# atan2 (3)

# NAME

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

#include <math.h>

Link withdouble atan2(doubley, doublex);float atan2f(floaty, floatx);long double atan2l(long doubley, long doublex);

*-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

|| /* 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. 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 |

# CONFORMING TO

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