Sorry, there's a "stream of thought" here... I guess the real post worth
of attention starts at the "OR..." below, and the rest is
just context ;)
On 2013-04-26 16:27, Jean-Pierre wrote:
Jim Klimov wrote:
On 2013-04-26 09:30, Jean-Pierre wrote:
struct io_stack_location {
uint8_t isl_major;
uint8_t isl_minor;
uint8_t isl_flags;
uint8_t isl_ctl;
...
union {
struct {
uint32_t isl_len;
/* "POINTER_ALIGNMENT" according to msdn */
uint32_t *isl_key;
uint64_t isl_byteoff;
} isl_read;
...
But in this case you have 4 1-byte values an 1 4-byte value
and then the pointer, doesn't this make the pointer 8-byte
aligned without fillers? (And the isl_len thus starts at
the offset of 4?)
No, because the inner structs have to stand by themselves.
For instance you must be able to define a pointer to the
inner struct "isl_read", and the fields must be aligned
from the pointer.
Hm... that is weird. I wonder if it is at all possible to define
the structure in such a manner that both the offset to the pointer
from start of the "mother" structure and from start of isl_read
structure would both be divisible by 8?
Should the start of the inner structure also be 8-aligned from the
start of its parent structure, or there is no such requirement?
For example, this test (based on your experience):
|| (offsetof(struct io_stack_location,
isl_parameters.isl_read.isl_len) != 8)
|| (offsetof(struct io_stack_location,
isl_parameters.isl_read.isl_byteoff) != 24)
implies that there is a 4-byte padding after the four single-bytes
before the unions (or, perhaps, should that be in the start of the
sub-structure itself?) Then there is a 4-byte isl_len which should
be at offset 8 from start of mother-structure, and the pointer
(8-bytes long?) isl_key...
The latter would be starting at either offset 8 or 4 from child
struct (depending on where we put the padding between isl_ctl and
isl_len, and at offset 12 from parent structure.
OR...
+ parent struct:
* 4 single-bytes (they do remain single, right?..)
* 4-byte padding (...or the single-bytes should be 4*2-byte words)
++ Sub-structure in the union (8-aligned itself):
** 4-byte isl_len (starts at offset 8 from parent struct)
** 4-byte padding
** 8-byte pointer (8-aligned in both structures)
** 8-byte offset (starts at offset 24 from parent struct)
Does this layout fit the actual drivers? :)
If so, some more #ifdef padding fields are needed, but I guess I can
craft them based on your offset numbers...
Does the first 4-byte padding belong in the child struct or in the
parent, prefixing all variations of the union sub-structures?
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