acos, acosf, acosl

Defined in header <math.h>
float       acosf( float arg );
(1) (since C99)
double      acos( double arg );
long double acosl( long double arg );
(3) (since C99)
Defined in header <tgmath.h>
#define acos( arg )
(4) (since C99)
1-3) Computes the principal value of the arc cosine of arg.
4) Type-generic macro: If the argument has type long double, acosl is called. Otherwise, if the argument has integer type or the type double, acos is called. Otherwise, acosf is called. If the argument is complex, then the macro invokes the corresponding complex function (cacosf, cacos, cacosl).


arg - floating point value

Return value

If no errors occur, the arc cosine of arg (arccos(arg)) in the range [0 ; π], is returned.

If a domain error occurs, an implementation-defined value is returned (NaN where supported).

If a range error occurs due to underflow, the correct result (after rounding) is returned.

Error handling

Errors are reported as specified in math_errhandling.

Domain error occurs if arg is outside the range [-1.0; 1.0].

If the implementation supports IEEE floating-point arithmetic (IEC 60559),

  • If the argument is +1, the value +0 is returned.
  • If |arg| > 1, a domain error occurs and NaN is returned.
  • if the argument is NaN, NaN is returned


#include <stdio.h>
#include <math.h>
#include <errno.h>
#include <fenv.h>
#include <string.h>
int main(void)
    printf("acos(-1) = %f\n", acos(-1));
    printf("acos(0.0) = %f 2*acos(0.0) = %f\n", acos(0), 2*acos(0));
    printf("acos(0.5) = %f 3*acos(0.5) = %f\n", acos(0.5), 3*acos(0.5));
    printf("acos(1) = %f\n", acos(1));
    // error handling 
    errno = 0; feclearexcept(FE_ALL_EXCEPT);
    printf("acos(1.1) = %f\n", acos(1.1));
    if(errno == EDOM) perror("    errno == EDOM");
    if(fetestexcept(FE_INVALID)) puts("    FE_INVALID raised");

Possible output:

acos(-1) = 3.141593
acos(0.0) = 1.570796 2*acos(0.0) = 3.141593
acos(0.5) = 1.047198 3*acos(0.5) = 3.141593
acos(1) = 0.000000
acos(1.1) = nan
    errno == EDOM: Numerical argument out of domain
    FE_INVALID raised


  • C11 standard (ISO/IEC 9899:2011):
    • The acos functions (p: 238)
    • 7.25 Type-generic math <tgmath.h> (p: 373-375)
    • F.10.1.1 The acos functions (p: 518)
  • C99 standard (ISO/IEC 9899:1999):
    • The acos functions (p: 218)
    • 7.22 Type-generic math <tgmath.h> (p: 335-337)
    • F.9.1.1 The acos functions (p: 455)
  • C89/C90 standard (ISO/IEC 9899:1990):
    • The acos function

See also

computes arc sine (\({\small\arcsin{x} }\)arcsin(x))
computes arc tangent (\({\small\arctan{x} }\)arctan(x))
computes arc tangent, using signs to determine quadrants
computes cosine (\({\small\cos{x} }\)cos(x))
computes the complex arc cosine
C++ documentation for acos

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.