difference between "rint" and "nearbyint"?

Question

what is the difference between rint and nearbyint? Will they give some different output in some cases? If not, is there a difference in the concept of calculations?

Answer 1

Since these are both C functions, we can check the man page for both of these. An excerpt:

The nearbyint() functions round their argument to an integer value in floating-point format, using the current rounding direction (see fesetround(3)) and without raising the inexact exception.

The rint() functions do the same, but will raise the inexact exception (FE_INEXACT, checkable via fetestexcept(3)) when the result differs in value from the argument.

In other words, rint allows you to do error checking while nearbyint does not. An example of error-checking:

#include <iostream>
#include <cmath>
#include <cfenv>

int main()
{
    std::feclearexcept(FE_INEXACT);
    double a = std::rint(93819.249);
    if (!std::fetestexcept(FE_INEXACT))
        std::cerr << "Bad rounding\n";
    else
        std::cout << a << '\n';
}