Re: When to (and not to) use sv_2mortal() [first question only]

av_push, as documented, takes ownership of one reference count. If you mortalise the scalars, their refcnt will be reduced to zero even though the array still hold a reference to them. Bug. Don't mortalise them.

AV* return_an_array() 
{
    AV* av = newAV();
    av_push( av, newSViv( 7  ) );
    av_push( av, newSviv( 42 ) );
    return av;
}
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The thing is, Perl has a bug of its own. The output typemap for AV*

Creates a reference to the AV.
Increases the refcount of the AV (REFCNT=2: BUG!!!)
Mortalises the reference.

So the AV and thus its content leak. Avoid it by using the following:

SV* return_an_array() 
{
    AV* av = newAV();
    av_push( av, newSViv( 7  ) );
    av_push( av, newSviv( 42 ) );
    return newRV_noinc(av);
}
[download]

The output typemap for SV* will mortialise the reference.

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Re^2: When to (and not to) use sv_2mortal() by davido (Cardinal) on Oct 28, 2011 at 18:14 UTC
av_push, as documented, takes ownership of one reference count. Ok, yes, I saw that documentation, and was unclear whether "takes ownership of one reference count" meant what it seems to mean (receives the existing reference count, up to a max of one), or whether it owns the responsibility of managing the ref-count by adding one to it. I knew I was trying to read too much into it but still managed to get it wrong. ;) Thanks for clarification, and thanks for pointing out the AV* bug! That one was the one I wasn't even considering to be an issue. :) Strange though; I had been passing back 150000 element arrays in a loop that had thousands of iterations (just doing some testing) and hadn't seen the memory leak. Given the quantity of data, in retrospect it should have been enough of a leak to cause some swapping to take place, which I never saw. Dave	[reply]

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Re^2: When to (and not to) use sv_2mortal()
by davido (Cardinal) on Oct 28, 2011 at 18:14 UTC

av_push, as documented, takes ownership of one reference count.

Ok, yes, I saw that documentation, and was unclear whether "takes ownership of one reference count" meant what it seems to mean (receives the existing reference count, up to a max of one), or whether it owns the responsibility of managing the ref-count by adding one to it. I knew I was trying to read too much into it but still managed to get it wrong. ;)

Thanks for clarification, and thanks for pointing out the AV* bug! That one was the one I wasn't even considering to be an issue. :) Strange though; I had been passing back 150000 element arrays in a loop that had thousands of iterations (just doing some testing) and hadn't seen the memory leak. Given the quantity of data, in retrospect it should have been enough of a leak to cause some swapping to take place, which I never saw.

Dave

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