acos

float       acosf( float arg );

double      acos( double arg );

long double acosl( long double arg );

#define acos( arg )

Parameters

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

Example

#include <stdio.h>
#include <math.h>
#include <errno.h>
#include <fenv.h>
#include <string.h>
 
#pragma STDC FENV_ACCESS ON
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");
}

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

References

• C11 standard (ISO/IEC 9899:2011):
  • 7.12.4.1 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):
  • 7.12.4.1 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):
  • 4.5.2.1 The acos function

See also