---
name: "ffsl"
section: "3"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/ffs"
---

# ffsl(3)

find first bit set in a word

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

## NAME

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

## LIBRARY

Standard C library (*libc*, *-lc*)

## SYNOPSIS

```
#include <strings.h>
```

**int ffs(int** *i***);** **int ffsl(long** *i***);** **int ffsll(long long** *i***);**

Feature Test Macro Requirements for glibc (see [**feature\_test\_macros**(7)](/feature_test_macros/7.md)):

**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

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*.

## ATTRIBUTES

For an explanation of the terms used in this section, see [**attributes**(7)](/attributes/7.md).

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **ffs**(), **ffsl**(), **ffsll**() | Thread safety | MT-Safe |

## STANDARDS

- **ffs**()

  POSIX.1-2001, POSIX.1-2008, 4.3BSD.

- **ffsl**() **ffsll**()

  GNU.

## SEE ALSO

[**memchr**(3)](/memchr/3.md)