NAME

ffs ffsl ffsll — find first bit set in a bit string

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <strings.h>

int
ffs(int value);

ffsl(long value);

ffsll(long long value);

DESCRIPTION

The ffs(), ffsl(), and ffsll() functions find the first bit set in value and return 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.

SEE ALSO

bitstring(3), ffs32(3), ffs64(3), popcount(3)

STANDARDS

The ffs() function conforms to IEEE Std 1003.1-2001 (“POSIX.1”). The ffsl() and ffsll() functions conform to IEEE Std 1003.1-2024 (“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.