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Re: Perl XS

by arkturuz (Curate)
on Nov 18, 2013 at 19:31 UTC ( #1063188=note: print w/ replies, xml ) Need Help??


in reply to Perl XS

You don't have to write XS code at all. Maybe the better solution would be to use excellent Inline::C module.

Also, post the slow Perl code, maybe we could optimize it a bit.

Edit: better link to Inline::C added.


Comment on Re: Perl XS
Re^2: Perl XS
by revendar (Novice) on Nov 18, 2013 at 19:50 UTC

    Thanks!. I have seen Inline::C. I wanted to give XS a try. I still wanted to use XS as it keeps an abstraction.

      Inline::C can still be helpful - as a quick way of discovering what's going wrong and testing proposed fixes.

      For example, I placed (copy'n'paste') your 2 functions in an Inline::C script:
      use warnings; use strict; use Inline C => Config => BUILD_NOISY => 1; use Inline C => <<'EOC'; void print_array_char(char * array) { int l; int i; l=strlen(array); printf("Length of array is %d\n",l); for(i=0;i < l;i++) { printf("Element array[%d] = %c\n",i,array[i]); } } void print_array_int(int array[], int l) { int i; for(i=0;i < l;i++) { printf("Element array[%d] = %d\n",i,array[i]); } } EOC print_array_char( "revendar" ); print_array_int (-1,12,23,3);
      When I run that script, it compiles, then outputs:
      Length of array is 8 Element array[0] = r Element array[1] = e Element array[2] = v Element array[3] = e Element array[4] = n Element array[5] = d Element array[6] = a Element array[7] = r Undefined subroutine &main::print_array_int_alt called at try.pl line +....
      The problem is that, although there's nothing syntactically wrong with print_array_int(), perl doesn't know how to pass the 'int array[]' type to XS.
      For a working solution, you need to know a little bit about the perl API. I suggest perlxs, perlxstut, and perlapi docs, though you'll perhaps also find some useful tips in the Inline::C cookbook and, no doubt, many other places.
      Anyway, here's one solution:
      void print_array_int(int x, ...) { dXSARGS; int i; for(i=0;i < items - 1; i++) { printf("Element array[%d] = %d\n",i,SvIV(ST(i))); } XSRETURN(0); }
      "items" is the number of elements on the stack - so there's really no need to pass the length of the array to the function. You could remove that arg and rewrite the for loop condition as (i=0; i<items; i++)

      Two things to note about Inline::C:
      1) It's really just XS - it takes your C code, autogenerates the XS code, then compiles and runs your program.
      2) Inline::C defines its own stack macros, all of which begin with "Inline_Stack_" and are defined in the Inline.h file that it also autogenerates. Other than that, it's the same as XS, and you don't *have* to use its stack macros. You can just use the normal XS terms - as I did above when I declared "dXSARGS" instead of "Inline_Stack_Vars" (and used "XSRETURN(0)" instead of "Inline_Stack_Void").

      Cheers,
      Rob

        I second this!

        Here's one way to do it...

        1. Prototype in Perl, thinking "This will be reimplemented in XS/C." (which will encourage you to keep the prototype simple.)
        2. Re-implement using Inline::C
        3. If there's some additional tweekery that isn't available when using Inline::C, grab the XS file that Inline::C generates, and tweak to your heart's content.

        It's really so much more convenient. One thing to keep in mind: Passing a char* string around is simple until you start dealing with Unicode. Eventually it becomes easier to pass an SV*, and avoid touching the internal PV string except with proper XS macros/functions, and even then with extreme care.

        Also, write your unit tests either as step zero, or in conjunction with step one. That way when you've re-implemented in Inline::C/XS you can verify behavior.


        Dave

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