Re: Multithreaded Programming

Jim wrote:
>Barbara,
>
>This type of locking has been used for a number of years for file
>and record locking in a number of applications without any problems,
>admittedly on PCs though.

You're probably referring to programming on old MS-Windows systems,
which use a "cooperative multitasking" model.  That meant that for
processes to multitask, processes had to regularly cede control to
the operating system.  (Of course, processes could always refuse to
cooperate, which often led to unresponsive systems.)  Your naive
model of locking may well work in that environment, but is of course
dangerous in the big leagues.

For at least the past 30 years, robust operating systems have used
"preemptive multitasking", where the system schedules machine cycles
for each process or thread.  When one thread is testing a flag, the
system may very well rip control away and give it to another thread
that sets the very same flag rendering that technique utterly
useless as a synchronization method.  (Even Windows NT uses
preemptive MT these days!)

In many cases, even single machine instructions cannot be guaranteed
to be performed without the possibility of interruption!  (That, of
course, complicates the job of the operating system developers.  But
CPUs are normally designed with at least one instruction guaranteed
to be atomic which can be used to implement semaphores.)

As Barbara stated, when dealing with threads, always expect the
worst.  ANY potentially shared resource within the thread should be
treated with respect and care must be taken to ensure proper
synchronization.

Cheers!  Hans

Hans Boldt, ILE RPG Development, IBM Toronto Lab, boldt@ca.ibm.com

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