Bit fields in NativeCall

I am trying to create Perl6 bindings for Cgraph, and one of the structs has bit fields set for some of its attributes with values under 8. How should I represent that in my module?

I have tried defining a custom type using the is nativesize(x) trait, but CStructs only support types that are a multiple of 8 bits wide.

C example code:

struct Agtag_s {
    unsigned objtype:2;
}

What I tried:

my native objtype is repr('P6int') is Int is nativesize(2) is export { }
class Agtag is repr('CStruct') is export {
    has objtype $.object-type;
}

Trying to use my module with that code fails with the following error message: CStruct only supports native types that are a multiple of 8 bits wide (was passed: 2)

Solution

Here is an example. I assume a function use_struct() is defined in a library libslib :

#include <stdio.h>

struct Agtag_s {
    unsigned objtype:2;
    unsigned footype:4;
    unsigned bartype:6;
};

void use_struct (struct Agtag_s *s) {
    printf("sizeof(struct Agtag_s): %ld\n", sizeof( struct Agtag_s ));
    printf("objtype = %d\n", s->objtype);
    printf("footype = %d\n", s->footype);
    printf("bartype = %d\n", s->bartype);
    s->objtype = 3;
    s->footype = 13;
    s->bartype = 55;
}

Then in Perl 6:

use v6;
use NativeCall;

class Agtag is repr('CStruct') is export {
    has int32 $.bitfield is rw;
}

sub use_struct(Agtag $s is rw) is native("./libslib.so") { * };

my $s = Agtag.new();
my $objtype = 1;
my $footype = 7;
my $bartype = 31;
$s.bitfield = $objtype +| ($footype +< 2 ) +| ($bartype +< 6);
say "Calling library function..";
say "--------------------------";
use_struct( $s );
say "After call..";
say "------------";
say "objtype = ", $s.bitfield +& 3;
say "footype = ", ($s.bitfield +> 2) +& 15;
say "bartype = ", ($s.bitfield +> 6) +& 63;

Output:

Calling library function..
--------------------------
sizeof(struct Agtag_s): 4
objtype = 1
footype = 7
bartype = 31
After call..
------------
objtype = 3
footype = 13
bartype = 55