round, roundf, roundl, lround, lroundf, lroundl, llround, llroundf, llroundl

 

C

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Headers

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Program utilities

Variadic function support

Error handling

Dynamic memory management

Strings library

Algorithms

Numerics

Date and time utilities

Input/output support

Localization support

Concurrency support (C11)

Technical Specifications

Symbol index

 

Numerics

Common mathematical functions

Floating-point environment (C99)

Pseudo-random number generation

Complex number arithmetic (C99)

Type-generic math (C99)

 

Common mathematical functions

Types
div_tldiv_tlldiv_timaxdiv_t (C99)(C99)     float_tdouble_t (C99)(C99)     _Decimal32_t_Decimal64_t (C23)(C23)
Functions
Basic operations abslabsllabsimaxabs (C99)(C99) fabs divldivlldivimaxdiv (C99)(C99) fmod remainder (C99) remquo (C99) fma (C99) fdim (C99) nannanfnanlnandN (C99)(C99)(C99)(C23) Maximum/minimum operations fmax (C99) fmaximum (C23) fmaximum_mag (C23) fmaximum_num (C23) fmaximum_mag_num (C23)     fmin (C99) fminimum (C23) fminimum_mag (C23) fminimum_num (C23) fminimum_mag_num (C23) Exponential functions exp exp10 (C23) exp2 (C99) expm1 (C99) exp10m1 (C23) exp2m1 (C23) log log10 log2 (C99) log1plogp1 (C99)(C23) log10p1 (C23) log2p1 (C23) Power functions sqrt cbrt (C99) rootn (C23) rsqrt (C23) hypot (C99) compound (C23) pow pown (C23) powr (C23) Trigonometric and hyperbolic functions sin cos tan asin acos atan atan2 sinpi (C23) cospi (C23) tanpi (C23) asinpi (C23) acospi (C23) atanpi (C23) atan2pi (C23) sinh cosh tanh asinh (C99) acosh (C99) atanh (C99)	Error and gamma functions erf (C99) erfc (C99) lgamma (C99) tgamma (C99) Nearest integer floating-point operations ceil floor roundlroundllround (C99)(C99)(C99) roundeven (C23) trunc (C99) nearbyint (C99) rintlrintllrint (C99)(C99)(C99) fromfpfromfpxufromfpufromfpx (C23)(C23)(C23)(C23) Floating-point manipulation functions ldexp frexp scalbnscalbln (C99)(C99) ilogbllogb (C99)(C23) logb (C99) modf nextafternexttoward (C99)(C99) nextupnextdown (C23)(C23) copysign (C99) canonicalize (C23) Narrowing operations fadd (C23) fsub (C23) fmul (C23) fdiv (C23) ffma (C23) fsqrt (C23) Quantum and quantum exponent functions quantizedN (C23) samequantumdN (C23) quantumdN (C23) llquantexpdN (C23) Decimal re-encoding functions encodedecdN (C23) decodedecdN (C23) encodebindN (C23) decodebindN (C23) Total order and payload functions totalorder (C23) getpayload (C23) setpayload (C23) setpayloadsig (C23) Classification fpclassify (C99) iscanonical (C23) isfinite (C99) isinf (C99) isnan (C99) isnormal (C99) signbit (C99) issubnormal (C23) iszero (C23) isgreater (C99) isgreaterequal (C99) isless (C99) islessequal (C99) islessgreater (C99) isunordered (C99) issignaling (C23) iseqsig (C23)
Macro constants
Special floating-point values HUGE_VALFHUGE_VALHUGE_VALLHUGE_VALDN (C99)(C99)(C23) INFINITYDEC_INFINITY (C99)(C23) NANDEC_NAN (C99)(C23) Arguments and return values FP_ILOGB0FP_ILOGBNAN (C99)(C99) FP_INT_UPWARDFP_INT_DOWNWARDFP_INT_TOWARDZEROFP_INT_TONEARESTFROMZEROFP_INT_TONEAREST (C23)(C23)(C23)(C23)(C23)     FP_LLOGB0FP_LLOGBNAN (C23)(C23) FP_NORMALFP_SUBNORMALFP_ZEROFP_INFINITEFP_NAN (C99)(C99)(C99)(C99)(C99)     Error handling MATH_ERRNOMATH_ERRNOEXCEPT (C99)(C99) math_errhandling (C99)	Fast operation indicators FP_FAST_FMAFFP_FAST_FMA (C99)(C99) FP_FAST_FADDFP_FAST_FADDLFP_FAST_DADDLFP_FAST_DMADDDN (C23)(C23)(C23)(C23)     FP_FAST_FMULFP_FAST_FMULLFP_FAST_DMULLFP_FAST_DMMULDN (C23)(C23)(C23)(C23) FP_FAST_FFMAFP_FAST_FFMALFP_FAST_DFMALFP_FAST_DMFMADN (C23)(C23)(C23)(C23) FP_FAST_FMALFP_FAST_FMADN (C99)(C23) FP_FAST_FSUBFP_FAST_FSUBLFP_FAST_DSUBLFP_FAST_DMSUBDN (C23)(C23)(C23)(C23) FP_FAST_FDIVFP_FAST_FDIVLFP_FAST_DDIVLFP_FAST_DMDIVDN (C23)(C23)(C23)(C23) FP_FAST_FSQRTFP_FAST_FSQRTLFP_FAST_DSQRTLFP_FAST_DMSQRTDN (C23)(C23)(C23)(C23)

 

Defined in header <math.h>
float       roundf( float arg );	(1)	(since C99)
double      round( double arg );	(2)	(since C99)
long double roundl( long double arg );	(3)	(since C99)
Defined in header <tgmath.h>
#define round( arg )	(4)	(since C99)
Defined in header <math.h>
long      lroundf( float arg );	(5)	(since C99)
long      lround( double arg );	(6)	(since C99)
long      lroundl( long double arg );	(7)	(since C99)
Defined in header <tgmath.h>
#define lround( arg )	(8)	(since C99)
Defined in header <math.h>
long long llroundf( float arg );	(9)	(since C99)
long long llround( double arg );	(10)	(since C99)
long long llroundl( long double arg );	(11)	(since C99)
Defined in header <tgmath.h>
#define llround( arg )	(12)	(since C99)

1-3) Computes the nearest integer value to arg (in floating-point format), rounding halfway cases away from zero, regardless of the current rounding mode.

5-7, 9-11) Computes the nearest integer value to arg (in integer format), rounding halfway cases away from zero, regardless of the current rounding mode.

4,8,12) Type-generic macros: If arg has type long double, roundl, lroundl, llroundl is called. Otherwise, if arg has integer type or the type double, round, lround, llround is called. Otherwise, roundf, lroundf, llroundf is called, respectively.

Parameters

arg

-

floating-point value

Return value

If no errors occur, the nearest integer value to arg, rounding halfway cases away from zero, is returned.

Return value

Argument

If a domain error occurs, an implementation-defined value is returned.

Error handling

Errors are reported as specified in math_errhandling.

If the result of lround or llround is outside the range representable by the return type, a domain error or a range error may occur.

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

For the round, roundf, and roundl function:

• The current rounding mode has no effect.
• If arg is ±∞, it is returned, unmodified.
• If arg is ±0, it is returned, unmodified.
• If arg is NaN, NaN is returned.

For lround and llround families of functions:

• FE_INEXACT is never raised.
• The current rounding mode has no effect.
• If arg is ±∞, FE_INVALID is raised and an implementation-defined value is returned
• If the result of the rounding is outside the range of the return type, FE_INVALID is raised and an implementation-defined value is returned.
• If arg is NaN, FE_INVALID is raised and an implementation-defined value is returned.

Notes

FE_INEXACT may be (but isn't required to be) raised by round when rounding a non-integer finite value.

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

POSIX specifies that all cases where lround or llround raise FE_INVALID are domain errors.

The double version of round behaves as if implemented as follows:

#include <math.h>
#pragma STDC FENV_ACCESS ON
 
double round(double x)
{
    return signbit(x) ? ceil(x - 0.5) : floor(x + 0.5);
}

Example

Run this code

#include <assert.h>
#include <fenv.h>
#include <float.h>
#include <limits.h>
#include <math.h>
#include <stdio.h>
// #pragma STDC FENV_ACCESS ON
 
double custom_round(double x)
{
    return signbit(x) ? ceil(x - 0.5) : floor(x + 0.5);
}
 
void test_custom_round()
{
    const double sample[] =
    {
        0.0, 2.3, 2.5 - DBL_EPSILON, 2.5, 2.5 + DBL_EPSILON, 2.7, INFINITY
    };
    for (size_t t = 0; t < sizeof sample / sizeof(double); ++t)
        assert(round(+sample[t]) == custom_round(+sample[t]) &&
               round(-sample[t]) == custom_round(-sample[t]));
}
 
int main(void)
{
    // round
    printf("round(+2.3) = %+.1f  ", round(2.3));
    printf("round(+2.5) = %+.1f  ", round(2.5));
    printf("round(+2.7) = %+.1f\n", round(2.7));
    printf("round(-2.3) = %+.1f  ", round(-2.3));
    printf("round(-2.5) = %+.1f  ", round(-2.5));
    printf("round(-2.7) = %+.1f\n", round(-2.7));
 
    printf("round(-0.0) = %+.1f\n", round(-0.0));
    printf("round(-Inf) = %+f\n",   round(-INFINITY));
 
    test_custom_round();
 
    // lround
    printf("lround(+2.3) = %+ld  ", lround(2.3));
    printf("lround(+2.5) = %+ld  ", lround(2.5));
    printf("lround(+2.7) = %+ld\n", lround(2.7));
    printf("lround(-2.3) = %+ld  ", lround(-2.3));
    printf("lround(-2.5) = %+ld  ", lround(-2.5));
    printf("lround(-2.7) = %+ld\n", lround(-2.7));
 
    printf("lround(-0.0) = %+ld\n", lround(-0.0));
    printf("lround(-Inf) = %+ld\n", lround(-INFINITY)); // FE_INVALID raised
 
    // error handling
    feclearexcept(FE_ALL_EXCEPT);
    printf("lround(LONG_MAX+1.5) = %ld\n", lround(LONG_MAX + 1.5));
    if (fetestexcept(FE_INVALID))
        puts("    FE_INVALID was raised");
}

Possible output:

round(+2.3) = +2.0  round(+2.5) = +3.0  round(+2.7) = +3.0
round(-2.3) = -2.0  round(-2.5) = -3.0  round(-2.7) = -3.0
round(-0.0) = -0.0
round(-Inf) = -inf
lround(+2.3) = +2  lround(+2.5) = +3  lround(+2.7) = +3
lround(-2.3) = -2  lround(-2.5) = -3  lround(-2.7) = -3
lround(-0.0) = +0
lround(-Inf) = -9223372036854775808
lround(LONG_MAX+1.5) = -9223372036854775808
    FE_INVALID was raised

References

• C23 standard (ISO/IEC 9899:2024):

• 7.12.9.6 The round functions (p: TBD)

• 7.12.9.7 The lround and llround functions (p: TBD)

• 7.25 Type-generic math <tgmath.h> (p: TBD)

• F.10.6.6 The round functions (p: TBD)

• F.10.6.7 The lround and llround functions (p: TBD)

• C17 standard (ISO/IEC 9899:2018):

• 7.12.9.6 The round functions (p: 184)

• 7.12.9.7 The lround and llround functions (p: 184-185)

• 7.25 Type-generic math <tgmath.h> (p: 272-273)

• F.10.6.6 The round functions (p: 384)

• F.10.6.7 The lround and llround functions (p: 385)

• C11 standard (ISO/IEC 9899:2011):

• 7.12.9.6 The round functions (p: 253)

• 7.12.9.7 The lround and llround functions (p: 253)

• 7.25 Type-generic math <tgmath.h> (p: 373-375)

• F.10.6.6 The round functions (p: 527)

• F.10.6.7 The lround and llround functions (p: 528)

• C99 standard (ISO/IEC 9899:1999):

• 7.12.9.6 The round functions (p: 233)

• 7.12.9.7 The lround and llround functions (p: 234)

• 7.22 Type-generic math <tgmath.h> (p: 335-337)

• F.9.6.6 The round functions (p: 464)

• F.9.6.7 The lround and llround functions (p: 464)

See also

floorfloorffloorl (C99)(C99)	computes largest integer not greater than the given value (function)
ceilceilfceill (C99)(C99)	computes smallest integer not less than the given value (function)
trunctruncftruncl (C99)(C99)(C99)	rounds to nearest integer not greater in magnitude than the given value (function)
C++ documentation for round