Messages in this thread | | | Date | Sun, 01 Feb 2009 10:35:50 -0600 | From | Eric Sandeen <> | Subject | Re: [xfs-masters] [PATCH] fs: Add new pre-allocation ioctls to vfs for compatibility with legacy xfs ioctls |
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Boaz Harrosh wrote: > Eric Sandeen wrote: >> Boaz Harrosh wrote: >> >> ... >> >>> 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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