_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE ||
_POSIX_C_SOURCE >= 200112L;
or
cc -std=c99

DESCRIPTION

The
atan2()
function calculates 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.

CONFORMING TO

C99, POSIX.1-2001.
The variant returning
double
also conforms to
SVr4, 4.3BSD, C89.

This page is part of release 3.27 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.