acos, acosf, acosl

Calculates the arccosine.


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


Value between -1 and 1, for which to calculate the arccosine (the inverse cosine).

Return Value

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

By default, if x is less than -1 or greater than 1, acos returns an indefinite.

Input SEH Exception Matherr Exception


Because C++ allows overloading, you can call overloads of acos that take and return float and long double types. In a C program, acos always takes and returns a double.


Routine Required header Optional headers
acos, acosf, acosl <math.h> <errno.h>


This program prompts for a value in the range -1 to 1. Input values outside this range 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,
    errno_t err;

    // argument checking
    if (ac != 2)
        fprintf_s( stderr, "Usage: %s <number between -1 and 1>\n",
        return 1;

    // Convert argument into a double value
    if ((err = sscanf_s( av[1], "%lf", &x )) != 1)
        fprintf_s( stderr, "Error converting argument into ",
                   "double value.\n");
        return 1;

    // Arcsine of X
    y = asin( x );
    printf_s( "Arcsine of %f = %f\n", x, y );

    // Arccosine of X
    y = acos( x );
    printf_s( "Arccosine of %f = %f\n", x, y );
Arcsine of 0.000000 = 0.000000
Arccosine of 0.000000 = 1.570796

See also

Floating-Point Support
asin, asinf, asinl
atan, atanf, atanl, atan2, atan2f, atan2l
cos, cosf, cosl
sin, sinf, sinl
tan, tanf, tanl