Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!apple!keith
From: keith@Apple.COM (Keith Rollin)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Another THINK Pascal gotcha - "With" instance vars
Message-ID: <48579@apple.Apple.COM>
Date: 28 Jan 91 22:51:12 GMT
References: <1991Jan25.233122.2825@fennel.cc.uwa.oz.au>
Organization: Apple Computer Inc., Cupertino, CA
Lines: 55

In article <1991Jan25.233122.2825@fennel.cc.uwa.oz.au> a_dent@fennel.cc.uwa.oz.au writes:
>
>As every keen Mac Pascal programmer knows, if you dereference a handle using
>"with handleName^^ do begin ...." you must lock the handle beforehand.

You probably know this, but your general statement above isn't quite
right. You only need to lock your handle if something in the body of
the statement moves memory. For instance, a statement like the
following is OK:

	WITH myRectHandle^^ DO BEGIN
	    top := 0;
	    left := 0;
	    bottom := 100;
	    right := 200;
	END;

However, the following is bad:

	WITH myGDHandle^^ DO BEGIN
	    gdPMap := NewPixMap;
	    ....
	END;

This is because the WITH statemnt will generate a pointer to the GDevice
stored in the myGDHandle. However, NewPixMap will allocate memory, which
could move the GDevice, invalidating the pointer Pascal has generated to
it.

>...
>This is a far subtler bug than passing instance variables by reference as
>the window of vulnerability is just whilst executing the code inside the
>"with" and may be quite small (depending on what you call).  However, even
>if you don't call things that move memory, the presence of networking drivers
>such as Tops can cause memory moves inside ANY sequence of code.

This isn't quite true, either. Drivers can only move memory at well
defined times (like at accRun time, which is done when you call
SystemTask).  Drivers cannot move memory between two arbitrary
statements in your application, because that would mean that they'd be
moving memory at interrupt time. In other words, something like the
following is OK:

	myPtr := myHandle^;
	myValue := myPtr^;

You don't have to worry about anything - including drivers - moving
memory in between those two statements.

-- 
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"Argue for your Apple, and sure enough, it's yours" - Keith Rollin, Contusions