RE: memcpy to an unaligned address

[Paul Koning <pkoning at equallogic dot com>]

>>>>> "Dave" == Dave Korn <dave.korn@artimi.com> writes:

 Dave> ----Original Message----
 >> From: Shaun Jackman Sent: 02 August 2005 20:26

 >> On 8/2/05, Paul Koning <pkoning@equallogic.com> wrote:
 >>> One of the things that continues to baffle me (and my colleagues)
 >>> is the bizarre way in which attributes such as "packed" work when
 >>> applied to structs.
 >>> 
 >>> It would be natural to assume, as Shaun did, that marking a
 >>> struct "packed" (or, for that matter, "packed,aligned(2)") would
 >>> apply that attribute to the fields of the struct.
 >>  This is exactly the behaviour suggested by the info docs:
 >> 
 >> $ info gcc 'C Ext' 'Type Attr' ...  Specifying this attribute for
 >> `struct' and `union' types is equivalent to specifying the
 >> `packed' attribute on each of the structure or union members.
 >> 


 Dave> There are two separate issues here:

 Dave> 1) Is the base of the struct aligned to the natural alignment,
 Dave> or can the struct be based at any address

 Dave> 2) Is there padding between the struct members to maintain
 Dave> their natural alignments (on the assumption that the struct's
 Dave> base address is aligned.)

Sure.  But in Shaun's case it looks like (2) has been applied, except
that the compiler doesn't adjust the generated code correctly.  I
would argue that "packed" applied to a whole struct should produce
BOTH effects 1 and 2.

There's a third case for which there appears to be no notation:

3) A pointer to a T that doesn't have the normal alignment of 
   the type T.

For example, as far as I can tell, GCC offers no way to say "pointer
to unaligned int" -- short of creating a one-member struct.

   paul