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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 <= 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). ββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ¬ββββββββββββββββ¬ββββββββββ βInterface β Attribute β Value β ββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββΌββββββββββββββββΌββββββββββ€ βffs(), ffsl(), ffsll() β Thread safety β MT-Safe β ββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ΄ββββββββββββββββ΄ββββββββββ 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) GNU 2021-03-22 FFS(3)