### _facos -- Calculate Arccosine

Format

>     #include <builtin.h>
>     double _facos(double x);

Language Level: Extension
_facos calculates the arccosine of x. This function causes the compiler to emit the appropriate 80387 instructions for the calculation of the arccosine.

Because it is a built-in function and has no backing code in the library:

- You cannot take the address of _facos.
- You cannot parenthesize a _facos function call, because parentheses specify a call to the backing code for the function.

Return Value
_facos returns the arccosine of x.

Example
This example prompts for a value for x. It prints an error message if x is greater than 1 or less than -1. Otherwise, it assigns the arccosine of x to y.

    #include <builtin.h>
    #include <stdio.h>

    #define MAX  1.0
    #define MIN -1.0

    int main(void)
    {
       double x;

       printf("Enter x:\n");
       scanf("%lf", &x);

       /* Output error if not in range */
       if (MAX < x)
               printf("Error: %lf too small for acos.\n", x);
            else
               printf("The arccosine of %lf is %lf.\n", x, _facos(x));
       return 0;

       /**************************************************************
          Assuming you enter: -1.0

          The output should be:

          The arccossine of -1.000000 is 3.141593.
       **************************************************************/
    }

acos -- Calculate Arccosine
cos -- Calculate Cosine
cosh -- Calculate Hyperbolic Cosine
_fcos -- Calculate Cosine
_fcossin -- Calculate Cosine and Sine
_fsincos -- Calculate Sine and Cosine
<builtin.h>