I won't pretend to understand all the subtleties of
__attribute__((packed)), but I did learn something about it when I
hacked USB/IP to run on ARM, for the NSLU2, last year. The issue
boiled down to this:
struct S {
int intfield;
char charfield;
} /* maybe add: __attribute__((packed)) here */ ;
f(struct S * s) {
...
(s->intfield)++;
...
}
g() {
char c;
struct S s;
(s.intfield)++;
f(&s);
}
If there are no attributes anywhere this will obviously work. There
will be a 3-byte gap between c and s.
What happens when we add __attribute__((packed)) to struct S? In g(),
s will now be placed immediately after c. So s.intfield is unaligned.
On x86, this doesn't matter. On ARM (etc.) this does matter, and
inside g() the compiler will use about 8 instructions to do the ++. It
will work, but a lot more slowly. However, the ++ in f() WILL NOT
WORK! The compiler assumes that the pointer that is passed IS
NATURALLY ALIGNED, and when you pass an unaligned pointer from g() IT
DOES NOT ISSUE A WARNING! I believe that the MMU can be configured to
generate a bus error in this case, but this seemed to be disabled on
the NSLU2 kernel and it just silently continued having not done the
store. There could be some difference between kernel and user code in
this respect.
Based on my experiece - it took me a long time to debug this - I would
suggest not adding __attribute__((packed)) to a struct unless it is
absolutely necessary, and if you do add it you should then check every
place where an address of a struct of that type is taken, and see how
that pointer is used. If the pointer is just cast to a char* (i.e. the
struct is treated as a lump of memory) you're safe; but if fields of
the struct are accessed via a pointer there could be problems (with no
compiler warnings).
Sorry if I'm just repeating stuff that you all already know....
Phil.
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