cos, cosf, cosl

Defined in header <math.h>
float       cosf( float arg );
(1) (since C99)
double      cos( double arg );
long double cosl( long double arg );
(3) (since C99)
Defined in header <tgmath.h>
#define cos( arg )
(4) (since C99)
1-3) Computes the cosine of arg (measured in radians).
4) Type-generic macro: If the argument has type long double, cosl is called. Otherwise, if the argument has integer type or the type double, cos is called. Otherwise, cosf is called. If the argument is complex, then the macro invokes the corresponding complex function (ccosf, ccos, ccosl).


arg - floating point value representing angle in radians

Return value

If no errors occur, the cosine of arg (cos(arg)) in the range [-1 ; +1], is returned.

The result may have little or no significance if the magnitude of arg is large.

(until C++11)

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.

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

  • if the argument is ±0, the result is 1.0
  • if the argument is ±∞, NaN is returned and FE_INVALID is raised
  • if the argument is NaN, NaN is returned


The case where the argument is infinite is not specified to be a domain error in C, but it is defined as a domain error in POSIX.


#include <stdio.h>
#include <math.h>
#include <errno.h>
#include <fenv.h>
int main(void)
    double pi = acos(-1);
    // typical usage
    printf("cos(pi/3) = %f\n", cos(pi/3));
    printf("cos(pi/2) = %f\n", cos(pi/2));
    printf("cos(-3*pi/4) = %f\n", cos(-3*pi/4));
    // special values
    printf("cos(+0) = %f\n", cos(0.0));
    printf("cos(-0) = %f\n", cos(-0.0));
    // error handling 
    printf("cos(INFINITY) = %f\n", cos(INFINITY));
    if(fetestexcept(FE_INVALID)) puts("    FE_INVALID raised");

Possible output:

cos(pi/3) = 0.500000
cos(pi/2) = 0.000000
cos(-3*pi/4) = -0.707107
cos(+0) = 1.000000
cos(-0) = 1.000000
cos(INFINITY) = -nan
    FE_INVALID raised


  • C11 standard (ISO/IEC 9899:2011):
    • The cos functions (p: 239)
    • 7.25 Type-generic math <tgmath.h> (p: 373-375)
    • F.10.1.5 The cos functions (p: 519)
  • C99 standard (ISO/IEC 9899:1999):
    • The cos functions (p: 220)
    • 7.22 Type-generic math <tgmath.h> (p: 335-337)
    • F.9.1.5 The cos functions (p: 456)
  • C89/C90 standard (ISO/IEC 9899:1990):
    • The cos function

See also

computes sine (sin(x))
computes tangent (tan(x))
computes arc cosine (arccos(x))
computes the complex cosine

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