RealView Compilation Tools Compiler Reference Guide - ARM ...

C and C++ Implementation Details
Packed structures
A packed structure is one where the alignment of the structure, and of the fields within
it, is always 1.
Packed structures are defined with the __packed qualifier. There is no command-line
option to change the default packing of structures.
Bitfields
In non-packed 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:
Little-endian
Big-endian
Lowest addressed means least significant.
Lowest addressed means most significant.
A bitfield container can be any of the integral types.
Note
In strict 1990 ISO Standard C, the only types permitted for a bit field are int, signed
int, and unsigned int. For non int bitfields, the compiler displays an error.
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:
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