FFS(3) Linux Programmer's Manual FFS(3)

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:

|| ! (_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.

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.
The ffsl() and ffsll() functions are glibc extensions.

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

2017-09-15 GNU