# atan2 - Man Page

arc tangent function of two variables

## Library

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

## Synopsis

#include <math.h> double atan2(doubley, doublex); float atan2f(floaty, floatx); long double atan2l(long doubley, long doublex);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

**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).

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

## Referenced By

acos(3), asin(3), atan(3), cos(3), perlfunc(1), sin(3), tan(3), unu-2op(1).

The man pages atan2f(3) and atan2l(3) are aliases of atan2(3).