< c‎ | numeric‎ | fenv
Defined in header <fenv.h>
#define FE_DOWNWARD     /*implementation defined*/
(since C99)
#define FE_TONEAREST    /*implementation defined*/
(since C99)
#define FE_TOWARDZERO   /*implementation defined*/
(since C99)
#define FE_UPWARD       /*implementation defined*/
(since C99)

Each of these macro constants expands to a nonnegative integer constant expression, which can be used with fesetround and fegetround to indicate one of the supported floating-point rounding modes. The implementation may define additional rounding mode constants in <fenv.h>, which should all begin with FE_ followed by at least one uppercase letter. Each macro is only defined if it is supported.

Constant Explanation
FE_DOWNWARD rounding towards negative infinity
FE_TONEAREST rounding towards nearest representable value
FE_TOWARDZERO rounding towards zero
FE_UPWARD rounding towards positive infinity

Additional rounding modes may be supported by an implementation.

The current rounding mode affects the following:

  • results of floating-point arithmetic operators outside of constant expressions
double x = 1;
x / 10; // 0.09999999999999999167332731531132594682276248931884765625 or
        // 0.1000000000000000055511151231257827021181583404541015625
sqrt(2); // 1.41421356237309492343001693370752036571502685546875 or
         // 1.4142135623730951454746218587388284504413604736328125
  • floating-point to floating-point implicit conversion and casts
double d = 1 + DBL_EPSILON;
float f = d; // 1.00000000000000000000000 or
             // 1.00000011920928955078125
strtof("0.1", NULL); // 0.0999999940395355224609375 or
                     // 0.100000001490116119384765625
lrint(2.1); // 2 or 3

The current rounding mode does NOT affect the following:

  • floating-point to integer implicit conversion and casts (always towards zero)
  • results of floating-point arithmetic operators in constant expressions executed at compile time (always to nearest)
  • the library functions round, lround, llround, ceil, floor, trunc

As with any floating-point environment functionality, rounding is only guaranteed if #pragma STDC FENV_ACCESS ON is set.

Compilers that do not support the pragma may offer their own ways to support current rounding mode. For example Clang and GCC have the option -frounding-math intended to disable optimizations that would change the meaning of rounding-sensitive code.

[edit] Example

#include <fenv.h>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
int main()
    puts("rounding down: ");
    printf("           pi = %.22f\n", acosf(-1));
    printf("strtof(\"1.1\") = %.22f\n", strtof("1.1", NULL));
    printf("    rint(2.1) = %.22f\n\n", rintf(2.1));
    puts("rounding up: ");
    printf("           pi = %.22f\n", acosf(-1));
    printf("strtof(\"1.1\") = %.22f\n", strtof("1.1", NULL));
    printf("    rint(2.1) = %.22f\n", rintf(2.1));


rounding down:
           pi = 3.1415925025939941406250
strtof("1.1") = 1.0999999046325683593750
    rint(2.1) = 2.0000000000000000000000
rounding up:
           pi = 3.1415927410125732421875
strtof("1.1") = 1.1000000238418579101563
    rint(2.1) = 3.0000000000000000000000

[edit] References

  • C23 standard (ISO/IEC 9899:2023):
  • 7.6/8 Floating-point environment <fenv.h> (p: TBD)
  • C17 standard (ISO/IEC 9899:2018):
  • 7.6/8 Floating-point environment <fenv.h> (p: 151)
  • C11 standard (ISO/IEC 9899:2011):
  • 7.6/8 Floating-point environment <fenv.h> (p: 207)
  • C99 standard (ISO/IEC 9899:1999):
  • 7.6/7 Floating-point environment <fenv.h> (p: 188)

[edit] See also

gets or sets rounding direction
(function) [edit]
C++ documentation for floating-point rounding macros