[fpc-pascal] Counting semaphore for free pascal
kapibara.pas at aol.com
Wed Jul 29 13:10:23 CEST 2015
@Serguei Thanks for your reply. What you write is probably right, but
let me explain the problem.
In an application you have N number of threads and they all want to
access a service. Now, for example, only five threads are allowed to
have access at the same time. I believe this is a job for a counting
semaphore. Delphi also has a semaphore. The code I wrote, as basic as it
is, limits the simultaneous thread accesses to the service. So the
semaphore is used here for synchronization of threads within the same
application, and that is one of the intended uses of semaphores. A
critical section blocks all threads except one, so I'm under the
impression that it can't be used for this task.
Reason I wrote is because I have a feeling the code I wrote could be
improved, or implemented differently altogether. A few weeks ago I never
even heard about "counting semaphores". There seem to be not so much
time spent on this subject in education, and many different opinions
about what semaphores are and what they are used for can be found.
On 2015-07-29 12:35, Serguei TARASSOV wrote:
> On 29/07/2015 12:00, fpc-pascal-request at lists.freepascal.org wrote:
>> A counting, or general, semaphore limits simultaneous access to a
>> resource according to the number of permits you specify. On the other
>> hand, a binary semaphore like a critical section limits the access to
>> "one at a time".
> Seems, you confuse a little semaphores and critical sections.
> In general, semaphores are inter-process but critical sections are not.
> I mean that semaphores allow synchronize processes whereas critical
> sections are designed to synchronize the threads within the same process.
> Some frameworks (i.e. .NET) provide classes like "threading semaphore"
> but it is very particular case.
> It is possible to use real semaphores to synchronize the threads too
> (with some overhead).
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