On MacOS Classic (no memory protection) you can happily write to address zero and replace whatever is stored there (I think Apple wisely left that memory address unused, though).
Although char should not be used to store numbers, since its signedness is implementation-defined (gcc has the switch -funsigned-chars I think to make them unsigned instead of signed). signed char/unsigned char are ok.
Edit: And most likely in the excellent book "Hacker's Delight" as well.