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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)