Path: utzoo!attcan!uunet!mcvax!hp4nl!janus!betty!casper
From: casper@betty.fwi.uva.nl (Casper H.S. Dik)
Newsgroups: comp.lang.c
Subject: Re: "do ... while ((NULL + 1) - 1);" -- valid C?
Message-ID: <781@janus.UUCP>
Date: 20 Aug 89 11:32:30 GMT
References: <1043@levels.sait.edu.au> <961@virtech.UUCP> <10684@smoke.BRL.MIL> <940@lakesys.UUCP> <18996@mimsy.UUCP> <597@targon.UUCP>
Sender: news@janus.UUCP
Reply-To: casper@fwi.uva.nl (Casper H.S. Dik)
Organization: Faculteit Wiskunde & Informatica, Universiteit van Amsterdam
Lines: 36

In article <597@targon.UUCP> ruud@targon.UUCP (Ruud Harmsen) writes:
>
>I suppose this is machine-dependent because of alignment: char-pointers can
>point to just about anywhere, but int-pointers on many machines have to be
>aligned properly.  My question is: can I make sure in my program, that
>though generally non-portable this IS portable?  I tried this once in the
>following way:
>The char-pointer gets its value from malloc, which the manual says gives
>pointers properly aligned for any type.  I never change that char-pointer
>other than by adding multiples of sizeof(int) to it.
>Is a "ip = cp" guaranteed safe under these conditions, so can I ignore
>the compiler-warning?

No. It is not safe. If you ever want to run your program on a Data General
MV, among others, you should use "ip = (int *) cp".

Since pointers to anything except char are word aligned on MV machines,
they decided that they could drop the last bit of the address and shift it.

A char pointer pointing to the second byte of memory is represented with
0x2. A word pointer to the same location is represented by 0x1.

This gave problems when porting programs. Most programmers write 
"newp = (type *) malloc (sizeof type)"

but many forget the cast to char with free:

      "free(oldp)" instead of "free((char *) oldp)"

This works fine in most cases, but not on machines that shift pointers
when casting.

  --cd
Casper H.S. Dik				VCP/HIP: +31205922022
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