Alignment of bit-fields
You can declare
a bit-field as a _Bool (C), 
bool (C++), char, signed
char, unsigned char, short, unsigned
short
, int, unsigned int, long, unsigned
long, long long, or unsigned long long data
type. The alignment of a bit-field depends on its base type and
the compilation mode.
The length
of a bit-field cannot exceed the length of its base type. In extended
mode, you can use the sizeof operator on a bit-field. The sizeof operator on a bit-field returns
the size of the base type.
The length of a bit-field can exceed the length of its base
type, but the remaining bits are used to pad the field, and do not
actually store any value.
However, alignment rules for aggregates containing bit-fields are different depending on the alignment mode in effect. These rules are described below.
Rules for Linux PowerPC® alignment
- Bit-fields are allocated from a bit-field container. The size of this container is determined by the declared type of the bit-field. For example, a char bit-field uses an 8-bit container, and an int bit-field uses 32 bits. The container must be large enough to contain the bit-field, as the bit-field will not be split across containers.
- Containers are aligned in the aggregate as if they start on a natural boundary for that type of container. Bit-fields are not necessarily allocated at the start of the container.
- If a zero-length bit-field is the first member of an aggregate, it has no effect on the alignment of the aggregate and is overlapped by the next data member. If a zero-length bit-field is a non-first member of the aggregate, it pads to the next alignment boundary determined by its base declared type but does not affect the alignment of the aggregate.
- Unnamed bit-fields do not affect the alignment of the aggregate.
Rules for bit-packed alignment
- Bit-fields have an alignment of 1 byte and are packed with no default padding between bit-fields.
- A zero-length bit-field causes the next member to start at the next byte boundary. If the zero-length bit-field is already at a byte boundary, the next member starts at this boundary. A non-bit-field member that follows a bit-field is aligned on the next byte boundary.
