Re: arm-coff-gcc: -fpack-struct and non-word-aligned ints?

[Toralf Lund <toralf at procaptura dot com>]

Richard Earnshaw wrote:

> On Mon, 2004-07-05 at 13:45, Toralf Lund wrote:
>  
>
> > I'm building some code with -fpack-struct on ARM cross-gcc because an 
> > old API expects it. This mostly works fine, but it turns out that I 
> > sometimes get word-alignment issues that I never saw on the platform I'm 
> > porting from (M68k, a completely different compiler.) I sometimes get 
> > int-type struct-members at addresses that are not word-aligned, although 
> > they are always *halfword*-aligned. In some, but not all (I'm not sure 
> > about the exact conditions) cases where this happens, I get into trouble 
> > if I assign a value to the variable via it's address. Simply put, it 
> > looks like the address is rounded down to the nearest word-boundary, so 
> > that if I do something like
> >
> > struct somestruct {
> >    short a;
> >    int b;
> > };
> > [ ... ]
> >
> > struct somestruct value;
> >
> > [ ... ]
> > int *bPtr=&value.b;
> >
> > *bPtr=-100;
> >
> > the assignment will (at least under certain conditions) not actually 
> > write to all 4 bytes of value.b's memory area, but rather value.a and 
> > the two first bytes of value.b.
> >
> > Is this expected to happen? 
> >    
>
> Yes.  You can't take the address of a packed structure member and then
> dereference it as a normal pointer.  If the underlying hardware/ABI only
> supports aligned types then only pointers that are correctly aligned can
> be dereferenced.  
>  
Fair enough.

I guess that it's logical that it would work on the 68K, then, as 32-bit 
accesses only need to be aligned to 16-bit words there, as far as I 
remember.


> > Is there another option or something that 
> > might make the problem go away?
> >    
>
> No.  GCC doesn't support pointer to unaligned fundamental type.
>  
OK. I also found out why I thought it sometimes worked; in fact I didn't 
take the address of a basic type in those cases, but of the struct 
itself instead. In other words, in my above example

struct somestruct *ptr=&value;

ptr->b=-100;


would probably do the right thing. I'm I right in assuming that this 
will always be allowed?

- T




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