The external functions defined in, or called within, a portable program can be named
whatever the programmer wishes, as long as these names are distinct from the
external names defined by the Standard library (§7). External names in a maximally
portable program must be distinct within the first 31 characters (in C95, the first 6
35 characters mapped into one case) (see §5.2.4.1 and §6.4.2).