### 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 ***y***, long double ***x***);**

Link with *-lm*.

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

**atan2f**(), **atan2l**(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE; or
*cc -std=c99*

### DESCRIPTION

The
**atan2**() function calculates 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.

### CONFORMING TO

C99, POSIX.1-2001.
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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