Path: utzoo!news-server.csri.toronto.edu!rutgers!sun-barr!apple!apple.com!rmh
From: rmh@apple.com (Rick Holzgrafe)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Think C Agony!
Message-ID: <12454@goofy.Apple.COM>
Date: 8 Mar 91 23:12:34 GMT
References: <1991Mar7.063549.14081@dartvax.dartmouth.edu>
Sender: usenet@Apple.COM
Organization: Apple Computer, Inc.
Lines: 27

In article <1991Mar7.063549.14081@dartvax.dartmouth.edu> 
sean@eleazar.dartmouth.edu (Sean P. Nolan) writes:
>         if (!((**hclusterNew).prgbiasstruct =
>                 (BIASSTRUCT**) NewHandle(NumNodes * sizeof(BIASSTRUCT))))

With some compilers, this can fail. A simplified example is:
         (**h).f = NewHandle (20);
If the handle h is not locked, and if the compiler evaluates **h before 
calling NewHandle, then the value of **h may become incorrect when 
NewHandle shuffles the heap.

There are three possible solutions. (1) Lock the handle. (2) Use a 
temporary variable:
        Handle theNewHandle = NewHandle (20);
        (**h).f = theNewHandle;
or (3) some compilers avoid the whole issue by not evaluating the 
left-hand side of an assignment until the right-hand side has been 
evaluated - check your manual (and maybe play it safe anyway).

Hope this helps.

==========================================================================
Rick Holzgrafe              |    {sun,voder,nsc,mtxinu,dual}!apple!rmh
Software Engineer           | AppleLink HOLZGRAFE1          rmh@apple.com
Apple Computer, Inc.        |  "All opinions expressed are mine, and do
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