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From: rmh@apple.com (Rick Holzgrafe)
Newsgroups: comp.sys.mac.programmer
Subject: Re: C function ptrs & Mac mem segments
Message-ID: <8018@goofy.Apple.COM>
Date: 2 May 90 19:08:06 GMT
Sender: usenet@Apple.COM
Organization: Apple Computer, Inc.
Lines: 38
References:<3550@dogie.macc.wisc.edu> <263DD5DE.1BCE@intercon.com>

In article <263DD5DE.1BCE@intercon.com> amanda@mermaid.intercon.com 
(Amanda Walker) writes:
> In article <3550@dogie.macc.wisc.edu>, yahnke@vms.macc.wisc.edu (Ross 
Yahnke,
> MACC) writes:
> >[about C function pointers]
> > Do I ever have to worry about the Mac memory manager purging and
> > then reloading this segment thereby possibly causing the address's
> > to become invalid?
> 
> I believe that both THINK C and MPW C implement function pointers as
> pointers to the jump table entry for the function, not as pointers to
> the beginning of the actual code for the function.  Since the jump
> table is always resident and never moves function pointers won't move.
> Also, CODE segments oonly get unloaded if you call UnloadSeg on them
> explicitly.
> 
> The thing you do have to watch out for is unloading a segment that
> contains a routine that will be called at interrupt time, since calling
> LoadSeg at interrupt time would be bad.

All perfectly correct. One other thing to watch out for: don't unload a 
segment containing a routine which is still on the stack. That is: if code 
in segment A calls a routine in segment B, segment B better not unload 
segment A. The reason is that the return address on the stack is a real 
pointer into segment A, and isn't one of these "magical" pointers into the 
jump table.

Often recommended is to keep your main event loop in segment 1 (which, 
like the jump table, is locked and can't be moved or unloaded), and each 
time through the loop call UnloadSeg on every other segment.

==========================================================================
Rick Holzgrafe              |    {sun,voder,nsc,mtxinu,dual}!apple!rmh
Software Engineer           | AppleLink HOLZGRAFE1          rmh@apple.com
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