What is the size of an enum in C?

Question

I'm creating a set of enum values, but I need each enum value to be 64 bits wide. If I recall correctly, an enum is generally the same size as an int; but I thought I read somewhere that (at least in GCC) the compiler can make the enum any width they need to be to hold their values. So, is it possible to have an enum that is 64 bits wide?

Answer 1

Yes, that's exactly right. An enum is at least as big as needed to hold the values. (Actually the size needed to hold all the value OR'd together, but that's usually the same as the size of the largest)

Answer 2

An enum is only guaranteed to be large enough to hold int values. The compiler is free to choose the actual type used based on the enumeration constants defined so it can choose a smaller type if it can represent the values you define. If you need enumeration constants that don't fit into an int you will need to use compiler-specific extensions to do so.

Answer 3

Taken from the current C Standard (C99): http://www.open-std.org/JTC1/SC22/WG14/www/docs/n1256.pdf

6.7.2.2 Enumeration specifiers
[...]
Constraints
The expression that defines the value of an enumeration constant shall be an integer constant expression that has a value representable as an int.
[...]
Each enumerated type shall be compatible with char, a signed integer type, or an unsigned integer type. The choice of type is implementation-defined, but shall be capable of representing the values of all the members of the enumeration.

Not that compilers are any good at following the standard, but essentially: If your enum holds anything else than an int, you're in deep "unsupported behavior that may come back biting you in a year or two" territory.