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acos, acosf

Calculates the arccosine.

double acos( 
   double x 
float acos(
   float x 
);   // C++ only
long double acos(
   long double x
);   // C++ only
float acosf(
   float x 


Value between –1 and 1 whose arccosine is to be calculated.

Return Value

The acos function returns the arccosine of x in the range 0 to π radians.

If x is less than –1 or greater than 1, acos returns an indefinite by default.

Input SEH Exception Matherr Exception


C++ allows overloading, so you can call overloads of acos. In a C program, acos always takes and returns a double.


Routine Required header Optional headers Compatibility
acos, acosf <math.h> <errno.h> ANSI, Win 98, Win Me, Win NT, Win 2000, Win XP

For additional compatibility information, see Compatibility in the Introduction.


All versions of the C run-time libraries.


This program prompts for a value in the range -1 to 1. Input values outside this range will produce _DOMAIN error messages. If a valid value is entered, the program prints the arcsine and the arccosine of that value.

// crt_asincos.c
// arguments: 0
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>

int main( int ac, char* av[] )
   double x, y;

   if (ac != 2) {
      fprintf(stderr, "Usage: %s <number between -1 and 1>\n", av[0]);
      return 1;
   sscanf( av[1], "%lf", &x );
   y = asin( x );
   printf( "Arcsine of %f = %f\n", x, y );
   y = acos( x );
   printf( "Arccosine of %f = %f\n", x, y );


Arcsine of 0.000000 = 0.000000
Arccosine of 0.000000 = 1.570796

See Also

Floating-Point Support Routines | asin | atan, cos | _matherr | sin | tan | Run-Time Routines and .NET Framework Equivalents