float acosf(float x);

acosf returns the principal value, in radians, of the inverse circular cosine of x. The principal value lies in the interval [0, PI] radians.

If |a| 1, errno is set to EDOM and acosf returns HUGE_VAL.

If x is NaN, acosf returns x. If |x| > 1, acosf returns NaN.