The C standard (see C23 5.2.1p3) requires the values of '0' through '9' to be contiguous, so it doesn't matter if you only care about round-tripping. '7' - '0' == 7 no matter the character set. Though, for round-tripping I suppose this isn't strictly necessary, but it certainly makes parsing and printing decimal notation very convenient. Notably, for both ASCII and EBCDIC 'A'..'F' and 'a'..'f' are also contiguous, so parsing and printing hexadecimal can be done much the same as decimal.