RealView Compilation Tools Compiler Reference Guide - ARM ...

C and C++ Implementation Details
Note
The same basic rules apply to bitfield declarations with different container types. For
example, adding an int bitfield to the example structure gives:
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 5-3.
Bit number
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
free
z
y
padding
x
Figure 5-3 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.
Bitfields in packed structures
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.
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