# atan2

```ATAN2(3)                   Linux Programmer's Manual                  ATAN2(3)

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);

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 tan-
gent 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  infin-
ity, +pi (-pi) is returned.

If  y is a finite value greater (less) than 0, and x is positive infin-
ity, +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 infin-
ity, +3*pi/4 (-3*pi/4) is returned.

If y is positive infinity (negative infinity) and x is positive  infin-
ity, +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 |
+-----------------------------+---------------+---------+
CONFORMING TO
C99, POSIX.1-2001, POSIX.1-2008.

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