printing very large floating point numbers
c, c++, floating-point, printf, standards
Solution
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.
Problem
``` #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`.