FFS(3) Linux Programmer's Manual 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 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 signifi-
cant) 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 func-
tions 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 at-
tributes(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>.
SEE ALSO
memchr(3)
COLOPHON
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GNU 2020-11-01 FFS(3)
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