In a scientific application, that exact floating point representation could be the literal you gave it, but it could also be the result of math that has been rounded to fit in the 53 bit mantissa. For example perl -MPOSIX -e “printf ‘%.0f’, POSIX::DBL_MAX - 10000000000000000;” Now the very precise answer from gcc is wrong. If you actually care about the digits beyond the first 17 then you need to use a BigFloat.

Anyway, but no I don’t know if there’s a way to ask ms libc for a different implementation.