Creating a mask with N least significant bits set

Question

I would like to create a macro or function¹ mask(n) which given a number n returns an unsigned integer with its n least sig

Answer 1

Here's one that is portable and conditional-free:

unsigned long long mask(unsigned n)
{
    assert (n <= sizeof(unsigned long long) * CHAR_BIT);
    return (1ULL << (n/2) << (n-(n/2))) - 1;
}