#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 <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
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.
| Interface | Attribute | Value |
| atan2(), atan2f(), atan2l() | Thread safety | MT-Safe |
The variant returning double also conforms to SVr4, 4.3BSD, C89.