Re: struct __attribute((packed));

["E. Weddington" <ericw at evcohs dot com>]

Ralf Corsepius wrote:

> On Fri, 2005-04-15 at 09:27 -0600, E. Weddington wrote:
>  
>
> > It seems that GCC will interpret the above as e1 and e2 is packed within 
> > the struct somestruct so that e2 "immediately follows e1" (according to 
> > the manual). The packed attribute in this case does not refer to what e1 
> > and e2 is pointing to.
> >    
> That's what I found out by experimenting and is the reason why I am
> asking. From what I see on i386,
>
> struct entrystruct * entry __attribute__ ((packed));
>
> is interpreted as "packed pointer to struct"
> not as "pointer to packed struct",
>  
I would interpret it that way too: packed pointer to struct. The 
"packed" attribute is on the variable itself, not the type of the 
variable (pointer to struct)

> i.e. this construct is not meaningful.
>  
Is it?

The manual says
"The |packed| attribute specifies that a variable or structure field 
should have the smallest possible alignment".

So it sounds like that:
struct entrystruct * entry __attribute__ ((packed));
by itself would mean that the variable entry has the smallest possible 
alignment. I don't know if that is meaningful just by itself or whether 
it is meaningful in your context.
And realise that I'm just postulating from looking at the manual. 
Somebody with more expertise would have to comment about this.

> You've got the point - I am collecting ammunition to fight a stubborn
> original author :-)
>  
Yeah, if that author was thinking that the packed attribute applied to 
what the variable pointed to, then I would think that it is wrong usage 
of the attribute.
I would suggest pointing the author to the description of "packed" in 
the section on attributes of types:
<http://gcc.gnu.org/onlinedocs/gcc-3.4.3/gcc/Type-Attributes.html#Type-Attributes>

HTH
Eric