Movatterモバイル変換

ffs(3) — Linux manual page

ffs(3)                   Library Functions Manualffs(3)

NAME        top

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

LIBRARY        top

       Standard C library (libc,-lc)

SYNOPSIS        top

#include <strings.h>int ffs(inti);int ffsl(longi);int ffsll(long longi);   Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):ffs():           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:               noneffsl(),ffsll():           Since glibc 2.27:               _DEFAULT_SOURCE           Before glibc 2.27:               _GNU_SOURCE

DESCRIPTION        top

       Theffs() function returns the position of the first (least       significant) bit set in the wordi.  The least significant bit is       position 1 and the most significant position is, for example, 32       or 64.  The functionsffsll() andffsl() do the same but take       arguments of possibly different size.

RETURN VALUE        top

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

ATTRIBUTES        top

       For an explanation of the terms used in this section, seeattributes(7).       ┌──────────────────────────────────────┬───────────────┬─────────┐       │Interface│Attribute│Value│       ├──────────────────────────────────────┼───────────────┼─────────┤       │ffs(),ffsl(),ffsll()               │ Thread safety │ MT-Safe │       └──────────────────────────────────────┴───────────────┴─────────┘

STANDARDS        top

ffs()  POSIX.1-2001, POSIX.1-2008, 4.3BSD.ffsl()ffsll()              GNU.

SEE ALSO        top

memchr(3)

COLOPHON        top

       This page is part of theman-pages (Linux kernel and C library       user-space interface documentation) project.  Information about       the project can be found at        ⟨https://www.kernel.org/doc/man-pages/⟩.  If you have a bug report       for this manual page, see       ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.       This page was obtained from the tarball man-pages-6.15.tar.gz       fetched from       ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on       2025-08-11.  If you discover any rendering problems in this HTML       version of the page, or you believe there is a better or more up-       to-date source for the page, or you have corrections or       improvements to the information in this COLOPHON (which isnot       part of the original manual page), send a mail to       man-pages@man7.orgLinux man-pages 6.15            2025-05-17ffs(3)

Pages that refer to this page:memchr(3), signal-safety(7)

HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH.