#include <math.h>
double atan(double x);
float atanf(float x);
long double atanl(long double x);
These functions shall compute the principal value of the arc tangent of their argument x.
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 x in the range [-pi/2,pi/2] radians.
If x is NaN, a NaN shall be returned.
If x is ±0, x shall be returned.
If x is ±Inf, ±pi/2 shall be returned.
If x is subnormal, a range error may occur and x should be returned.
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.
atan2(), 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>