FFS

Section: Linux Programmer's Manual (3)
Updated: 2020-11-01
Page Index

NAME

ffs, ffsl, ffsll - find first bit set in a word

SYNOPSIS

#include <strings.h>

int ffs(int i);

#include <string.h>

int ffsl(long i);

int ffsll(long long i);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

ffs():

Since glibc 2.12::     _XOPEN_SOURCE >= 700
    || ! (_POSIX_C_SOURCE >= 200809L)
    || /* Glibc since 2.19: */ _DEFAULT_SOURCE
    || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
Before glibc 2.12:: none

ffsl(), ffsll():

Since glibc 2.27:: _DEFAULT_SOURCE
Before glibc 2.27:: _GNU_SOURCE

DESCRIPTION

The ffs() function returns the position of the first (least significant) bit set in the word i. The least significant bit is position 1 and the most significant position is, for example, 32 or 64. The functions ffsll() and ffsl() do the same but take arguments of possibly different size.

RETURN VALUE

These functions return the position of the first bit set, or 0 if no bits are set in i.

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).

Interface	Attribute	Value
ffs(), ffsl(), ffsll()	Thread safety	MT-Safe

CONFORMING TO

ffs(): POSIX.1-2001, POSIX.1-2008, 4.3BSD.

The ffsl() and ffsll() functions are glibc extensions.

NOTES

BSD systems have a prototype in <string.h>.

COLOPHON

This page is part of release 5.10 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

FFS