Datasheet

ARM Compiler Reference

Packed structures

A packed structure is one where the alignment of the structure, and of the fields within

it, is always 1. Floating-point types cannot be fields of packed structures.

Packed structures are defined with the

__packed

qualifier. (See ARM-specific keywords

on page 3-10.) There is no command-line option to change the default packing of

structures.

Bitfields

In nonpacked structures, the ARM compilers allocate 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:

Little-endian Lowest addressed means least significant.

Big-endian Lowest addressed means most significant.

A bitfield container can be any of the integral types.

Note

The compiler warns about non

int

bitfields. You can disable this warning with the

-Wb

compiler option.

A plain bitfield, declared without either

signed

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

. At the second

declaration, the compiler finds the existing integer container with a sufficient number

of unallocated bits, and allocates

in the same container as

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 above: