FFS(3)

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 int i);

int ffsll(long long int 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
 | 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>.

SEE ALSO
memchr(3)

COLOPHON
This page is part of release 4.15 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/.