[U-Boot] packed attribute problem

[Detlev Zundel <dzu@denx.de>]

Hi Scott,

> On Thu, 7 Oct 2010 21:31:43 +0200
> Wolfgang Denk <wd@denx.de> wrote:
>
>> Dear Scott Wood,
>> 
>> In message <20101007135257.05a93975@udp111988uds.am.freescale.net> you wrote:
>> >
>> > > It is a pretty common method to use a pointer to some struct (for
>> > > example, some form of PDU) and make it point to some I/O buffer.
>> > 
>> > Yes, but at that point we are not talking about well-defined C, but
>> > rather implementation-specific behavior.  There's nothing wrong with
>> > it, but the C standard is no longer authoritative on what happens in
>> > such cases.
>> 
>> Huch?  Which part of that is not well-defined (or even not
>> standard-conforming) C?
>
> Well, how do you obtain the unaligned pointer?

[...]

> Extensions like __attribute__((packed)) are obviously not
> well-defined C.

In ISO-IEC9899:TC3[1] I find this about dereferencing a pointer:

,----[ 6.5.3.2 footnote 87 ]
| [...]
| Among the invalid values for dereferencing a pointer by the unary *
| operator are a null pointer, an address inappropriately aligned for the
| type of object pointed to, and the address of an object after the end of
| its lifetime.
`----

So dereferencing such a pointer - however we arrive at it - is simply
invalid.  The same sentence is in the ANSI spec[2] also.  The C FAQ[3]
notes in section 16.8 that using unaligned pointers may very well lead
to segmentation faults, bus errors or general protection faults.  Not
much documentation actually...

Cheers
  Detlev

[1] http://www.open-std.org/JTC1/SC22/WG14/www/docs/n1256.pdf
[2] http://flash-gordon.me.uk/ansi.c.txt
[3] http://www.c-faq.com/versions.html


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