Extending Perl with C: how to use an array reference?

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

Dear fellow monks,
I am trying to use one of my own c files to extend (and ultimately speed up) one of my perl script.
I read the perlxstut and friends
I would like to pass a reference to an array of integers to a C function I have. The protype of the function looks like that:
double foo_in_c(int *, int);
I did something in XS that works but that in my opinion is not very elegant, and also not very efficient (I have to copy the array).

double
foo(aref)
    SV * aref
  INIT:
    int i, last_index;
    int *array;
    if ((!SvROK(aref)) || (SvTYPE(SvRV(aref)) != SVt_PVAV) || ((last_i
+ndex = av_len((AV *)SvRV(aref))) < 0))
    {
      XSRETURN_UNDEF;
    }
    array = (int*)calloc(last_index+1, sizeof(int)
  CODE:
    for(i=0;i<=last_index;i++)
      array[i] = SvIV(*av_fetch((AV *)SvRV(array), i, 0));
    RETVAL = foo_in_c(array, last_index+1);
  OUTPUT:
    RETVAL
[download]

My question is simple: does anyone know of a better way to do that? Especially, how is it possible to avoid the copying part?
Thanks in advance,

--
jey

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Re: Extending Perl with C: how to use an array reference? by ikegami (Patriarch) on Apr 11, 2007 at 21:26 UTC
Especially, how is it possible to avoid the copying part? I don't know if there's an existing function to do this for you, but you must inevitably convert from an AV to an int* unless you change `foo_in_c` to work with the AV directly. On the bright side, I seriously doubt this is as inefficient as think. The time it takes to do the copy is surely dwarfed by the time `foo_in_c` spends processing it. You could always make a little helper function if you're concerned about neatness.	[reply] [d/l] [select]
Re: Extending Perl with C: how to use an array reference? by djp (Hermit) on Apr 12, 2007 at 04:35 UTC
Randy Kobes solved this problem years ago for Math::FFT, have a look at http://search.cpan.org/src/RKOBES/Math-FFT-1.28/arrays.c which maps between Perl and C arrays.	[reply]
Re: Extending Perl with C: how to use an array reference? by renodino (Curate) on Apr 12, 2007 at 02:33 UTC
Not certain if it'd be faster, and you might consider it uglier, but... Have you considered just using pack() in Perl to turn the Perl array into an int[], then passing the resulting binary scalar into your XS, grabbing the SvPV off the scalar, and treating it like an int[] ? That leaves most of the heavy lifting to Perl. BTW: In your version, I fear you've got a memory leak unless foo_in_c() free()'s your temp array... Perl Contrarian & SQL fanboy	[reply]
Re: Extending Perl with C: how to use an array reference? by BrowserUk (Patriarch) on Apr 11, 2007 at 23:15 UTC
What is your C functions doing? Would it be very hard to move that functions code into the XS above and operate directly upon the IVs? Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. "Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."	[reply]
Re: Extending Perl with C: how to use an array reference? by salva (Canon) on Apr 12, 2007 at 08:27 UTC
Another solution is to create the int array from Perl using vec, pack or some module like Tie::Array::PackedC or Tie::Array::Packed. On most cases this solution will be slower than converting the regular AV to an int array on the XS function wrapper. But, for instance, if you are just passing the arrays around between C functions, and manipulating them directly from Perl only in a few cases it can be faster.	[reply]
Re: Extending Perl with C: how to use an array reference? by aufflick (Deacon) on Apr 12, 2007 at 06:03 UTC
For ad-hoc use of C, it might pay to take a look at Inline::C - it's pretty good at munging the parameters for you.	[reply]