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Content-type: text/plain
In simple terms what do these two emails mean?
Are you both correct?
Is there a (unnecessary) safety problem with objects
and unloading that could have been avoided if the
compiler has been written differently?
(I think I have occasionally experienced the
problem Wojtek describes, but did not investigate
throughly. I have not considered the inability of
some program to survive 'malicious' unloading
of some of it's Modules as a bug. After all
unloading is not something a 'user' can do; it
requires a 'Developers' license.)
Robert.
>> 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 - 10:28:21 UTC