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nearbyint, nearbyintf, nearbyintl

Defined in header `<math.h>`
float nearbyintf( float arg );	(1)	(since C99)
double nearbyint( double arg );	(2)	(since C99)
long double nearbyintl( long double arg );	(3)	(since C99)
Defined in header `<tgmath.h>`
#define nearbyint( arg )	(4)	(since C99)

1-3) Rounds the floating-point argument arg to an integer value in floating-point format, using the current rounding mode.

4) Type-generic macro: If arg has type long double, nearbyintl is called. Otherwise, if arg has integer type or the type double, nearbyint is called. Otherwise, nearbyintf is called, respectively.

Parameters

arg

floating point value

Return value

The nearest integer value to arg, according to the current rounding mode, is returned.

Error handling

This function is not subject to any of the errors specified in math_errhandling.

If the implementation supports IEEE floating-point arithmetic (IEC 60559),

FE_INEXACT is never raised
If arg is ±∞, it is returned, unmodified
If arg is ±0, it is returned, unmodified
If arg is NaN, NaN is returned

Notes

The only difference between nearbyint and rint is that nearbyint never raises FE_INEXACT.

The largest representable floating-point values are exact integers in all standard floating-point formats, so nearbyint never overflows on its own; however the result may overflow any integer type (including intmax_t), when stored in an integer variable.

If the current rounding mode is FE_TONEAREST, this function rounds to even in halfway cases (like rint, but unlike round).

Example

#include <stdio.h>
#include <math.h>
#include <fenv.h>
 
int main(void)
{
#pragma STDC FENV_ACCESS ON
    fesetround(FE_TONEAREST);
    printf("rounding to nearest:\nnearbyint(+2.3) = %+.1f  ", nearbyint(2.3));
    printf("nearbyint(+2.5) = %+.1f  ", nearbyint(2.5));
    printf("nearbyint(+3.5) = %+.1f\n", nearbyint(3.5));
    printf("nearbyint(-2.3) = %+.1f  ", nearbyint(-2.3));
    printf("nearbyint(-2.5) = %+.1f  ", nearbyint(-2.5));
    printf("nearbyint(-3.5) = %+.1f\n", nearbyint(-3.5));
 
    fesetround(FE_DOWNWARD);
    printf("rounding down: \nnearbyint(+2.3) = %+.1f  ", nearbyint(2.3));
    printf("nearbyint(+2.5) = %+.1f  ", nearbyint(2.5));
    printf("nearbyint(+3.5) = %+.1f\n", nearbyint(3.5));
    printf("nearbyint(-2.3) = %+.1f  ", nearbyint(-2.3));
    printf("nearbyint(-2.5) = %+.1f  ", nearbyint(-2.5));
    printf("nearbyint(-3.5) = %+.1f\n", nearbyint(-3.5));
 
    printf("nearbyint(-0.0) = %+.1f\n", nearbyint(-0.0));
    printf("nearbyint(-Inf) = %+.1f\n", nearbyint(-INFINITY));
}

Output:

rounding to nearest:
nearbyint(+2.3) = +2.0  nearbyint(+2.5) = +2.0  nearbyint(+3.5) = +4.0
nearbyint(-2.3) = -2.0  nearbyint(-2.5) = -2.0  nearbyint(-3.5) = -4.0
rounding down: 
nearbyint(+2.3) = +2.0  nearbyint(+2.5) = +2.0  nearbyint(+3.5) = +3.0
nearbyint(-2.3) = -3.0  nearbyint(-2.5) = -3.0  nearbyint(-3.5) = -4.0
nearbyint(-0.0) = -0.0
nearbyint(-Inf) = -inf

References

C11 standard (ISO/IEC 9899:2011):

7.12.9.3 The nearbyint functions (p: 251-252)
7.25 Type-generic math <tgmath.h> (p: 373-375)
F.10.6.3 The nearbyint functions (p: 526)

C99 standard (ISO/IEC 9899:1999):

7.12.9.3 The nearbyint functions (p: 232)
7.22 Type-generic math <tgmath.h> (p: 335-337)
F.9.6.3 The nearbyint functions (p: 463)

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)
roundlroundllround (C99)(C99)(C99)	rounds to nearest integer, rounding away from zero in halfway cases (function)
fegetroundfesetround (C99)(C99)	gets or sets rounding direction (function)
C++ documentation for `nearbyint`