Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!usc!ginosko!uunet!kddlab!ccut!titcca!sragwa!wsgw!socslgw!diamond
From: diamond@csl.sony.co.jp (Norman Diamond)
Newsgroups: comp.lang.c
Subject: Re: effect of free()
Message-ID: <10828@riks.csl.sony.co.jp>
Date: 13 Sep 89 03:49:43 GMT
References: <319@cubmol.BIO.COLUMBIA.EDU> <3756@buengc.BU.EDU> <1989Aug17.005548.745@twwells.com> <16022@vail.ICO.ISC.COM> <248@seti.inria.fr> <1010@m3.mfci.UUCP> <10973@smoke.BRL.MIL> <247@bbxeng.UUCP> <10985@smoke.BRL.MIL>
Reply-To: diamond@ws.sony.junet (Norman Diamond)
Organization: Sony Computer Science Laboratory Inc., Tokyo, Japan
Lines: 35

In article <247@bbxeng.UUCP> scott@bbxeng.UUCP (Scott-Engineering) writes:
>>In other words - you cannot even *test* a pointer unless you are *sure*
>>there is a valid address in it.  I don't think so.

In article <10985@smoke.BRL.MIL> gwyn@brl.arpa (Doug Gwyn) writes:

>There is no way a strictly conforming C program can create an invalid
>pointer value, apart from free()ing a valid one that it got from malloc()
>or realloc().  The moment an improper pointer value is created via
>arithmetic operations, the program has violated the guarantees of the
>Standard, and the implementation would be free to trap immediately.

Another way to create a pointer that does not point to an object is to
assign a NULL constant to it.

A pointer variable containing a valid pointer value may be compared to
a NULL constant.

Scott-Engineering's posting implies these questions (among others):
(1) if a pointer variable contains a NULL value, then may that pointer
     be compared to a NULL constant?
(2) if a pointer variable contains a NULL value, then may that pointer
     be compared to a valid pointer value?

Scott-Engineering's reading of the standard implied negative answers,
which he finds hard to believe.  (I still can't get a copy of the
proposed standard and cannot form an opinion.)  Mr. Gwyn, perhaps you
could please post a better answer.

--
-- 
Norman Diamond, Sony Corporation (diamond@ws.sony.junet)
  The above opinions are inherited by your machine's init process (pid 1),
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