Calculates the arc tangent of a floating-point number.
long double atanl (long double d);
d Floating-point number for which arc tangent is to be computed
The atan function returns the arc tangent in the range (–π/2, +π/2) by the radian.
If the function call is replaced by code that uses a trigonometric function unit, the function might no longer be reentrant. For details, refer to the item about -tfu in the section on compile options.