## NAME

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

## SYNOPSIS

**#include
<math.h>**

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

*y*

**, float**

*x*

**);**

long double atan2l(long double

long double atan2l(long double

*y*

**, long double**

*x*

**);**

Link with
*−lm*.

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 versions <= 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).

## CONFORMING TO

C99, POSIX.1-2001, POSIX.1-2008.

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

## SEE ALSO

acos(3), asin(3), atan(3), carg(3), cos(3), sin(3), tan(3)

## COLOPHON

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part of release 4.15 of the Linux *man-pages* project.
A description of the project, information about reporting
bugs, and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.