FE_DOWNWARD, FE_TONEAREST, FE_TOWARDZERO, FE_UPWARD

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.

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

• results of standard library mathematical functions

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

• string conversions such as strtod or printf

strtof("0.1", NULL); // 0.0999999940395355224609375
                  // or 0.100000001490116119384765625

• the library rounding functions nearbyint, rint, lrint

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.

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));
}

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

References