Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!shadooby!samsung!ginosko!aplcen!haven!adm!smoke!gwyn
From: gwyn@smoke.BRL.MIL (Doug Gwyn)
Newsgroups: comp.lang.c
Subject: Re: sizeof a struc field
Message-ID: <11426@smoke.BRL.MIL>
Date: 27 Oct 89 18:10:31 GMT
References: <7710@microsoft.UUCP> <4700046@m.cs.uiuc.edu>
Reply-To: gwyn@brl.arpa (Doug Gwyn)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 21

In article <4700046@m.cs.uiuc.edu> kenny@m.cs.uiuc.edu writes:
>#define NEW(p) ((p) = malloc_and_check ((size_t) (sizeof *(p))))
>[appears to be claimed to be] illegal under the proposed
>standard, because it dereferences the uninitialized pointer p.

No, there is nothing wrong with that; the argument of sizeof is
not evaluated, and so a reference via the uninitialized pointer
cannot occur.

Since I'm the one claiming that sizeof(((foo*)0)->member) is invalid,
let me try ONE MORE TIME to clarify what I think makes it invalid.
It is invalid because the operand of sizeof is invalid, and the
operand of sizeof is invalid because you are not allowed to
write ((foo*)0)->member EVEN THOUGH the null pointer does have
type "pointer to foo" (and we are assuming that foo objects do have
"member" member designators).  I believe that a compiler is allowed
to diagnose that construct as an abuse of a null pointer constant.
There may be Standard-wise people who disagree with me on this,
but I haven't been getting much valid counterargument from them.
There HAS been a lot of argument against points that I did not
think I was trying to make!