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


#include <strings.h>

int ffs(int i);

#include <string.h>

int ffsl(long int i);

int ffsll(long long int i);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):


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:


ffsl(), ffsll():

Since glibc 2.27:


Before glibc 2.27:



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.


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


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

See Also



This page is part of release 5.04 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

Referenced By

memchr(3), signal-safety(7).

The man pages ffsl(3) and ffsll(3) are aliases of ffs(3).

2017-09-15 GNU Linux Programmer's Manual