In C, the element type of a file-scope array can be an incomplete struct or union type. The element type must be completed before its size is needed, for example, if the array is subscripted. If the array is not extern, the element type must be completed by the end of the compilation.

The final semicolon preceding the closing brace } of a struct or union specifier can be omitted. A warning is issued.

An initializer expression that is a single value and is used to initialize an entire static array, struct, or union, does not have to be enclosed in braces. ISO C requires the braces.

An extension is supported to enable constructs similar to C++ anonymous unions, including the following:

To enable support for anonymous structures and unions, you must use the anon_unions pragma.

An extra comma is permitted at the end of an enum list but a remark is issued.

enum tags can be incomplete. You can define the tag name and resolve it later, by specifying the brace-enclosed list.

The values of enumeration constants can be given by expressions that evaluate to unsigned quantities that fit in the unsigned int range but not in the int range. For example:

/* When ints are 32 bits: */
enum a { w = -2147483648 };  /* No error */
enum b { x = 0x80000000 };   /* No error */
enum c { y = 0x80000001 };   /* No error */
enum d { z = 2147483649 };   /* Error */

Bit fields can have base types that are enum types or integral types besides int and unsigned int.