### 1.3 `acos`, `acosf`—arc cosine ¶

Synopsis

    #include <math.h>
    double acos(double x);
    float acosf(float x);

Description

`acos` computes the inverse cosine (arc cosine) of the input value. Arguments to `acos` must be in the range −1 to 1.

`acosf` is identical to `acos`, except that it performs its calculations on `floats`.

Returns
`acos` and `acosf` return values in radians, in the range of 0 to pi.

If x is not between −1 and 1, the returned value is NaN (not a number), and the global variable `errno` is set to `EDOM`.