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[ANSI C X3.1591989]
ATAN2(3) OpenBSD Programmer's Manual ATAN2(3)
NAME
atan2, atan2f  arc tangent functions of two variables
SYNOPSIS
#include <math.h>
double
atan2(double y, double x);
float
atan2f(float y, float x);
DESCRIPTION
The atan2() and atan2f() functions compute the principal value of the arc
tangent of y/x, using the signs of both arguments to determine the quad
rant of the return value.
RETURN VALUES
The atan2 function, if successful, returns the arc tangent of y/x in the
range [pi, +pi] radians. If both x and y are zero, the global variable
errno is set to EDOM. On the VAX:
atan2(y, x):= atan(y/x) if x > 0,
sign(y)*(pi  atan(y/x)) if x < 0,
0 if x = y = 0, or
sign(y)*pi/2 if x = 0 y.
NOTES
The function atan2() defines "if x > 0," atan2(0, 0) = 0 on a VAX despite
that previously atan2(0, 0) may have generated an error message. The
reasons for assigning a value to atan2(0, 0) are these:
1. Programs that test arguments to avoid computing atan2(0, 0)
must be indifferent to its value. Programs that require it to
be invalid are vulnerable to diverse reactions to that inva
lidity on diverse computer systems.
2. The atan2() function is used mostly to convert from rectangu
lar (x,y) to polar (r,theta) coordinates that must satisfy x =
r*cos theta and y = r*sin theta. These equations are satis
fied when (x=0,y=0) is mapped to (r=0,theta=0) on a VAX. In
general, conversions to polar coordinates should be computed
thus:
r := hypot(x,y); ... := sqrt(x*x+y*y)
theta := atan2(y,x).
3. The foregoing formulas need not be altered to cope in a rea
sonable way with signed zeros and infinities on a machine that
conforms to IEEE 754; the versions of hypot(3) and atan2()
provided for such a machine are designed to handle all cases.
That is why atan2(+0, 0) = +pi for instance. In general
the formulas above are equivalent to these:
r := sqrt(x*x+y*y); if r = 0 then x := copysign(1,x);
SEE ALSO
acos(3), asin(3), atan(3), cos(3), cosh(3), math(3), sin(3),
sinh(3), tan(3), tanh(3)
STANDARDS
The atan2() function conforms to ANSI X3.1591989 (``ANSI C'').
OpenBSD 2.6 May 2, 1991 1
Source: OpenBSD 2.6 man pages. Copyright: Portions are copyrighted by BERKELEY SOFTWARE DESIGN, INC., The Regents of the University of California, Massachusetts Institute of Technology, Free Software Foundation, FreeBSD Inc., and others. 
(Corrections, notes, and links courtesy of RocketAware.com)
FreeBSD Sources for atan2(3) functions FreeBSD Sources for atan2(3) functions OpenBSD sources for atan2(3)
Up to: Trigonometric  Trigonometric functions (sin, cos, tan, et al)
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[ANSI C X3.1591989]
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