I'd be very grateful for any feedback from experts on Inline C and Perl internals. I'm working with bit vectors and creating several Inline C functions which (among other things) need to test/set bits.

The example code below illustrates how I am passing a bit vector from Perl into C, and testing/setting bits in C, such that the original Perl variable is changed (=intended).

use strict;
use warnings;

use Inline C => 'DATA';

my $vector = "\0" x 100;            # Bit vector with 800 bits
mytest($vector, 12);                # Pass it to Inline C function
print "Result:" . vec($vector, 12, 1) . "\n";    # Prints '1'

__DATA__
__C__

int mytest(SV* sv_vec, unsigned int bit) {

    STRLEN vecbytes; // Length of vector in bytes
    unsigned char *myvec = (unsigned char *) SvPV(sv_vec, vecbytes);

    if (bit/8 >= vecbytes) return 0; // Check in range
    
    if (myvec[bit/8] & 1U<<(bit%8)) return 1; // Test if a bit is set

    myvec[bit/8] |= 1U<<(bit%8); // Set bit (CHANGES $vector)

    return 1;
}

[download]

This works, but I am wondering if this approach is safe & correct, or if there are hidden gotchas or better/more efficient ways to do this with Inline C?

In reply to Bit vector fiddling with Inline C by oxone

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