haiku/docs/user/support/Locker.dox

/*
 * Copyright 2007-2014 Haiku, Inc. All rights reserved.
 * Distributed under the terms of the MIT License.
 *
 * Authors:
 *		Niels Sascha Reedijk, niels.reedijk@gmail.com
 *		John Scipione, jscipione@gmail.com
 *
 * Corresponds to:
 *		headers/os/support/Locker.h rev 36218
 *		src/kits/support/Locker.cpp rev 32758
 */


/*!
	\file Locker.h
	\ingroup support
	\ingroup libbe
	\brief Provides locking class BLocker.
*/


/*!
	\class BLocker
	\ingroup support
	\ingroup libbe
	\brief Semaphore-type class for thread safety.

	The BLocker interface is not merely a wrapper around a semaphore, but it
	also has two advantages. First of all, it implements a benaphore.
	A benaphore is in some ways more speed efficient,
	because before it uses the internal semaphore, it first checks against a
	variable that is only operated on with atomic operations. Setting a variable
	is a lot more efficient than acquiring a semaphore, thus this type of locking
	is much preferred.

	It basically works as follows. Whenever you newly created BLocker object
	receives a locking request, it atomically sets the benaphore variable to
	\c 1. Then only additional calls from different threads will utilize the
	semaphore. You can imagine that in many cases where you protect
	of data that \em might be accessed by two or more concurrent threads, but
	the chances of it happening being very small, the benaphore benefits the
	most from it's speed.

	The other feature of BLocker that improves basic semaphore handling is that
	it allows for recursive locks. The following piece of code works with a
	BLocker, but block inevitably with a semaphore. Let's pretend I call
	\c Water():

\code
status_t
Flower::Grow(int length)
{
    if (fLock->Lock()) {
        fLength += length;
        fLock->Unlock();
        return B_OK;
    } else
        return B_ERROR;
}

status_t
Flower::Water(int amount)
{
    if (fLock->Lock()) {
        status_t status = Grow(amount * 2);
        fLock->Unlock();
        return status;
    } else
        return B_ERROR;
}
\endcode

	This code would work because BLocker keeps track of the amount of lock
	requests from the same thread. A normal semaphore would block in \c Grow()
	because the semaphore would be acquired already. Please do make sure you
	pair every Lock() with an Unlock() though, or you'll create a deadlock.

	\sa BAutolock

	\since BeOS R3
*/


/*!
	\fn BLocker::BLocker()
	\brief Create a new BLocker with the default name "some BLocker" and
	       benaphore-style locking.

	This BLocker will use the benaphore-style locking.

	\note For debugging purposes, it's extremely convenient to actually give a
	      name to the object. In case of a deadlock, it's easier to track down
	      which BLocker object might have caused the problems.

	\see BLocker(const char*, bool) for all the options.

	\since BeOS R3
*/


/*!
	\fn BLocker::BLocker(const char* name)
	\brief Creates a new BLocker with the given \a name and benaphore-style
	       locking.

	\param name A NULL-terminated string that contains the name of the
	       semaphore. Note that the length of the names are limited to
	       \c B_OS_NAME_LENGTH constant, which includes the \c \\0
	       character.

	\see BLocker(const char* name, bool benaphoreStyle) for all the options.

	\since BeOS R3
*/


/*!
	\fn BLocker::BLocker(bool benaphoreStyle)
	\brief Creates a BLocker with the default name "some BLocker" and the given
	       locking style.

	\note For debugging purposes, it's extremely convenient to actually give a
	      name to the object. In case of a deadlock, it's easier to track down
	      which BLocker object might have caused the problems.

	\param benaphoreStyle If you pass \c true, the locker will be in benaphore
	       style (which is the default option for other constructors). If you
	       pass \c false, the object will completely rely on semaphores for
	       it's functioning.

	\see BLocker(const char* name, bool benaphoreStyle) if you also want
	     to set a name.

	\since BeOS R4
*/


/*!
	\fn BLocker::BLocker(const char* name, bool benaphoreStyle)
	\brief Creates a new BLocker with the given \a name and locking style.

	\param name A NULL-terminated string that contains the name of the
	       semaphore. Note that the length of the names are limited to
	       \c B_OS_NAME_LENGTH constant, which includes the \c \\0
	       character.
	\param benaphoreStyle If you pass \c true, the locker will be in benaphore
	       style (which is the default option for other constructors). If
	       you pass \c false, the object will completely rely on semaphores
	       for its functioning.

	\since BeOS R4
*/


/*!
	\fn virtual BLocker::~BLocker()
	\brief Destructor.

	Release the internal semaphore. Because of this, any pending Lock() calls
	from other threads be cancelled. The return code will be \c false for 
	those calls.

	\since BeOS R3
*/


/*!
	\fn status_t BLocker::InitCheck() const
	\brief Check whether the locker has properly initialized

	\return A status code, \c B_OK if the semaphore has been properly
	        initialized or any other error (negative) value related
	        to semaphore initialization.

	\since Haiku R1
*/


/*!
	\fn bool BLocker::Lock()
	\brief Add a lock request and block on it until we get it.

	\retval true Lock acquired successfully.
	\retval false Failed to acquire the lock. Most probable cause is that the
	        object is deleted. This frees the semaphore and releases the
	        pending Lock() requests.

	\see LockWithTimeout(bigtime_t timeout), Unlock()

	\since BeOS R3
*/


/*!
	\fn status_t BLocker::LockWithTimeout(bigtime_t timeout)
	\brief Add a lock request and block until we get it or until it times out.

	\param timeout This is a timeout in microseconds (one millionth of a
	       second) relative to now.

	\see Lock(), Unlock()

	\since BeOS R3
*/


/*!
	\fn void BLocker::Unlock(void)
	\brief Release the lock that's currently held.

	\since BeOS R3
*/


/*!
	\fn thread_id BLocker::LockingThread(void) const
	\brief Return the \c thread_id of the thread that's currently holding the 
	       lock.

	\since BeOS R3
*/


/*!
	\fn bool BLocker::IsLocked(void) const
	\brief Check if the calling thread is actually holding the lock.

	\return Whether or not the calling thread is holding the lock.
	\retval true The thread from which this method is called from is currently
	        holding the lock.
	\retval false The object is unlocked or the lock is held by another thread.

	\since BeOS R3
*/


/*!
	\fn int32 BLocker::CountLocks(void) const
	\brief Return the number of recursive locks that are currently held.

	\return the number of currently held recursive locks as an int32.

	\since BeOS R3
*/


/*!
	\fn int32 BLocker::CountLockRequests(void) const
	\brief Return the number of threads with a pending lock request.

	\return The number of threads with a pending lock request as an int32.

	\since BeOS R3
*/


/*!
	\fn sem_id BLocker::Sem(void) const
	\brief Return the sem_id of the semaphore this object holds.

	\warning Like any other internal objects that the Haiku API might expose,
	         this semaphore id should in general be left alone. You should not
	         use any of the public low-level semaphore functions on this
	         semaphore, because it will harm the internal consistency of the
	         object.

	\return The sem_id of the semaphore this object holds.

	\since BeOS R3
*/