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:
- 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
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.
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.
[edit] 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[edit] References
- 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)
[edit] See also
| (C99)(C99) |
gets or sets rounding direction (function) |
| C++ documentation for floating-point rounding macros
| |
