---
name: "fls"
section: "3"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["freebsd"]
url: "https://new.carta.tech/ffs"
---

# fls(3)

find first or last bit set in a bit string

Part of [ffs(3)](/ffs/3.md).

## 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), [stdc\_first\_trailing\_one(3)](/stdc_first_trailing_one/3.md), [stdc\_trailing\_zeros(3)](/stdc_trailing_zeros/3.md), [bitset(9)](/bitset/9.md)

## STANDARDS

The **ffs**() function conforms to IEEE Std 1003.1-2008 (“POSIX.1”). The **ffsl**() and **ffsll**() functions conform to IEEE Std 1003.1-2024 (“POSIX.1”).

## 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.