What is the size of an enum in C?

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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.

Update: The latest publicly available draft of the C Standard (C11): http://www.open-std.org/JTC1/SC22/WG14/www/docs/n1570.pdf contains the same clauses. Hence, this answer still holds for C11.

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