The next trick is to get it to work with $a and $b. The following seems to work, but I'm not sure it isn't corrupting the stack. Any improvements? 
#! perl -slw
use strict;
use List::Util qw[ shuffle ];

use Inline C => 'DATA';

$^W = 0;

my $max = 120;
my @values = 1 .. $max;
my @values_mixed = shuffle( @values );
my @top = topN( 10, sub { $a <=> $b; }, \@values_mixed );
print "@top";

__END__
__C__

int callComp( SV* cmp, SV* a, SV* b ) {
    dSP;
    int rv;
    ENTER; 
    SAVETMPS; 

    GvSV(gv_fetchpv("main::a", TRUE, SVt_PV)) = a;
    GvSV(gv_fetchpv("main::b", TRUE, SVt_PV)) = b;
    
    PUSHMARK(SP);
    PUTBACK;

    if( call_sv( cmp, G_SCALAR|G_NOARGS ) != 1 )
        croak( "Bad comparator" );
    SPAGAIN;

    rv = POPi;

    PUTBACK; // << Was missing and is required!
    FREETMPS;LEAVE;
    return rv;
}

void topN( int n, SV* comp, AV*data ) {
    int *topN;
    int len = av_len( data );
    int i, j, k;

    Inline_Stack_Vars;
    Newz( 1, topN, n + 1, SV* );
    for( i = 0; i < n+1; i++ ) topN[ i ] = newSViv( 0 );
    
    for( i = 0; i <= len; i++ )    {
        SV* val = *av_fetch( data, i, 0 );
        for( j = 0; j < n; j++ ) {
            int cmp = callComp( comp, topN[ j ], val );
            if( cmp >= 0 ) continue;
            if( cmp < 0 ) {
                for( k = n; k > j; k-- ) topN[ k ] = topN[ k-1 ];
                topN[ j ] = val;
                break;
            }
        }
    }
    
    Inline_Stack_Reset;
    for( i = 0; i < n; i++ )
        Inline_Stack_Push( sv_2mortal( newSVsv( topN[ i ] ) ) );
    
    Safefree( topN );
    Inline_Stack_Done;
}

[download]

In reply to Re: Using $a and $b from XS by tall_man
in thread Using $a and $b from XS by tall_man

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.