Path: utzoo!utgpu!water!watmath!clyde!rutgers!husc6!purdue!gatech!bbn!lawrence
From: lawrence@bbn.COM (Gabriel Lawrence)
Newsgroups: comp.sys.mac
Subject: Re: Philosophical programming question:  handles
Message-ID: <6713@ccv.bbn.COM>
Date: 9 Feb 88 17:39:13 GMT
References: <870130@hpcilzb.HP.COM> <5192@well.UUCP>
Reply-To: lawrence@ccv.bbn.com.BBN.COM (Gabriel Lawrence)
Organization: Bolt, Beranek and Newman Inc., Cambridge MA
Lines: 32

(Michael Zentner) writes:
>In terms of straight "C" programming, a Handle is just a pointer to a 
>pointer...

Not to point any fingers but I've yet to see anyone answer this question
directly.  The above statement is incomplete, substituting "Pascal" or
"Assembly Language" or "Forth" for "C" would have precisely the same
connotation.  Although the simplest way to represent the Handle as a data
structure in 'C' is to use 'typedef char **handle', that does not take the
complete definition of a handle into consideration.  A handle is defined as
a pointer to a master pointer which points to a relocatable block of memory.
Because the memory manager may relocate these blocks of memory at it's
own convenience, a programmer must always be aware that his/her dereferenced
pointer to his/her data may or may not still be valid.  A data structure which
was truly representative of a handle would have to include references to 
master pointers, lock/unlock bits and all that nonsense.  Of course to 
preserve a transparent memory management interface (ie. to allow software to
retain compatibility whether or not a MMU is utilized) you are better off 
'hiding' portions of your data structure (such as the lock/unlock mechanisms)
and providing data structure specific subroutines or macros which manipulate
certain portions of your elementary data  structure (Right Apple??!! :-) ).
I'm beginning to ramble but I hope my point is clear - the method (or language)
by which access to a data structure is used does _not_ necessarily define the
actual structure of the data.

Now then, who can cite actual instances where a handle-based data structure
has been used within a contemporary operating system? Extra points awarded
if you can also cite the data structure (in any language) used to represent
it.

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