---
name: "ffs"
section: "3"
platform: "macos"
package: "libplatform"
version: "375.120.2"
release: "macOS 26.5"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/ffs"
---

# ffs(3)

find first or last bit set in a bit string

## NAME

**ffs**, **ffsl**, **ffsll**, **fls**, **flsl**, **flsll** – find first or last bit set in a bit string

## LIBRARY

Standard C Library (libc, -lc)

## SYNOPSIS

**#include \<strings.h\>**

*int*\

**ffs**(*int value*);

*int*\

**ffsl**(*long value*);

*int*\

**ffsll**(*long long value*);

*int*\

**fls**(*int value*);

*int*\

**flsl**(*long value*);

*int*\

**flsll**(*long long value*);

## DESCRIPTION

The **ffs**(), **ffsl**() and **ffsll**() functions find the first (least significant) bit set in *value* and return the index of that bit.

The **fls**(), **flsl**() and **flsll**() functions find the last (most significant) bit set in *value* and return the index of that bit.

Bits are numbered starting at 1, the least significant bit. A return value of zero from any of these functions means that the argument was zero.

## SEE ALSO

[bitstring(3)](/bitstring/3.md)

## HISTORY

The **ffs**() function appeared in 4.3BSD. Its prototype existed previously in ⟨string.h⟩ before it was moved to ⟨strings.h⟩ for IEEE Std 1003.1-2001 (“POSIX.1”) compliance.

The **ffsl**(), **fls**() and **flsl**() functions appeared in FreeBSD 5.3. The **ffsll**() and **flsll**() functions appeared in FreeBSD 7.1.