nearbyint, nearbyintf, nearbyintl
From cppreference.com
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) 
13) Rounds the floatingpoint argument
arg
to an integer value in floatingpoint format, using the current rounding mode.
4) Typegeneric 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.
Contents 
[edit] Parameters
arg    floating point value 
[edit] Return value
The nearest integer value to arg
, according to the current rounding mode, is returned.
[edit] Error handling
This function is not subject to any of the errors specified in math_errhandling.
If the implementation supports IEEE floatingpoint 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
[edit] Notes
The only difference between nearbyint
and rint is that nearbyint
never raises FE_INEXACT.
The largest representable floatingpoint values are exact integers in all standard floatingpoint 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).
[edit] Example
Run this code
#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
[edit] References
 C11 standard (ISO/IEC 9899:2011):

 7.12.9.3 The nearbyint functions (p: 251252)

 7.25 Typegeneric math <tgmath.h> (p: 373375)

 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 Typegeneric math <tgmath.h> (p: 335337)

 F.9.6.3 The nearbyint functions (p: 463)
[edit] See also
(C99)(C99)(C99)(C99)(C99)(C99)(C99)(C99)(C99)

rounds to an integer using current rounding mode with exception if the result differs (function) 
(C99)(C99)(C99)

rounds to nearest integer, rounding away from zero in halfway cases (function) 
(C99)(C99)

gets or sets rounding direction (function) 
C++ documentation for nearbyint
