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FE_DOWNWARD, FE_TONEAREST, FE_TOWARDZERO, FE_UPWARD

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.

- - - - - -
Constant Explanation
FE_DOWNWARD rounding towards negative infinity
FE_TONEAREST rounding towards nearest representable value
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:

- 
double x = 1;
-x / 10; // 0.09999999999999999167332731531132594682276248931884765625 or
-        // 0.1000000000000000055511151231257827021181583404541015625
sqrt(2); // 1.41421356237309492343001693370752036571502685546875 or
-         // 1.4142135623730951454746218587388284504413604736328125
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:

-

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.

-

Example

#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

References

See also

- -
-
(C99)(C99)
 gets or sets rounding direction
(function)
C++ documentation for floating-point rounding macros
-

- © cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
- https://en.cppreference.com/w/c/numeric/fenv/FE_round -

-
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