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

Parameters

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

#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");
}

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

See also