Date: Tue, 27 Oct 1998 00:04:27 +0100 From: Eivind Eklund <eivind@yes.no> To: Garrett Wollman <wollman@khavrinen.lcs.mit.edu> Cc: freebsd-net@FreeBSD.ORG Subject: Re: questions of taste.. mbuf header change Message-ID: <19981027000427.59289@follo.net> In-Reply-To: <199810261712.MAA19008@khavrinen.lcs.mit.edu>; from Garrett Wollman on Mon, Oct 26, 1998 at 12:12:55PM -0500 References: <199810252219.RAA10102@whizzo.transsys.com> <199810261712.MAA19008@khavrinen.lcs.mit.edu>
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On Mon, Oct 26, 1998 at 12:12:55PM -0500, Garrett Wollman wrote:
> (You have stumbled into one of the most annoying misfeatures of the C
> language: there is no way to actually USE an incomplete structure that
> is even marginally useful; in order to have opaque objects it is
> necessary to insert a level of indirection which is less than
> desirable in a low-level context such as this.)
Well, there is one nasty trick: Create a non-opaque definition of a
variable as an anonymous structure, then fill in a char[] with the
size of the structure, minus the size of the first element (which you
need to get the correct alignement). Like this:
struct something;
static struct {
struct something *something;
... more struct defintion here ...
} anonymous_stuff;
struct stuff {
struct something *do_not_use_this_under_any_circumstance;
char filler[sizeof(anonymous_stuff) - sizeof(anonymous_stuff.something)];
};
This is not allowed to fail in standard C. However, it may well
produce a bunch of warnings, and I don't particularly like it. It
will make any cross-file checker (like a good lint) SCREAM (at least
until you set up the necessary filters for those warning messages).
Eivind.
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