atan2 (3)  Linux Manuals
atan2: arc tangent function of two variables
NAME
atan2, atan2f, atan2l  arc tangent function of two variables
SYNOPSIS
#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():

_ISOC99_SOURCE  _POSIX_C_SOURCE >= 200112L
 /* 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  MTSafe 
CONFORMING TO
C99, POSIX.12001, POSIX.12008.The variant returning double also conforms to SVr4, 4.3BSD, C89.
COLOPHON
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