Signedness of enum in C/C99/C++/C++x/GNU C/GNU C99

An enum is guaranteed to be represented by an integer, but the actual type (and its signedness) is implementation-dependent.

You can force an enumeration to be represented by a signed type by giving one of the enumerators a negative value:

enum SignedEnum { a = -1 };

In C++0x, the underlying type of an enumeration can be explicitly specified:

enum ShortEnum : short { a };

(C++0x also adds support for scoped enumerations)

For completeness, I'll add that in The C Programming Language, 2nd ed., enumerators are specified as having type int (p. 215). K&R is not the C standard, so that's not normative for ISO C compilers, but it does predate the ISO C standard, so it's at least interesting from a historical standpoint.