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Re: Alignement on ARM

On 05/12/2010 01:21 PM, Vincent Bernat wrote:

While I have fixed my warnings, I  am still a bit astonished that such a
common code as :
  sa = (struct sockaddr_in *)ifa->ifa_addr;
  if (... sa->sin_addr ...) ...
can  lead to  a  misaligned access  on  ARM.  Maybe  there is  something
somewhere that  ensure that struct  sockaddr and struct  sockaddr_ll are
always aligned correctly on a 4-byte boundary?

Because with enough casting and whatnot, any magic number can get plugged into ifa_addr.

Look carefully at where the value that gets assigned to ifa_addr comes from. If it truly is the address of an independent structure that's allocated by the compiler (stack, data or code), then it's highly unlikely to be misaligned because IIRC all structures begin on 32-bit address boundaries. I think there's even some language in the C standard about that. I don't think your example code will exhibit any problems.

If ifa_addr is instead the address of something inside of another structure, and that structure is "packed", then ifa_addr could be a misaligned pointer because of the structure packing. And if ifa_addr is a casted pointer from a location in a byte stream, or a calculated value, then only $deity$ knows what you can say about it other than it's bad, bad code. :)


Bill Gatliff

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