---
name: "log2l"
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/log2"
---

# log2l(3)

base-2 logarithmic function

Part of [log2(3)](/log2/3.md).

## NAME

log2, log2f, log2l - base-2 logarithmic function

## LIBRARY

Math library (*libm*, *-lm*)

## SYNOPSIS

```
#include <math.h>
```

**double log2(double** *x***);** **float log2f(float** *x***);** **long double log2l(long double** *x***);**

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

**log2**(), **log2f**(), **log2l**():

```
    _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
```

## DESCRIPTION

These functions return the base-2 logarithm of *x*.

## RETURN VALUE

On success, these functions return the base-2 logarithm of *x*.

For special cases, including where *x* is 0, 1, negative, infinity, or NaN, see [**log**(3)](/log/3.md).

## ERRORS

See [**math\_error**(7)](/math_error/7.md) for information on how to determine whether an error has occurred when calling these functions.

For a discussion of the errors that can occur for these functions, see [**log**(3)](/log/3.md).

## ATTRIBUTES

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

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **log2**(), **log2f**(), **log2l**() | Thread safety | MT-Safe |

## STANDARDS

C11, POSIX.1-2008.

## HISTORY

glibc 2.1. C99, POSIX.1-2001.

The variant returning *double* also conforms to SVr4, 4.3BSD.

## SEE ALSO

[**cbrt**(3)](/cbrt/3.md), [**clog2**(3)](/clog2/3.md), [**log**(3)](/log/3.md), [**log10**(3)](/log10/3.md), [**sqrt**(3)](/sqrt/3.md)