#include <math.h>
double atan2(double y, double x);
float atan2f(float y, float x);
long double atan2l(long double y, long double x);
These functions shall compute the principal value of the arc tangent of y/ x, using the signs of both arguments to determine the quadrant of the return value.
An application wishing to check for error situations should set errno to zero and call feclearexcept(FE_ALL_EXCEPT) before calling these functions. On return, if errno is non-zero or fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an error has occurred.
Upon successful completion, these functions shall return the arc tangent of y/ x in the range [-pi,pi] radians.
If y is ±0 and x is < 0, ±pi shall be returned.
If y is ±0 and x is > 0, ±0 shall be returned.
If y is < 0 and x is ±0, -pi/2 shall be returned.
If y is > 0 and x is ±0, pi/2 shall be returned.
If x is 0, a pole error shall not occur.
If either x or y is NaN, a NaN shall be returned.
If the result underflows, a range error may occur and y/ x should be returned.
If y is ±0 and x is -0, ±pi shall be returned.
If y is ±0 and x is +0, ±0 shall be returned.
For finite values of ± y > 0, if x is -Inf, ±pi shall be returned.
For finite values of ± y > 0, if x is +Inf, ±0 shall be returned.
For finite values of x, if y is ±Inf, ±pi/2 shall be returned.
If y is ±Inf and x is -Inf, ±3pi/4 shall be returned.
If y is ±Inf and x is +Inf, ±pi/4 shall be returned.
If both arguments are 0, a domain error shall not occur.
These functions may fail if:
If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then errno shall be set to [ERANGE]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the underflow floating-point exception shall be raised.
The following sections are informative.
On error, the expressions (math_errhandling & MATH_ERRNO) and (math_errhandling & MATH_ERREXCEPT) are independent of each other, but at least one of them must be non-zero.
atan(), feclearexcept(), fetestexcept(), isnan(), tan(), the Base Definitions volume of IEEE Std 1003.1-2001, Section 4.18, Treatment of Error Conditions for Mathematical Functions, <math.h>