On Mon, 20 Jun 2011, Alan Stern wrote:

On Mon, 20 Jun 2011, Nicolas Pitre wrote:

quoted

On Mon, 20 Jun 2011, Alan Stern wrote:

quoted

On Sun, 19 Jun 2011, Nicolas Pitre wrote:

quoted

quoted

quoted

The question is: does the structure really has to be packed?

What do you mean?  The structure really does need to be allocated
without padding between the fields; is that the same thing?  So do a
bunch of other structures that currently have no annotations at all.

Yes, that's the same thing.  The packed attribute tells the compiler 
that you don't want it to insert padding in it as it sees fit.

I thought the packed attribute does more than that.  For example, on
some architectures doesn't it also force the compiler to use
byte-oriented instructions for accessing the structure's fields?

Yes, but that's a consequence of not being able to access those fields 
in their naturally aligned position anymore.  Hence the addition of the 
align attribute to tell the compiler that we know that the structure is 
still aligned to a certain degree letting the compiler to avoid 
byte-oriented instructions when possible.

Not exactly.  As far as I can tell, the ((packed)) attribute caused the 
compiler to change the structure's alignment from its natural value to 
1.  That's why the fields weren't in their naturally aligned positions 
and why removing ((packed)) fixed the problem.

Are we talking past each other?

Remember that I was the one asking if the align attribute was needed in 
the first place.  If it is not then by all means please get rid of it!

But if it _is_ needed, then the generated code can be much better if the 
packed attribute is _also_ followed by the align attribute to 
increase it from 1.


Nicolas