Using the GNU Compiler Collection (GCC): Structures unions enumerations and bit-fields implementation

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### 4.9 Structures, unions, enumerations, and bit-fields

- A member of a union object is accessed using a member of a different type (C90 6.3.2.3).

  The relevant bytes of the representation of the object are treated as an object of the type used for the access. See Type-punning. This may be a trap representation.

- Whether a “plain” `int` bit-field is treated as a `signed int` bit-field or as an `unsigned int` bit-field (C90 6.5.2, C90 6.5.2.1, C99 and C11 6.7.2, C99 and C11 6.7.2.1).

  By default it is treated as `signed int` but this may be changed by the `-funsigned-bitfields` option.

- Allowable bit-field types other than `_Bool`, `signed int`, and `unsigned int` (C99 and C11 6.7.2.1).

  Other integer types, such as `long int`, and enumerated types are permitted even in strictly conforming mode.

- Whether atomic types are permitted for bit-fields (C11 6.7.2.1).

  Atomic types are not permitted for bit-fields.

- Whether a bit-field can straddle a storage-unit boundary (C90 6.5.2.1, C99 and C11 6.7.2.1).

  Determined by ABI.

- The order of allocation of bit-fields within a unit (C90 6.5.2.1, C99 and C11 6.7.2.1).

  Determined by ABI.

- The alignment of non-bit-field members of structures (C90 6.5.2.1, C99 and C11 6.7.2.1).

  Determined by ABI.

- The integer type compatible with each enumerated type (C90 6.5.2.2, C99 and C11 6.7.2.2).

  Normally, the type is `unsigned int` if there are no negative values in the enumeration, otherwise `int`. If `-fshort-enums` is specified, then if there are negative values it is the first of `signed char`, `short` and `int` that can represent all the values, otherwise it is the first of `unsigned char`, `unsigned short` and `unsigned int` that can represent all the values.

  On some targets, `-fshort-enums` is the default; this is determined by the ABI.

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