# 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():

_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.

`x` or `y` is NaN, a NaN is returned.

`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.

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

# CONFORMING TO

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

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

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