ffs(3) — Linux manual page

ffs(3)                  Library Functions Manual                  ffs(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(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 >= 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         top

       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         top

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

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       attributes(7).

       ┌──────────────────────────────────────┬───────────────┬─────────┐
       │Interface                             │ Attribute     │ Value   │
       ├──────────────────────────────────────┼───────────────┼─────────┤
       │ffs(), ffsl(), ffsll()                │ Thread safety │ MT-Safe │
       └──────────────────────────────────────┴───────────────┴─────────┘

VERSIONS         top

       BSD systems have a prototype in <string.h>.

STANDARDS         top

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

       ffsl()
       ffsll()
              GNU.

SEE ALSO         top

       memchr(3)

Linux man-pages 6.04           2023-03-30                         ffs(3)

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

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