haiku/docs/user/support/Locker.dox

/*
 * Copyright 2007, Haiku, Inc. All Rights Reserved.
 * Distributed under the terms of the MIT License.
 *
 * Documentation by:
 *  Niels Sascha Reedijk <niels.reedijk@gmail.com>
 * Corresponds to:
 * /trunk/headers/os/support/Locker.h rev 19972
 * /trunk/src/kits/support/Locker.cpp rev 13826
 */

/*!
  \file Locker.h
  \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 prefered. 

  It basically works as follows. Whenever you newly created BLocker object
  recieves 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.
*/

/*!
  \fn BLocker::BLocker()
  \brief Constructor.

  Create a new BLocker with the default name of <tt>some BLocker</tt>. 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* name, bool benaphoreStyle) for all the options.
*/

/*!
  \fn BLocker::BLocker(const char* name)
  \brief Constructor.
  
  Create a new BLocker with 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 B_OS_NAME_LENGTH constant,
    which includes the \c \\0 character.
  
  \see BLocker(const char* name, bool benaphoreStyle) for all the options.
*/

/*!
  \fn BLocker::BLocker(bool benaphoreStyle)
  \brief Constructor.
  
  Creates a BLocker with the default name of <tt>some BLocker</tt>.
  
  \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.
*/

/*!
  \fn BLocker::BLocker(const char* name, bool benaphoreStyle)
  \brief Constructor.
  
  \param name A NULL-terminated string that contains the name of the semaphore.
    Note that the length of the names are limited to 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 it's
    functioning.   
*/

/*!
  \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. 
*/

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

/*!
  \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),
    \e relative from now. 
  
  \see Lock(), Unlock()
*/

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

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

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

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

/*!
  \fn nt32 BLocker::CountLockRequests(void) const
  \brief Return the number of threads with a pending lock request.
*/

/*!
  \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.
*/