fegetround, fesetround

Defined in header `<fenv.h>`
int fesetround( int round );	(1)	(since C99)
int fegetround();	(2)	(since C99)

1) Attempts to establish the floating-point rounding direction equal to the argument round, which is expected to be one of the floating-point rounding macros.

2) Returns the value of the floating-point rounding macro that corresponds to the current rounding direction.

Parameters

round

rounding direction, one of floating-point rounding macros

Return value

1) 0 on success, non-zero otherwise.

2) the floating-point rounding macro describing the current rounding direction or a negative value if the direction cannot be determined.

Notes

The current rounding mode, reflecting the effects of the most recent fesetround, can also be queried with FLT_ROUNDS.

Example

#include <stdio.h>
#include <math.h>
#include <fenv.h>
 
#pragma STDC FENV_ACCESS ON
void show_fe_current_rounding_method(void)
{
    printf("current rounding method:  ");
    switch (fegetround()) {
           case FE_TONEAREST:  printf ("FE_TONEAREST");  break;
           case FE_DOWNWARD:   printf ("FE_DOWNWARD");   break;
           case FE_UPWARD:     printf ("FE_UPWARD");     break;
           case FE_TOWARDZERO: printf ("FE_TOWARDZERO"); break;
           default:            printf ("unknown");
    };
    printf("\n");
}
 
int main(void)
{
    /* Default rounding method */
    show_fe_current_rounding_method();
    printf("+11.5 -> %+4.1f\n", rint(+11.5)); /* midway between two integers */
    printf("+12.5 -> %+4.1f\n", rint(+12.5)); /* midway between two integers */
 
    /* Save current rounding method. */
    int curr_method = fegetround();
 
    /* Temporarily change current rounding method. */
    fesetround(FE_DOWNWARD);
    show_fe_current_rounding_method();
    printf("+11.5 -> %+4.1f\n", rint(+11.5));
    printf("+12.5 -> %+4.1f\n", rint(+12.5));
 
    /* Restore default rounding method. */
    fesetround(curr_method);
    show_fe_current_rounding_method(); 
 
    return 0;
}

Possible output:

current rounding method:  FE_TONEAREST
+11.5 -> +12.0
+12.5 -> +12.0
current rounding method:  FE_DOWNWARD
+11.5 -> +11.0
+12.5 -> +12.0
current rounding method:  FE_TONEAREST

References

C11 standard (ISO/IEC 9899:2011):

7.6.3.1 The fegetround function (p: 212)
7.6.3.2 The fesetround function (p: 212-213)

C99 standard (ISO/IEC 9899:1999):

7.6.3.1 The fegetround function (p: 193)
7.6.3.2 The fesetround function (p: 193-194)

nearbyintnearbyintfnearbyintl (C99)(C99)(C99)	rounds to an integer using current rounding mode (function)
rintrintfrintllrintlrintflrintlllrintllrintfllrintl (C99)(C99)(C99)(C99)(C99)(C99)(C99)(C99)(C99)	rounds to an integer using current rounding mode with exception if the result differs (function)