acos, acosf, acosl - arc cosine function
double acos(double x);
float acosf(float x);
long double acosl(long double x);
Link with -lm.
The acos() function calculates the arc cosine of x; that is the value whose cosine is x. If x falls outside the range -1 to 1, acos() fails and errno is set.
The acos() function returns the arc cosine in radians and the value is mathematically defined to be between 0 and PI (inclusive).
SVr4, POSIX.1-2001, 4.3BSD, C89. The float and long double variants are C99 requirements.