'\" t .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk) .\" .\" SPDX-License-Identifier: Linux-man-pages-copyleft .\" .\" References consulted: .\" Linux libc source code .\" Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991) .\" 386BSD man pages .\" Modified Sat Jul 24 19:39:35 1993 by Rik Faith (faith@cs.unc.edu) .\" .\" Modified 2003 Walter Harms (walter.harms@informatik.uni-oldenburg.de) .\" .TH ffs 3 2024-05-02 "Linux man-pages 6.9.1" .SH NAME ffs, ffsl, ffsll \- find first bit set in a word .SH LIBRARY Standard C library .RI ( libc ", " \-lc ) .SH SYNOPSIS .nf .B #include .P .BI "int ffs(int " i ); .BI "int ffsl(long " i ); .BI "int ffsll(long long " i ); .fi .P .RS -4 Feature Test Macro Requirements for glibc (see .BR feature_test_macros (7)): .RE .P .BR ffs (): .nf Since glibc 2.12: _XOPEN_SOURCE >= 700 || ! (_POSIX_C_SOURCE >= 200809L) || /* glibc >= 2.19: */ _DEFAULT_SOURCE || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE Before glibc 2.12: none .fi .P .BR ffsl (), .BR ffsll (): .nf Since glibc 2.27: .\" glibc commit 68fe16dd327c895c08b9ee443b234c49c13b36e9 _DEFAULT_SOURCE Before glibc 2.27: _GNU_SOURCE .fi .SH DESCRIPTION The .BR ffs () function returns the position of the first (least significant) bit set in the word \fIi\fP. The least significant bit is position 1 and the most significant position is, for example, 32 or 64. The functions .BR ffsll () and .BR ffsl () do the same but take arguments of possibly different size. .SH RETURN VALUE These functions return the position of the first bit set, or 0 if no bits are set in .IR i . .SH ATTRIBUTES For an explanation of the terms used in this section, see .BR attributes (7). .TS allbox; lbx lb lb l l l. Interface Attribute Value T{ .na .nh .BR ffs (), .BR ffsl (), .BR ffsll () T} Thread safety MT-Safe .TE .SH STANDARDS .TP .BR ffs () POSIX.1-2001, POSIX.1-2008, 4.3BSD. .TP .BR ffsl () .TQ .BR ffsll () GNU. .SH SEE ALSO .BR memchr (3)