What is the fastest/most efficient way to find the highest set bit (msb) in an integer in C?

Question

If I have some integer n, and I want to know the position of the most significant bit (that is, if the least significant bit is on the right, I want to know the position of

Answer 1

What about

int highest_bit(unsigned int a) {
    int count;
    std::frexp(a, &count);
    return count - 1;
}

?