in reply to Arrays and Inline C

For some reason, people tend to think of accessing Perl arrays from C code. I think you'll run into a lot fewer bugs if you instead deal with C arrays from Perl code, since the code is so trivial to write: 

use Inline ( 'C', <<END_C );

void _munge( char *outlist, int size, char *inlist1, char *inlist2 )
{
    double *output = (double *) outlist;
    double *list1  = (double *) inlist1;
    double *list2  = (double *) inlist2;
    /* use the "arrays" just like normal for C code: */
    int i;
    for( i= 0; i < size; ++i ) {
        output[i] = combine( list1[i], list2[i] );
    }
}

END_C

sub munge {
    my( $l1, $l2 ) = @_;
    my $size = 0+@$l1;
    my $out = "\0" x ( $size * length pack "d", 0.0 );
    my $pack1 = pack "d$size", @$l1;
    my $pack2 = pack "d$size", @$l2;
    _munge( $out, $size, $pack1, $pack2 );
    return unpack "d$size", $out;
}

[download]

- tye        

Replies are listed 'Best First'.
Re^2: Arrays and Inline C (pack)
by ed (Initiate) on Jun 23, 2010 at 15:07 UTC
    Thanks for the example - but somehow the trick doesn't seem to work: 
    use Inline 'C', <<'END_C'

void _munge( char *outlist, int size, char *inlist1, char *inlist2 )
{
    double *output = (double *) outlist;
    double *list1  = (double *) inlist1;
    double *list2  = (double *) inlist2;
    int i;
    for( i= 0; i < size; ++i ) {
        printf("%f %f\n", inlist1[i], inlist2[i]);
    }
}

END_C
    ;

my $size = 3;
my $out = "\0" x ( $size * length pack "d", 0.0 );
my $pack1 = pack "d$size", 1, 2, 3;
my $pack2 = pack "d$size", 4, 5, 6;
_munge( $out, $size, $pack1, $pack2 );

    [download]
    This prints 
    0.000000 0.000000
0.000000 0.000000
0.000000 0.000000

    [download]
    I expected it to print the numbers passed in. What happened?
[reply]
[d/l]
[select]

      You are passing 'char' values to your printf(), not 'double' values. Here is tested code that also includes the returning of values back to Perl: 

      #!/usr/bin/perl -w
use strict;

use Inline 'C', <<'END_C';

void _munge(
    char *outbuf,
    int   size,
    char *inbuf1,
    char *inbuf2 )
{
    double *output = (double *) outbuf;
    double *list1  = (double *) inbuf1;
    double *list2  = (double *) inbuf2;
    int i;
    for(  i= 0;  i < size;  ++i  ) {
        printf( "%f %f\n", list1[i], list2[i] );
        output[i]= list1[i] * list2[i];
    }
}

END_C


my $size = 3;
my $out  = "\0" x ( $size * length pack "d", 0.0 );
my $pack1= pack "d$size", 1, 2, 3;
my $pack2= pack "d$size", 4, 5, 6;
_munge( $out, $size, $pack1, $pack2 );
print $_, $/ for unpack "d$size", $out;

      [download]
       $ perl double.pl
1.000000 4.000000
2.000000 5.000000
3.000000 6.000000
4
10
18

      [download]

      - tye        

[reply]
[d/l]
[select]

In Section Seekers of Perl Wisdom