownership of one reference count". This means it assumes you are relinquishing your hold on the scalar, and it will adjust the reference count accordingly. There's nothing else to be done (unless you store a copy of the SV locally, in which case you'd want to increment the reference count).

or they are automatically destroyed on XS function return?

That would be bad, since the array still references them. They are destroyed when their reference count becomes zero.

Does the use of mPUSHs(AV) correct?

Subs can only return scalars, so no. You need to place a reference to the array on the stack.

mPUSHs(newRV_noinc(MUTABLE_SV(av)));
[download]

Like with av_push, newRV_noinc takes ownership of a reference count.

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