Since spaces are in fact not allowed, choosing between 0 and 0x20 is pretty arbitrary here. You’d probably go for 0 today because C became popular and 0-padding

Since spaces are in fact not allowed, choosing between 0 and 0x20 is pretty arbitrary here. You’d probably go for 0 today because C became popular and 0-padding has the useful property of also 0-terminating the string, but at the time that was less of a consideration.