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

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 the furthest left bit that is a 1), what is the quickest/most efficient method of finding out?

I know that POSIX supports a ffs() method in strings.h to find the first set bit, but there doesn't seem to be a corresponding fls() method.

Is there some really obvious way of doing this that I'm missing?

What about in cases where you can't use POSIX functions for portability?

Edit: What about a solution that works on both 32 and 64 bit architectures (many of the code listings seem like they'd only work on 32 bit ints).