Re: [9fans] 8c padding problem

[Skip Tavakkolian <9nut@9netics.com>]

i am guessing that the requirement of being able to demote a type to
one of its anonymouse constituents -- as in Node to Lock in Ken's example --
would require a uniform layout that makes it easy to calculate the offset.

> i've got a structure that looks like this
> 
> 	/* ethernet header */
> 	typedef struct{
> 		uchar	d[6];
> 		uchar	s[6];
> 		uchar	type[2];
> 	}Ehdr;
> 
> 	typedef struct{
> 		Ehdr;
> 		uchar	op[2];
> 		...
> 	}Pkt;
> 
> the problem is that 8c adds two bytes of padding between
> the end of pkt->type and pkt->op.  since sizeof Ehdr%wordsize != 0
> this isn't totally unexpected.  suprisingly, adding the
> appropriate packed pragmas doesn't eliminate the padding.
> is this a bug?
> 
> of course, if my original definition had been
> 
> 	typedef struct{
> 		uchar	d[6];
> 		uchar	s[6];
> 		uchar	type[2];
> 		uchar	op[2];
> 		...
> 	}Pkt;
> 
> i would have been fine.  but doesn't this defeat
> the point of unnamed structures?
> 
> also, the fact that there is padding contradicts
> my mental model of unnamed structures.  i thought
> that all the elements of unnamed structures were
> laid down as if they were elements of the enclosing
> structure.  my reading /sys/doc/compilers.ps doesn't
> say much.  is there another reference on unnamed
> structures?
> 
> - erik