From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <1ccc8ef58858daba6badd7543df078f4@quanstro.net>
From: erik quanstrom <quanstro@quanstro.net>
Date: Fri, 21 Dec 2007 18:10:10 -0500
To: 9fans@cse.psu.edu
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Subject: [9fans] 8c padding problem
Topicbox-Message-UUID: 1ffe9502-ead3-11e9-9d60-3106f5b1d025

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