On 29 May 2013 14:24, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> On Wed, May 29, 2013 at 07:52:37AM -0500, Anthony Liguori wrote:
>> 1) C makes no guarantees about structure layout beyond the first
>> member. Yes, if it's naturally aligned or has a packed attribute,
>> GCC does the right thing. But this isn't kernel land anymore,
>> portability matters and there are more compilers than GCC.
>
> You expect a compiler to pad this structure:
>
> struct foo {
> uint8_t a;
> uint8_t b;
> uint16_t c;
> uint32_t d;
> };
>
> I'm guessing any compiler that decides to waste memory in this way
> will quickly get dropped by users and then we won't worry
> about building QEMU with it.
Structure alignment is a platform ABI choice. Oddly enough people
choose operating systems on more criteria than the amount of
padding their ABI mandates in structures.
In any case it's really tedious to read a structure and wade
through working out whether it's going to have all its members
naturally aligned, especially once it's more than a handful
of members in size. And getting it wrong is "silent portability
problem".
thanks
-- PMM
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