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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.867589873@merv>#1/1
X-Deja-AN: 253415524
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>
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 says

<<Perhaps it was someone who actually knows C.  Whoever said it was
correct.  Objects may be padded in that there may be bytes that are
not used, but there must be no additional padding between array
elements.  For any type T,

        T a;
        T b[5];

we must have sizeof b == 5 * sizeof a. There can be no unused space
between b[0] and b[1].
>>

Ah, a confusion. I thought the statement was about Ada.

But I am interested in how you could construct a valid ANSI C program
with portable semantics that could detect the difference, and in particular
how exactly the ANSI C standard formally prohibits such "additional padding".
This is the sort of requirement that is very easy to state informally, but
much harder to state formally.

What would one expect on a PDP10 with 5 characters per word and an unused
bit left over, or does the ANSI C standard require 36 bits/character on 
this machine?

<<I don't have the C standard at hand, so i would be interested if someone
could quote chapter and verse on this>>

This sort of statement is the kind of statement that Ada would certainly
like to make in the packed case, but it is hard to make it more than
implementation advice.

I also don't see how your conditoin above can be correct

suppose that type T is a struct with a double field and an int field.
The size of this struct would be 12 bytes, but its alignment requirement
is 8, so it would not be possible to laout the array b without alignment
gaps???