🚀 go-pugleaf

In article <43ugdt$ihe@newshost.lanl.gov>,
Tanmoy Bhattacharya <tanmoy@qcd.lanl.gov> wrote:
>In article <43s7k4$j2v@gatekeeper.atml.co.uk>, jrr@atml.co.uk (John Rickard)
>writes: 
>|> Stan Ryckman (sgr@ae-sun01.alden.com) wrote:
>|> : All pointers-to-struct must have the same "shape"
>|> 
>|> Is that so?  I've often written code that makes that assumption, but I
>|> can't find any justification for it in the standard.  Can you give
>|> references?

It was somewhat beaten to death in this group a while ago, I think,
as derivable from the standard (though not stated there as such)
in order to make pointers to incomplete structures work, but
Tanmoy writes...

>I think the statement is strictly true only if one makes the assumption that
>the linkers are not incredibly smart. All proofs I have seen are based on
>translation units containing incomplete struct pointers (which, by the way
>are not incomplete types): and in the absence of smart linkers, all struct
     ^^^^^^^^^^^^^^^^^^^^
     (That's why I'm using the term "pointer to incomplete struct")
>pointers need to be the same shape for things to work.

The following seems legal [is it? gcc likes it] but not handleable
by a linker however smart unless "shape" is known:
	struct S;
	struct cs { char c; };
	struct is { int i; };
	void bar(struct cs*);
	void baz(struct is*);
	void foo( int is_cs, struct S *inc)
	{
	    is_cs ? bar((struct cs *)inc) : baz((struct is *)inc);
	}

In other words, if casting a (pointer to incomplete struct) to
(pointer to some other struct type) is legal, then more than linker
issues are involved, I think.

(Of course, "proofs" derived from inferences from the standard
must assume arbitrarily smart linkers, at least in this group :-)

Cheers,
Stan.
-- 
HELP!  I'M BEING HELD PRISONER IN A .SIGNATURE FACTORY!
    Stan Ryckman   sgr@alden.com