Technical Note ME10

MultiFinder and _SetGrowZone

CONTENTS MultiFinder References Downloadables

MultiFinder patches the _SetGrowZone trap, and this patch can cause your program to crash if you attempt to save and restore the grow zone procedure. [Jun 01 1989]

MultiFinder

MultiFinder gives each application its own heap in which to run. Because it wants to do some fairly tricky memory management, MultiFinder installs its own grow zone procedure (gzProc) in the application heap, and patches _SetGrowZone to store your application's gzProc in a temporary variable inside of itself.

A problem arises when you want to allocate some memory without invoking the application's gzProc. This can be useful if you are writing a library of routines that does its own internal caching, and you do not want that cache to purge the application's reserved memory. Let's say that to do this, you write a pair of routines, KillGZProc and RestoreGZProc, which look like this:

    #include <Memory.h>

    GrowZoneProcPtr savedGZProc;

    pascal void KillGZProc(void)
    {
        THZ myZone;

        myZone = GetZone();
        /*    since there is no GetGrowZone trap, we have to pull it directly
            from the zone header (Ugh! Very gross!) */
        savedGZProc = myZone->gzProc;
        /* we don't want a grow zone proc */
        SetGrowZone( (GrowZoneProcPtr) nil);
    }

    pascal void RestoreGZProc(void)
    {
        /* set to saved value */
        SetGrowZone(savedGZProc);
    }

Now let's say that you bracket your call to _NewHandle with these two routines. When MultiFinder is active, you get the following:

• When the application starts, you set your gzProc to the routine MyGZProc. MultiFinder stores the procedure pointer inside of your application's MultiFinder data area.

  
  
  

• You call KillGZProc. The global variable savedGZProc now contains a pointer to MultiFinder's gzProc, which MultiFinder installed in your zone header before your application started.

  
  
  

• You do your memory allocation, and your gzProc (MyGZProc) doesn't get called, just as you intended.

  
  
  

• You call RestoreGZProc, which stores a pointer to MultiFinder's gzProc in your application's MultiFinder data area.

  
  
  

• The next time you do a memory allocation that causes the gzProc to be called, MultiFinder's gzProc will be called. One of the things this gzProc does is to see if there is a valid gzProc stored in your application's MultiFinder data area. If there is a valid gzProc, it gets called. But the gzProc in your application's MultiFinder data area is MultiFinder's gzProc, so we go into an infinite loop. Oops...

  
  
  

The only solution to work around this problem is to avoid reading the value of the gzProc out of the zone header, since it isn't valid when MultiFinder is active. (Reading the fields of the zone header is dangerous, compatibility wise as well.) Your application should only have one grow zone procedure, so you should change your KillGZProc and RestoreGZProc to restore your application's grow zone procedure directly. The corrected code would look like the following:

    #include <Memory.h>

    pascal long MyGrowZone(Size cbNeeded);

    pascal void KillGZProc(void)
    {
        /* we don't want a grow zone proc */
        SetGrowZone( (GrowZoneProcPtr) nil);
    }

    pascal void RestoreGZProc(void)
    {
        /* set to my routine */
        SetGrowZone(MyGrowZone);
    }

As you can see, the code is simpler, though not quite as flexible, but at least it won't throw your machine for a loop.

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References

Inside Macintosh, Volume II, Memory Manager

Programmers Guide To MultiFinder

Technical Note M.TB.Multifinder - MultiFinder Questions

Technical Note M.OV.Multifinder - MultiFinder Revisited

Technical Note M.OV.32BitClean - The Joy Of Being 32-Bit Clean

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Downloadables

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