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From: hook@cscsun3.larc.nasa.gov (Ed Hook)
Subject: Re: HELP ! need to insert value in array !!
Date: 1997/06/30
Message-ID: <5p8bd0$rhc$2@reznor.larc.nasa.gov>#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>
 <dewar.867608134@merv>
Organization: CSC/NASA Langley Research Center
Reply-To: hook@cscsun3.larc.nasa.gov
Newsgroups: 
 comp.lang.c,comp.lang.c++,comp.os.msdos.programmer,comp.lang.asm.x86,comp.lang.ada
Date: 1997-06-30T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <dewar.867608134@merv>, dewar@merv.cs.nyu.edu (Robert Dewar) writes:
|> 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];

   Actually, the line above should probably be:

             unsigned long d = (char *)(a + 1) - (char *)a;

|>           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?
|>

   The compiler is allowed to insert padding bytes _in_the_element_;
   it's just not permitted to add additional padding _between_ elements.
   That's needed to guarantee (for instance) that an allocation of
   n * sizeof(<type>) bytes can successfully hold an array of n elements
   of type <type>.
 
|> Or are you saying that the ANSI standard requires that the size be a
                                            .allows.
|> multiple of the required alignment. If so, please quote chapter and
|> verse.
|> 

  I don't have a copy of the Standard handy, so I'll let someone else
  provided citations.

|> 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.
|>

  I'm fairly certain that this (=="no padding between array elements") _is_
  guaranteed by the Standard (but, again, I can't provide the requested
  "chapter and verse").
 
|> if the latter, relevant references to the standard would be helpful in
|> following your argument.
|> 

-- 
 Ed Hook                              |       Copula eam, se non posit
 Computer Sciences Corporation        |         acceptera jocularum.
 NASA Langley Research Center         | Me? Speak for my employer?...<*snort*>
 Internet: hook@cscsun3.larc.nasa.gov |        ... Get a _clue_ !!! ...