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

```
#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)):

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.

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.

No errors occur.

For an explanation of the terms used in this section, see attributes(7).

Interface | Attribute | Value |

atan2(), atan2f(), atan2l() | Thread safety | MT-Safe |

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

The variant returning `double`

also conforms to SVr4, 4.3BSD,
C89.

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