atan2 (3)
Leading comments
Copyright 1993 David Metcalfe (david@prism.demon.co.uk) and Copyright 2008, Linux Foundation, written by Michael Kerrisk <mtk.manpages@gmail.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting ...
NAME
atan2, atan2f, atan2l  arc tangent function of two variablesSYNOPSIS
#include <math.h> double atan2(double y, double x); float atan2f(float y, float x); long double atan2l(long double y, long double x);Link with lm.
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
atan2f(), atan2l():

_BSD_SOURCE  _SVID_SOURCE  _XOPEN_SOURCE >= 600  _ISOC99_SOURCE 
_POSIX_C_SOURCE >= 200112L;
or cc std=c99
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  MTSafe 
CONFORMING TO
C99, POSIX.12001, POSIX.12008.The variant returning double also conforms to SVr4, 4.3BSD, C89.