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Synchronisation of Threads in VC++

Posted on 1998-06-10
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Last Modified: 2008-02-01
I am creating threads using API calls. So I cannot use the seamophores class using MFC.

So is there any way of synchronising the threads. I have few global variables which are being accessed by two or more threads. So how do I synchronise them.

Thanks in advance

nikhilh

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Question by:nikhilh
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chensu earned 100 total points
ID: 1165580
You can use the MFC classes CSemaphore, CSingleLock or CMultiLock to synchronize your threads no matter whether you are using MFC to create the threads. These classes are the wrapper for Semaphore object and the waiting functions. You can also directly use them (CreateSemaphore, WaitForSingleObject, etc). Visaul C++ comes with several multithread samples (MFC and non-MFC).
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by:nietod
ID: 1165581
That is MFC, though.

You can use CreateMutex() to create mutexes, CreateSemaphore() to create semaphores, and CreateEvent(0 to create events.  These are all regulat API calls.

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by:nietod
ID: 1165582
Oh I see.  chensu interpretted this as an MFC program that doesn't use MFC to create threads.  I interpretted it as a non-MFC program.  Which is right?
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Expert Comment

by:alexo
ID: 1165583
Critical sections are much more efficient than mutexes and have similar functionality.

APIs:
  InitializeCriticalSection()
  EnterCriticalSection()
  LeaveCriticalSection()
  DeleteCriticalSection()

MFC object: CCriticalSection
  CCriticalSection() - Constructs a CCriticalSection object.
  Unlock() - Releases the CCriticalSection object.
  Lock() - Use to gain access to the CCriticalSection object.

Tell me it it helps.
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by:nietod
ID: 1165584
Do you know for a fact that they are more efficient?  I always assumed they were implimented using mutexes.
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by:alexo
ID: 1165585
Yes, I do know that for a fact.

A CS only works between threads of a single process so no kernel call (ring 3 --> ring 0 --> ring 3 transition) needs to be made if the CS is free.

I once did an emphirical timing and, if I remember correctly, CS calls were about 20x faster than mutex calls with no blocking (timing with blocking is all but meaningless).

References:
http://www.dejanews.com/dnquery.xp?search=thread&recnum=%3c337b4b70.83813086@neptune%3e%231/1
http://www.dejanews.com/dnquery.xp?search=thread&recnum=%3c34983a37.166416534@news.netvision.net.il%3e%231/1
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by:abesoft
ID: 1165586
alexo:
Any data on Events, Semaphores, etc?  I assume that these are in the same category as CS, wrt speed.
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by:alexo
ID: 1165587
>> I assume that these are in the same category as CS, wrt speed.
Same order of magnitude as *mutexes*.  Don't remember the exact details but the results were pretty close.  CSs are much faster because you avoid the kernell call that you must take when working with "real" sync. objects (those you can pass to WaitFor...() APIs).
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