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Clarion 6: Threading - 1 2002-12-15 -- Andy Ireland Newsgroups: softvelocity.products.c55ee
> That leaves a question for the gurus: How do you maintain similar
> static global data integrity in the new model as was done
> (automatically) in the old model? Let's say I have Glo:SomeVariable
> of type Long, non-threaded, that I have been using to pass information
> between the main window and another procedure that can be started
> multiple times on separate threads?
Pass it as a parameter or use synchronisation.
Here's the bottom line....
Global data that is read / write / shared is not safe
Global data that is read only is safe
Global data that is threaded ',THREAD' is safe
But also consider the affects of changes....
Changing Global read / write / shared to ',THREAD' breaks the app logic as
it is no longer shared and not inited on each thread which means the change
to the app is to either sync the code or redesign how these vars are used
and the code that uses them. Therefore it is not in most cases safe to just
make them threaded in 6.0 as this breaks the application logic. The answer
is to roll out globals completely in favour of vars that are passed around
and all objects cooperate.
The next problem comes in the form of deadlocks and all the pitfulls that
should be understood in order to avoid it etc but that is not something that
can be taught on the NG's. Deadlock is where all threads are blocked waiting
on another and none are running. This can be caused by DLL init code
blocking on an event as this grabs the process mutex as do other things or
entering a critical section that has already been entered etc so it's not
going to be all plain sailing for people. IMO the pre-emptive threading
issue has been far over simplified so as not to appear daunting but these
are issues people will hit sometime or other now that they are expected to
use threading and synchronisation functionality from the API.
Regards
Andy
Plugware Solutions.com Ltd, taking the puzzle out of I.T
Tel : +44 1249 813335
Fax : +44 1249 813462
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