Header File 

math.h  

Category 

Math Routines 

Prototype 

double acos(double x); 

long double acosl(long double x); 

Description 

Calculates the arc cosine. 

acos returns the arc cosine of the input value.  

acosl is the long double version; it takes a long double argument and returns a long double result. 

Arguments to acos and acosl must be in the range -1 to 1, or else acos and acosl return NAN and set the global variable errno to: 

EDOM Domain error 

Return Value 

acos and acosl of an argument between -1 and +1 return a value in the range 0 to pi. Error handling for these routines can be modified through the functions _matherr_matherr and _matherrl. 

Example  

#include <stdio.h>
#include <math.h>
int main(void)
{
  double result;
  double x = 0.5;
  result = acos(x);
  printf("The arc cosine of %lf is %lf\n", x, result);
  return 0;
}

Portability