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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | STANDARDS | SEE ALSO | COLOPHON |
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ffs(3) Library Functions Manual ffs(3)
ffs, ffsl, ffsll - find first bit set in a word
Standard C library (libc, -lc)
#include <strings.h>
int ffs(int i);
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 >= 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 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
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.
These functions return the position of the first bit set, or 0 if
no bits are set in i.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ ffs(), ffsl(), ffsll() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
ffs() POSIX.1-2001, POSIX.1-2008, 4.3BSD.
ffsl()
ffsll()
GNU.
memchr(3)
This page is part of the man-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
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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-
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man-pages@man7.org
Linux man-pages 6.15 2025-05-17 ffs(3)
Pages that refer to this page: memchr(3), signal-safety(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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