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From: dewar@merv.cs.nyu.edu (Robert Dewar)
Subject: Re: HELP ! need to insert value in array !!
Date: 1997/06/29
Message-ID: <dewar.867608134@merv>#1/1
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References: <33A9C27C.22F7@post4.tele.dk> <5oci49$97g@crl.crl.com>
 <866920621snz@genesis.demon.co.uk> <5p0v7l$9uc@nntp.seflin.org>
 <dewar.867554609@merv> <33b64c2f.43589878@nntp.ix.netcom.com>
 <dewar.867589873@merv> <33b67602.54296884@nntp.ix.netcom.com>
Organization: New York University
Newsgroups: 
 comp.lang.c,comp.lang.c++,comp.os.msdos.programmer,comp.lang.asm.x86,comp.lang.ada
Date: 1997-06-29T00:00:00+00:00
List-Id: <comp.lang.ada>


Michael said

<<It's quite easy to detect the difference.  For example,

        #include <stdio.h>

        int main(void)
        {
          int a[2];
          unsigned long d = (char*) a[1] - (char*) a[0];
          printf ("%lu %lu\n", d, sizeof(int));
          return 0;
        }

This must print the same number twice.
>>


I am unable to prove that the above code has well defined semantics
according to the ANSI standard. The casts of integer pointers into
character pointers seem quite implementation dependent in their
exact meaning to me. In fact it seems quite hard yto implement
this natural semantics on some word oriented machines.

Also, you did not answer my point about alignment. Suppose the array
element type is one whose size is not a multiple of its alignment.
How can gaps be avoided in such a case?

Or are you saying that the ANSI standard requires that the size be a
multiple of the required alignment. If so, please quote chapter and
verse.

It would be useful to know whether you are talking about things that "everyone
knows must be so", or things that are guaranteed to have the semantics you claim
from the standard.

if the latter, relevant references to the standard would be helpful in
following your argument.