Re: ARM compiler misbehaves ?

[Richard Earnshaw <Richard dot Earnshaw at arm dot com>]

On Thu, 2004-04-29 at 02:22, Bill Gatliff wrote:
> Grant:
> 
> 
> >test_struct.test_short = 0x1234;
> >printf("%04x\n", test_struct.test_short);
> >
> >_may_ work.  I don't think it's guaranteed to work, though.
> >
> 
> Actually, I'm pretty sure it is guaranteed to work as long as the 
> compiler can see the "packed" attribute during compilation (if it 
> couldn't, that would be a serious structural problem in your source code!).
> 
> >>	test_pointer = &test_var.test_short;
> >>    
> >>
> >
> >test_pointer now points to an odd address. Dereferencing it
> >will almost certainly not work right, since the compiler
> >doesn't generate run-time checks for misaligned pointers.
> >

Wrong.  You can't take the address of a packed element and store it in a
normal pointer.  Normal pointers can't handle unaligned objects.

You'd need to create a structure containing only a packed short and then
play games using that.

It's sometimes a real shame that gcc doesn't have 

short attribute((packed)) *f;

but it doesn't, so the test case is not valid GNU C.

R.

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