ffs() - find first bit set in a bit string
#include <strings.h>
int ffs (int value)
The ffs(3) function finds the first bit set in value and returns the index of that bit. Bits are numbered starting from 1, starting at the right-most bit.
Ffs(3) returns the index to the first bit set; the right-most bit is numbered 1. A return value of 0 means that the argument was zero.