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FFS(3) Library Functions Manual FFS(3)

ffs
find first bit set in a bit string

#include <strings.h>

int
ffs(int value);

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 rightmost bit. A return value of 0 means that the argument was zero.

bitstring(3)

The ffs() function conforms to IEEE Std 1003.1-2008 (“POSIX.1”).

The ffs() function first appeared in 4.2BSD.
November 24, 2015 OpenBSD-current