Getting rid of alignment faults in userspace
paul at codesourcery.com
Sat Jun 18 08:42:01 UTC 2011
> > > char buf;
> > > void *v = &buf;
> > > unsigned int *p = (unsigned int *)v;
> > This does not (reliably) do what you expect. The compiler need not align
> > buf.
> Printing the value of p should clarify this.
> And, as we can see above, the "simple" accesses are left to the hardware
> to fix up. However, if the misaligned access is performed using a
> 64-bit value pointer, then the kernel will trap an exception and the
> access will be simulated.
I think you've missed my point. gcc may (though unlikely in this case) choose
to place buf at an odd address. In which case p will happen to be properly
I'm not sure where you get "64-bit value pointer" from. *p is only a word
sized access, and memcpy is defined in terms of bytes so will only be promoted
to wider accesses when the compiler believes it is safe.
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