On Wednesday 06 December 2006 16:48, Phil Endecott wrote:
> Let me make this concrete again. With this test program:
>
> struct test {
> int a __attribute__((packed));
> int b __attribute__((packed));
> };
>
> char c = 1;
> struct test t = { .a=2, .b=3 };
>
> what can I assume about the alignment of t.a? As I now understand it,
> t.a and t.b both have alignment of 1, so t has alignment of 1, so t can
> be packed immediately after c with no gap. It is then unsafe (on some
> hardware) to take &t.a and pass it to a function that wants an int*.
Correct. It may be unsafe on all hardware because the compiler is allowed to
assume that the low 2 bits of an int* are zero.
> On the other hand, on ARM it leaves a gap:
> struct test {
> int a;
> int b;
> } __attribute__((packed));
>
> then the ARM compiler generates the same layout as the x86 compiler,
> without the gap.
This is because the ARM ABI you're using specifies that all structures have a
minimum alignment of 4 bytes. ie.
struct foo {
char c;
};
has size and alignmet 4.
Making the structure packed overrides that.
Paul
