Use `signed char' explicitly where needed. It is important because:
- It is implementation-dependent if plain `char' signed or not.
- C standard defines three *distinct* types: char, signed char,
  and unsigned char.
- Assuming that char is always unsigned or signed can lead to
  compile-time and run-time errors.

You can also use int8_t, but then it would be less obvious for developers
to never assume that char is always unsigned (or always signed).