5.7.3.11 Unsigned-char Option

The -funsigned-char option forces a plain char objects to have an unsigned type.

By default, the plain char type is equivalent to unsigned char. The -funsigned-char (or -fno-signed-char option) makes this type explicit.

Consider explicitly stating the signedness of char objects when they are defined, rather than relying on the type assigned to a plain char type by the compiler.