ffs - FreeBSD 15.1-RELEASE and Ports.quarterly Manual Pages

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FFS(3)			     Library Functions Manual			  FFS(3)

NAME
     ffs,  ffsl,  ffsll, fls, flsl, flsll -- find first or last bit set in a bit
     string

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <strings.h>

     int
     ffs(int value);

     int
     ffsl(long value);

     int
     ffsll(long long value);

     int
     fls(int value);

     int
     flsl(long value);

     int
     flsll(long long value);

DESCRIPTION
     The ffs(), ffsl() and ffsll() functions find the first (least  significant)
     bit set in value and return the index of that bit.

     The  fls(),  flsl()  and flsll() functions find the last (most significant)
     bit set in value and return the index of that bit.

     Bits are numbered starting at 1, the least significant bit.  A return value
     of zero from any of these functions means that the argument was zero.

SEE ALSO
     bitstring(3), stdc_first_trailing_one(3), stdc_trailing_zeros(3), bitset(9)

STANDARDS
     The ffs() function conforms  to  IEEE  Std  1003.1-2008  ("POSIX.1").   The
     ffsl() and ffsll() functions conform to IEEE Std 1003.1-2024 ("POSIX.1").

HISTORY
     The ffs() function appeared in 4.3BSD.  Its prototype existed previously in
     <string.h>  before  it  was  moved  to <strings.h> for IEEE Std 1003.1-2001
     ("POSIX.1") compliance.

     The ffsl(), fls() and flsl() functions appeared in FreeBSD  5.3.	The  ff-
     sll() and flsll() functions appeared in FreeBSD 7.1.

FreeBSD 15.1			October 25, 2025			  FFS(3)

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