Structures, unions, enumerations, and bitfields

When a member of a union is accessed using a member of a different type, the resulting value can be predicted from the representation of the original type. No error is given.

An object of type enum is implemented in the smallest integral type that contains the range of the enum.

In C mode, and in C++ mode without --enum_is_int, if an enum contains only positive enumerator values, the storage type of the enum is the first unsigned type from the following list, according to the range of the enumerators in the enum. In other modes, and in cases where an enum contains any negative enumerator values, the storage type of the enum is the first of the following, according to the range of the enumerators in the enum:

Implementing enum in this way can reduce data size. The command-line option --enum_is_int forces the underlying type of enum to at least as wide as int.

See the description of C language mappings in the Procedure Call Standard for the ARM Architecture specification for more information.

#66: enumeration value is out of "int" range

armcc --diag_error=66 ...

The following points apply to:

Structures can contain padding to ensure that fields are correctly aligned and that the structure itself is correctly aligned. Figure 3 shows an example of a conventional, nonpacked structure. Bytes 1, 2, and 3 are padded to ensure correct field alignment. Bytes 11 and 12 are padded to ensure correct structure alignment. The sizeof() function returns the size of the structure including padding.

Figure 3. Conventional nonpacked structure example

The compiler pads structures in one of the following ways, according to how the structure is defined:

Use the --remarks option to view the messages that are generated when the compiler inserts padding in a struct.

Structures with empty initializers are permitted in C++:

struct
{
    int x;
} X = { };

However, if you are compiling C, or compiling C++ with the --cpp and--c90 options, an error is generated.

A packed structure is one where the alignment of the structure, and of the fields within it, is always 1.

You can pack specific structures with the __packed qualifier. Alternatively, you can use #pragma pack(n) to make sure that any structures with unaligned data are packed. There is no command-line option to change the default packing of structures.

In nonpacked structures, the ARM compiler allocates bitfields in containers. A container is a correctly aligned object of a declared type.

Bitfields are allocated so that the first field specified occupies the lowest-addressed bits of the word, depending on configuration:

A bitfield container can be any of the integral types.

A plain bitfield, declared without either signed or unsigned qualifiers, is treated as unsigned. For example, int x:10 allocates an unsigned integer of 10 bits.

A bitfield is allocated to the first container of the correct type that has a sufficient number of unallocated bits, for example:

struct X
{
    int x:10;
    int y:20;
};

The first declaration creates an integer container and allocates 10 bits to x. At the second declaration, the compiler finds the existing integer container with a sufficient number of unallocated bits, and allocates y in the same container as x.

A bitfield is wholly contained within its container. A bitfield that does not fit in a container is placed in the next container of the same type. For example, the declaration of z overflows the container if an additional bitfield is declared for the structure:

struct X
{
    int x:10;
    int y:20;
    int z:5;
};

The compiler pads the remaining two bits for the first container and assigns a new integer container for z.

Bitfield containers can overlap each other, for example:

struct X
{
    int x:10;
    char y:2;
};

The first declaration creates an integer container and allocates 10 bits to x. These 10 bits occupy the first byte and two bits of the second byte of the integer container. At the second declaration, the compiler checks for a container of type char. There is no suitable container, so the compiler allocates a new correctly aligned char container.

Because the natural alignment of char is 1, the compiler searches for the first byte that contains a sufficient number of unallocated bits to completely contain the bitfield. In the example structure, the second byte of the int container has two bits allocated to x, and six bits unallocated. The compiler allocates a char container starting at the second byte of the previous int container, skips the first two bits that are allocated to x, and allocates two bits to y.

If y is declared char y:8, the compiler pads the second byte and allocates a new char container to the third byte, because the bitfield cannot overflow its container. Figure 4 shows the bitfield allocation for the following example structure:

struct X
{
    int x:10;
    char y:8;
};

Figure 4. Bitfield allocation 1

struct X
{
    int x:10;
    char y:8;
    int z:5;
}

The compiler allocates an int container starting at the same location as the int x:10 container and allocates a byte-aligned char and 5-bit bitfield, see Figure 5.

Figure 5. Bitfield allocation 2

You can explicitly pad a bitfield container by declaring an unnamed bitfield of size zero. A bitfield of zero size fills the container up to the end if the container is not empty. A subsequent bitfield declaration starts a new empty container.

Bitfield containers in packed structures have an alignment of 1. Therefore, the maximum bit padding for a bitfield in a packed structure is 7 bits. For an unpacked structure, the maximum padding is 8*sizeof(container-type)-1 bits.