The ffs() 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. A return value of 0 means that the argument was zero.
The ffs() function conforms to IEEE Std 1003.1-2001 (“POSIX.1”).
HISTORY
The ffs() function appeared in 4.3BSD. The prototype for it existed previously in the <string.h> header before it was moved to <strings.h> for POSIX compliance.