5.7.3.6 Signed-bitfields Option

The -fsigned-bitfield option control the signedness of a plain int bit-field type.

By default, the plain int type, when used as the type of a bit-field, is equivalent to signed int. This option specifies the type that will be used by the compiler for plain int bit-fields. Using -fsigned-bitfield or the -fno-unsigned-bitfield option forces a plain int bit-field to be signed.

Consider explicitly stating the signedness of bit-fields when they are defined, rather than relying on the type assigned to a plain int bit-field type.