Boaz Harrosh wrote:

Eric Sandeen wrote:

quoted

Boaz Harrosh wrote:

...

quoted

I don't understand

if you have a structure like
struct foo {
	u32 one;
	u32 two;
};
vs
struct foo_packed {
	u32 one;
	u32 two;
} __packed;

Just adding an __attribute__((packed)) to it clearly does not change
the layout of the structure. Are you saying the __attribute__((packed))
is an hint to the compiler that foo_packed might be used unaligned. This
is just brain-dead, because I can use an unaligned pointer to foo just as
I can to foo_packed. Otherwise there is no difference what-so-ever between
the two. I have to see it to believe. It is totally the wrong hint in the
wrong place taking away valuable meaning of saying "please don't use padding
holes in this structure"

Sorry for been so slow, I just don't get it.
Boaz

While I'm no gcc guru, I can confirm that gratuitous use of the packed
attribute is suboptimal; adding "packed" to every ondisk structure made
obdump -d xfs.ko | wc -l explode by about 15,000 lines on ia64.

Yes! but are the structures the same? that is sizeof(foo_packed) == sizeof(foo) ?
If not then clearly above is expected.

Yes, they are the same.  They're disk structure definitions after all;
ia64 doesn't *need* the packing, but adding the packed attribute changes
the code that gcc generates.

See also, perhaps,
http://digitalvampire.org/blog/index.php/2006/07/31/why-you-shouldnt-use-__attribute__packed/

For an interface like this maybe it's fine, but sprnkling it around like
pixie dust may not be a good plan.  :)

-Eric

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