Question

#include <stdio.h>
#include <float.h>

int main()
{
    printf("%f\n", FLT_MAX);
}

Output from GNU:

340282346638528859811704183484516925440.000000

Output from Visual Studio:

340282346638528860000000000000000000000.000000

Do the C and C++ standards allow both results? Or do they mandate a specific result?

Note that FLT_MAX = 2^128-2^104 = 340282346638528859811704183484516925440.

Answer 1

I think the relevant part of the C99 standard is the “Recommended practice” from 7.19.6.1 p.13:

For e, E, f, F, g, and G conversions, if the number of significant decimal digits is at most
DECIMAL_DIG, then the result should be correctly rounded. If the number of
significant decimal digits is more than DECIMAL_DIG but the source value is exactly
representable with DECIMAL_DIG digits, then the result should be an exact
representation with trailing zeros. Otherwise, the source value is bounded by two
adjacent decimal strings L < U, both having DECIMAL_DIG significant digits; the value
of the resultant decimal string D should satisfy L <= D <= U, with the extra stipulation that
the error should have a correct sign for the current rounding direction.

My impression is that this allows some leeway in what may be printed in this case; so my conclusion is that both VS and GCC are compliant here.