## table of contents

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

# NAME¶

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

# SYNOPSIS¶

#include <math.h>

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

Link with *-lm*.

**atan2f**(), **atan2l**():

|| /* 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¶

# COLOPHON¶

This page is part of release 5.10 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/.

2017-09-15 |