Re: ARM compiler misbehaves ?

[Bill Gatliff <bgat at billgatliff dot com>]

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.
>
>  
>
> > 	*test_pointer = 0x789A;
> >    
>
> Doing a 16-bit access to an odd address is undefined on ARM.

Yep.  The above is 'bad day' code waiting to happen.  Legal in "straight 
C", gets potentially broken when a non-portable, non-standard operation 
like the packed attribute is used.

If you aren't sure we're right, take out the "packed" attribute and try 
again.  It should work fine then.  Also, stop the debugger after the 
assignment to test_pointer and examine its value: it should be 16-bit 
aligned.



b.g.

--
Bill Gatliff
Professional embedded GNU training.
bgat@billgatliff.com




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