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Content-type: text/plain
Wojtek,
After you unload the module TestUpper,
the object on the heap will continue to exits just fine.
It contains a hidden tag -- the pointer to the type descriptor (so it will be garbage collected just fine, when its time comes).
The type descriptor need not be unloaded along with TestUpper. TestUpper knows its type descriptors, so it can do things like NILlify back references from them to itself when unloading, as well as NILlify pointers to methods.
So if an attempt to call the object's method (whose code is in the unloaded TestUpper), the situation is easily detected (e.g. the NILlified pointers to methods) and the module can be reloaded.
All the information is available when it's needed, no implementation difficulties.
A procedure variable is just an address of the entry point into the procedure: there is no auxiliary information to help the run-time system.
Cheers
Fyodor
> > That is, a procedure variable may contain a dynamically created reference
> > to a procedure in the module being unloaded, even if the latter is not
> > related by import with the variable's module.
>
> Is the same not true with objects as well?
>
> MODULE TestLower;
> TYPE hookType* = POINTER TO EXTENSIBLE RECORD END;
> VAR hook* : hookType;
> END TestLower.
>
> MODULE TestUpper;
> IMPORT TestLower;
> TYPE myHook = POINTER TO RECORD (TestLower.hookType) END;
> VAR hook: myHook;
> BEGIN
> NEW (hook);
> TestLower.hook := hook;
> END TestUpper.
>
> As you see, I have installed a "hook object" in a module Lower.
> I can now unload module Upper because it does not have clients.
> What happens if TestUpper.hook uses resources from module Upper?
>
> Naturally, one should take care of this in the CLOSE section of module
> TestUpper, but what if there is no CLOSE section? Can you guarantee there
> are no safety problems if one unloads such a sloppy module, even though I
> have not used procedure variables?
>
> What I am driving at is that possibly procedure variables are no more
> dangerous than the O-O solution would have been, in the hands of a sloppy
> programmer of course.
>
> Comments?
>
> W.
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Received on Fri Nov 19 2004 - 06:56:04 UTC