---
name: "atan2"
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/atan2"
---

# atan2(3)

arc tangent function of two variables

## NAME

atan2, atan2f, atan2l - arc tangent function of two variables

## LIBRARY

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

## SYNOPSIS

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

**double atan2(double** *y***, double** *x***);** **float atan2f(float** *y***, float** *x***);** **long double atan2l(long double** *y***, long double** *x***);**

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

**atan2f**(), **atan2l**():

```
    _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 *y/x*, using the signs of the two arguments to determine the quadrant of the result.

## RETURN VALUE

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

If *y* is +0 (-0) and *x* is less than 0, +pi (-pi) is returned.

If *y* is +0 (-0) and *x* is greater than 0, +0 (-0) is returned.

If *y* is less than 0 and *x* is +0 or -0, -pi/2 is returned.

If *y* is greater than 0 and *x* is +0 or -0, pi/2 is returned.

If either *x* or *y* is NaN, a NaN is returned.

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

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

If *y* is a finite value greater (less) than 0, and *x* is negative infinity, +pi (-pi) is returned.

If *y* is a finite value greater (less) than 0, and *x* is positive infinity, +0 (-0) is returned.

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

If *y* is positive infinity (negative infinity) and *x* is negative infinity, +3\*pi/4 (-3\*pi/4) is returned.

If *y* is positive infinity (negative infinity) and *x* is positive infinity, +pi/4 (-pi/4) 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** |
| **atan2**(), **atan2f**(), **atan2l**() | 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), [**atan**(3)](/atan/3.md), [**carg**(3)](/carg/3.md), [**cos**(3)](/cos/3.md), [**sin**(3)](/sin/3.md), [**tan**(3)](/tan/3.md)