Path: utzoo!mnetor!uunet!husc6!bbn!mit-eddie!uw-beaver!cornell!batcomputer!eacj
From: eacj@batcomputer.tn.cornell.edu (Julian Vrieslander)
Newsgroups: comp.sys.mac.programmer
Subject: How do you count unused master pointers?
Message-ID: <4362@batcomputer.tn.cornell.edu>
Date: 10 Apr 88 05:36:27 GMT
Reply-To: eacj@tcgould.tn.cornell.edu (Julian Vrieslander)
Organization: Cornell Theory Center, Cornell University, Ithaca NY
Lines: 24
Keywords: master pointers

The "standard" approach to allocating master pointer blocks is to
(empirically) determine the largest number of handles an application is
likely to need, and then to place enough calls of MoreMasters() in the
initialization code to cover that worst case.  A more sophisticated
approach that I have seen described in a couple of places (including Scott
Knaster's first book) involves periodically counting the number of unused
master pointers.  When you see them running low you call MoreMasters()
again, making sure to do the call when it will not cause fragmentation
(eg. from the event loop, with all segments unlocked).

That sounds like a good idea, but I am not sure how to do it in a way that
will not be broken by future changes to the master pointer format.  Inside
Mac says that the unused master pointers are kept in a linked list, and
the address of the first one can be found in the heap zone header.  But
the master pointers themselves are not normal pointers - the high byte is
used for flags, the low 3 bytes for the address.  So it seems that special
code is needed to traverse the list.  Can this be done in a way that won't
fail if the master pointer format changes?  And how likely is it that the
format will change?
-- 
Julian Vrieslander (607)255-3594 
Neurobiology & Behavior, W250 Mudd Hall, Cornell University, Ithaca NY 14853    
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