The unsigned keyword indicates that the most significant bit of an integer variable represents a data bit rather than a signed bit. This keyword is optional and can be used with any of the character and integer types char, wchar_t, int, long, short, and small.
When you use the MIDL compiler switch /char, character and integer types that appear in the IDL file without explicit sign keywords can appear with the signed or unsigned keyword in the generated header file. To avoid confusion, explicitly specify the sign of the integer and character types.
base_types, /char, IDL, int, long, short, signed, small