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Showing content from http://en.cppreference.com/w/c/numeric/math/../fenv/FE_round.html below:

FE_DOWNWARD, FE_TONEAREST, FE_TOWARDZERO, FE_UPWARD - cppreference.com

#define FE_DOWNWARD     /*implementation defined*/

#define FE_TONEAREST    /*implementation defined*/

#define FE_TOWARDZERO   /*implementation defined*/

#define FE_UPWARD       /*implementation defined*/

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

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.

#include <fenv.h>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
 
// #pragma STDC FENV_ACCESS ON
 
int main()
{
    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

• C23 standard (ISO/IEC 9899:2024):

• 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)

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