# 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.

```

#### RETURNVALUE

```   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).

Interface                     Attribute      Value

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

```

#### CONFORMINGTO

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

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

```

#### SEEALSO

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

```

#### COLOPHON

```   This page is part of release 4.09 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/.

2016-03-15                          ATAN2(3)
```

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