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FE_DOWNWARD, FE_TONEAREST, FE_TOWARDZERO, FE_UPWARD

From cppreference.com
< 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.

On most implementations, these macro constants expand to the values equal to the values of FLT_ROUNDS and float_round_style

Constant Explanation
FE_DOWNWARD rounding towards negative infinity
FE_TONEAREST rounding towards nearest integer
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 (always to nearest)
  • the library functions round, lround, ceil, floor, trunc

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

[edit] Example

#include <stdio.h>
#include <stdlib.h>
#include <fenv.h>
#include <math.h>
int main()
{
#pragma STDC FENV_ACCESS ON
    fesetround(FE_DOWNWARD);
    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));
    fesetround(FE_UPWARD);
    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));
}

Output:

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] See also

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