Path: utzoo!attcan!uunet!snorkelwacker!apple!goofy.apple.com!chesley
From: chesley@goofy.apple.com (Harry Chesley)
Newsgroups: comp.sys.mac.programmer
Subject: Re: FSRead hangs on the serial driver
Message-ID: <6155@internal.Apple.COM>
Date: 12 Jan 90 18:37:52 GMT
Sender: usenet@Apple.COM
Organization: Apple Computer, Inc.
Lines: 20
References:<7770@unix.SRI.COM> <4713@hydra.gatech.EDU <6113@internal.Apple.COM> <3915@atr-la.atr.co.jp>

In article <3915@atr-la.atr.co.jp> alain@atr-la.atr.co.jp (Alain de 
Cheveigne) writes:
> I currently use a 8172 byte buffer, and I don't unset the buffer on 
> exit.  I have never witnessed such a crash.

It probably doesn't happen if you close the driver before exiting. I 
always deallocate the buffer, just to be paranoid.

What happens is the driver keeps on putting received characters into the 
place it thinks the buffer is. In the meantime, however, the system has 
cleared out the application heap (where the buffer was allocated) and 
given it to the next application to allocate from. So the input characters 
go into an essentially random memory location in the middle of a running 
program. This will cause problems if you get more characters on the port, 
and if they happen to get stuck into a critical location for the next 
program. Therefore, this can cause random and unpredictable crashes some 
arbitrary time after the program you wrote was run. In short: a 
programmer's nightmare.

The same situation arises in other drivers, such as MacTCP.