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From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.sys.ibm.pc,comp.lang.c
Subject: Re: Assigning to Pointers
Message-ID: <6495@mimsy.UUCP>
Date: Wed, 29-Apr-87 13:23:43 EDT
Article-I.D.: mimsy.6495
Posted: Wed Apr 29 13:23:43 1987
Date-Received: Sat, 2-May-87 02:46:56 EDT
References: <3537@vrdxhq.UUCP>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 73
Xref: mnetor comp.sys.ibm.pc:3635 comp.lang.c:1979

In article <3537@vrdxhq.UUCP> tom@vrdxhq.UUCP (Tom Welsh) writes:
>This may be a dumb question,

No, pointers and arrays and their interconversions are a common
confusion point in C.

>but could someone please explain to me how I assign an absolute
>address to a pointer to an array?

Given your example, you should use a pointer, not a pointer to an
array.  The C language assumes a flat memory space*, and given this
assumption, a single pointer can tell you everything you need to
know about a one-dimensional array save its size (which C does not
use anyway, in this case).

(*The flat space is only per data object, which allows segmented
implementations on the 80x86.)

>int *aa[];

% cdecl
explain int *aa[]
declare aa as array of pointer to int
% 

In other words, this is an array of unspecified size (which is treated
as though it were an `extern' definition, and allocates no memory,
but, at least on Unix compilers, does set the symbol), each member of
which points to zero or more integers.

>-- or even better --

>struct POINTER_PARTS
>  {
>    unsigned offset_part;
>    unsigned segment_part;
>   }

>union POINTER_ACCESS
>  {
>     struct POINTER_PARTS pp;
>     int *far_pointer;
>   }

>union POINTER_ACCESS upa;

>upa.pp.segment_part = 0x5000;
>upa.pp.offset_part = 0x0;

>aa = upa.far_pointer;

This is indeed better:  Simply leave out the final assignment, and use

	<int_lvalue> = upa.far_pointer[<int_expr>];

If you need a separate variable:

	int *aa;
	union POINTER_ACCESS upa;

	... set upa ...
	aa = upa.far_pointer;

`int *aa' is a pointer to a block of memory just as much as it is a
pointer to a single `int'.  The only thing that distinguishes the
two is the programmer.

(Yes, one can argue for `int (*aa)[]', but the current dpANS does
not allow this declaration---though it does allow `int ar[]; &ar',
in which &ar has type int (*)[]!)
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)
UUCP:	seismo!mimsy!chris	ARPA/CSNet:	chris@mimsy.umd.edu