---
name: "atanl"
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/atan"
---

# atanl(3)

arc tangent function

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

## NAME

atan, atanf, atanl - arc tangent function

## LIBRARY

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

## SYNOPSIS

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

**double atan(double** *x***);** **float atanf(float** *x***);** **long double atanl(long double** *x***);**

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

**atanf**(), **atanl**():

```
    _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
        || /* Since glibc 2.19: */ _DEFAULT_SOURCE
        || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
```

## DESCRIPTION

These functions calculate the principal value of the arc tangent of *x*; that is the value whose tangent is *x*.

## RETURN VALUE

On success, these functions return the principal value of the arc tangent of *x* in radians; the return value is in the range \[-pi/2, pi/2\].

If *x* is a NaN, a NaN is returned.

If *x* is +0 (-0), +0 (-0) is returned.

If *x* is positive infinity (negative infinity), +pi/2 (-pi/2) is returned.

## ERRORS

No errors occur.

## ATTRIBUTES

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

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **atan**(), **atanf**(), **atanl**() | Thread safety | MT-Safe |

## STANDARDS

C11, POSIX.1-2008.

## HISTORY

C99, POSIX.1-2001.

The variant returning *double* also conforms to SVr4, 4.3BSD, C89.

## SEE ALSO

[**acos**(3)](/acos/3.md), [**asin**(3)](/asin/3.md), [**atan2**(3)](/atan2/3.md), [**carg**(3)](/carg/3.md), [**catan**(3)](/catan/3.md), [**cos**(3)](/cos/3.md), [**sin**(3)](/sin/3.md), [**tan**(3)](/tan/3.md)