@c ----------------------------------------------------------------------
@node atan2, math
@subheading Syntax

@example
#include <math.h>

double atan2(double y, double x);
@end example

@subheading Description

This function computes the angle, in the range @code{[-Pi..Pi]} radians,
whose tangent is @code{@var{y}/@var{x}}.  In other words, it computes
the angle, in radians, of the vector (@var{x},@var{y}) with respect to
the @code{+x} axis, reckoning the counterclockwise direction as
positive, and returning the value in the range @code{[-Pi, Pi]}.

@subheading Return Value

The arc tangent, in radians, of @code{@var{y}/@var{x}}.  @code{Pi} is
returned if @var{x} is negative and @var{y} is a negative zero,
@code{-0.0}.  @code{-Pi} is returned, if @var{x} is negative, and
@var{y} is a positive zero, @code{+0.0}.

If either @var{x} or @var{y} is infinite, @code{atan2} returns,
respectively, @code{Pi} with the sign of @var{y} or zero, and
@code{errno} is left unchanged.  However, if @emph{both} arguments are
infinite, the return value is @code{NaN} and @code{errno} is set to
@code{EDOM}.

A @code{NaN} is returned, and @code{errno} is set to @code{EDOM}, if
either @var{x} and @var{y} are both zero, or if either one of the
arguments is a @code{NaN}.


@subheading Portability

@portability ansi, posix