FE_DOWNWARD, FE_TONEAREST, FE_TOWARDZERO, FE_UPWARD

C

Language

Headers

Type support

Program utilities

Variadic function support

Error handling

Dynamic memory management

Date and time utilities

Strings library

Algorithms

Numerics

Input/output support

Localization support

Atomic operations (C11)

Thread support (C11)

Technical Specifications

Numerics

Common mathematical functions

Floating-point environment

Complex number arithmetics

Random number generation

Type-generic math

Floating point environment

Functions

feclearexcept (C99)

fetestexcept (C99)

feraiseexcept (C99)

fegetexceptflagfesetexceptflag (C99)(C99)

fegetroundfesetround (C99)(C99)

fegetenvfesetenv (C99)

feholdexcept (C99)

feupdateenv (C99)

Macro constants

FE_ALL_EXCEPTFE_DIVBYZEROFE_INEXACTFE_INVALIDFE_OVERFLOWFE_UNDERFLOW (C99)

FE_DOWNWARDFE_TONEARESTFE_TOWARDZEROFE_UPWARD (C99)

FE_DFL_ENV (C99)

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.

On most implementations, these macro constants expand to the values equal to the values of FLT_ROUNDS and float_round_style

Constant	Explanation
FE_DOWNWARD	rounding towards negative infinity
FE_TONEAREST	rounding towards nearest integer
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

• string conversions such as strtod or printf

strtof("0.1", NULL); // 0.0999999940395355224609375
                  // or 0.100000001490116119384765625

• the library rounding functions nearbyint, rint, lrint

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.

Example

Run this code

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

• 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)

See also

fegetroundfesetround (C99)(C99)

gets or sets rounding direction (function)