An enum type is represented by an underlying integer type. The size of the integer type and whether it is signed is based on the range of values of the enumerated constants.
In strict C89/C99/C11 mode, the compiler allows only enumeration constants with values that will fit in "int" or "unsigned int".
For C++ and relaxed C89/C99/C11, the compiler allows enumeration constants up to the largest integral type (64 bits). The default, which is recommended, is for the underlying type to be the first type in the following list in which all the enumerated constant values can be represented: int, unsigned int, long, unsigned long, long long, unsigned long long.