2002-07-09 16:24:59 +04:00
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2009-10-20 02:42:52 +04:00
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<!-- $Id: BLockerUseCases.html 10 2002-07-09 12:24:59Z ejakowatz $ -->
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2002-07-09 16:24:59 +04:00
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<TITLE>BLocker Use Cases and Implementation Details</TITLE>
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<H1>BLocker Use Cases and Implementation Details:</H1>
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<P>This document describes the BLocker interface and some basics of how it is implemented.
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The document has the following sections:</P>
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<OL>
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<LI><A HREF="#interface">BLocker Interface</A></LI>
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<LI><A HREF="#usecases">BLocker Use Cases</A></LI>
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<LI><A HREF="#implement">BLocker Implementation</A></LI>
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</OL>
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<A NAME="interface"></A><H2>BLocker Interface:</H2>
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<P>The BLocker class is a simple class for handling synchronization between threads. The best
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source of information for the BLocker interface can be found
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<A HREF="file:///boot/beos/documentation/Be%20Book/The%20Support%20Kit/Locker.html">here in the Be Book</A>.
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</P>
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<A NAME="usecases"></A><H2>BLocker Use Cases:</H2>
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<P>The following use cases cover the BLocker functionality:</P>
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<OL>
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<LI><P><B>Construction 1:</B> A BLocker can be created by specifying a name for the semaphore used
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internally. If a name is specified during construction, then that name is given to the internal
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semaphore. If no name is given at construction, the semaphore is given the name "some BLocker".
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</P></LI>
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<LI><P><B>Construction 2:</B> A BLocker can use a semaphore or a "benaphore" internally depending
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on a flag passed when the BLocker is created. If the flag is false, the BLocker uses a semaphore
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to do synchronization. If the flag is true, the BLocker uses a benaphore internally to do
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synchronization. If no flag is specified, the BLocker uses a benaphore internally.</P></LI>
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<LI><P><B>Destruction:</B> When a BLocker is destructed, any threads waiting for the lock are
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immediately unblocked. The threads are notified by the return code of the locking member function
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that the lock was not successfully acquired. Any threads blocked on Lock() will return with
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a false value. Any threads blocked on LockWithTimeout() will return with B_BAD_SEM_ID.</P></LI>
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<LI><P><B>Locking 1:</B> When a thread acquires the BLocker using the Lock() or LockWithTimeout()
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member functions, no other thread can acquire the lock until this thread releases it.</P></LI>
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<LI><P><B>Locking 2:</B> When a thread holds the BLocker and it calls Lock() or LockWithTimeout(),
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the member function returns immediately. The thread must call Unlock() the same number of times
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it calls Lock...() before the BLocker is released. At any time, the thread can call CountLocks()
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to get the number of times it must call Unlock() to release the BLocker.</P></LI>
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<LI><P><B>Locking 3:</B> When a thread calls Lock(), the thread blocks until it can acquire the
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lock. Once the lock has been acquired or an unrecoverable error has occurred, the Lock() member
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function completes. If the lock has been acquired, Lock() returns true. If the lock has not
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been acquired, Lock() returns false.</P></LI>
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<LI><P><B>Locking 4:</B> When a thread calls LockWithTimeout(), the thread blocks until it can
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acquire the lock, the time specified in microseconds expires, or an unrecoverable error occurs.
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If the timeout specified is B_INFINTE_TIMEOUT, there is no timeout and the member function will
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either acquire the lock or fail due to an unrecoverable error. If the lock is acquired, the
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member function returns B_OK. If the timeout is reached, B_TIMED_OUT is returned and the lock is
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not acquired. If a serious error occurs, a non B_OK code is returned.</P></LI>
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<LI><P><B>Unlocking:</B> The Unlock() member function takes no arguments and returns no value.
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If the thread currently holds the lock, the lock count is reduced by the call to Unlock(). If
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the lock count reaches zero, then another thread may acquire the lock. If the thread does not
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hold the lock and it calls Unlock(), the call will have no affect at all on the BLocker.</P></LI>
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<LI><P><B>Locking Thread:</B> The LockingThread() member function returns the thread_id of the
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thread that is holding the lock. If no thread holds the lock, then B_ERROR is returned.</P></LI>
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<LI><P><B>Is Locked:</B> The IsLocked() member function returns true if the BLocker is currently
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held by the calling thread. If the BLocker is not acquired by any thread or it is acquired by a
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different thread, IsLocked() returns false.</P></LI>
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<LI><P><B>Count Locks:</B> The CountLocks() member function returns the number of times the lock
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has been acquired by the thread which holds the lock. If no thread holds the lock, then 0 is
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returned. If the BLocker is held by any thread, including a thread which is not the thread
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making the CountLocks() request, the number of times the lock has been acquired by the thread which
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holds the lock is returned.</P></LI>
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<LI><P><B>Count Lock Requests:</B> The CountLockRequests() member function returns the number of
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threads currently attempting to lock the BLocker. If no thread holds the lock and no thread is
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waiting for the lock, then 0 is returned. If one thread holds the lock and no other threads
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are waiting for the lock, then 1 is returned. If one thread holds the lock and x threads are
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waiting for the lock, then x+1 is returned. The call to CountLockRequests() can be made by any
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thread including threads which do not have the lock.</P>
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<P><B>NOTE:</B> Reading the Be Book, that would seem like what is returned by this member
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function. In actuality, the value returned is just the "benaphore count" for the BLocker.
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If the BLocker is semaphore style, then the benaphore count is set to 1 at construction time
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to ensure that the semaphore is always tested when a lock is acquired. The return value is
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just this count. So, the return value for benaphore style is:</P>
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<PRE>
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numThreadsWaitingForTheLock + numThreadsHoldingTheLock + numOfTimeoutsOccuredOnTheLock
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</PRE>
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<P>The return value for a semaphore style is:</P>
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<PRE>
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numThreadsWaitingForTheLock + numThreadsHoldingTheLock + numOfTimeoutsOccuredOnTheLock + 1
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</PRE>
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<P>Again, this is what we are implementing but the above description is what appears in the
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BeBook as far as I understand it.</P>
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</LI>
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<LI><P><B>Sem:</B> The Sem() member function returns the sem_id of the semaphore used by the
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BLocker. If the BLocker is a benaphore, then the sem_id returned is the semaphore used to
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implement the benaphore. If the BLocker is a not a benaphore, then the sem_id returned is the
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semaphore which the BLocker represents.</P></LI>
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</OL>
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<A NAME="implement"></A><H2>BLocker Implementation:</H2>
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<P>For more information about how to implement a benaphore, you can reference an implementation
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found on Be's website at
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<A HREF="http://www-classic.be.com/aboutbe/benewsletter/Issue26.html">http://www-classic.be.com/aboutbe/benewsletter/Issue26.html</A>.</P>
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