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    41 

    42 #include "qplatformdefs.h"

    43 #include "qreadwritelock.h"

    44 

    45 #ifndef QT_NO_THREAD

    46 #include "qmutex.h"

    47 #include "qthread.h"

    48 #include "qwaitcondition.h"

    49 

    50 #include "qreadwritelock_p.h"

    51 

    52 QT_BEGIN_NAMESPACE

    53 

    54 /*! \class QReadWriteLock

    55     \brief The QReadWriteLock class provides read-write locking.

    56 

    57     \threadsafe

    58 

    59     \ingroup thread

    60 

    61     A read-write lock is a synchronization tool for protecting

    62     resources that can be accessed for reading and writing. This type

    63     of lock is useful if you want to allow multiple threads to have

    64     simultaneous read-only access, but as soon as one thread wants to

    65     write to the resource, all other threads must be blocked until

    66     the writing is complete.

    67 

    68     In many cases, QReadWriteLock is a direct competitor to QMutex.

    69     QReadWriteLock is a good choice if there are many concurrent

    70     reads and writing occurs infrequently.

    71 

    72     Example:

    73 

    74     \snippet doc/src/snippets/code/src_corelib_thread_qreadwritelock.cpp 0

    75 

    76     To ensure that writers aren't blocked forever by readers, readers

    77     attempting to obtain a lock will not succeed if there is a blocked

    78     writer waiting for access, even if the lock is currently only

    79     accessed by other readers. Also, if the lock is accessed by a

    80     writer and another writer comes in, that writer will have

    81     priority over any readers that might also be waiting.

    82 

    83     Like QMutex, a QReadWriteLock can be recursively locked by the

    84     same thread when constructed in

    85     \l{QReadWriteLock::RecursionMode}recursive mode}.  In such cases,

    86     unlock() must be called the same number of times lockForWrite() or

    87     lockForRead() was called. Note that the lock type cannot be

    88     changed when trying to lock recursively, i.e. it is not possible

    89     to lock for reading in a thread that already has locked for

    90     writing (and vice versa).

    91 

    92     \sa QReadLocker, QWriteLocker, QMutex, QSemaphore

    93 */

    94 

    95 /*! 

    96     \enum QReadWriteLock::RecursionMode

    97     \since 4.4

    98 

    99     \value Recursive In this mode, a thread can lock the same

   100     QReadWriteLock multiple times and the mutex won't be unlocked

   101     until a corresponding number of unlock() calls have been made.

   102 

   103     \value NonRecursive In this mode, a thread may only lock a

   104     QReadWriteLock once.

   105 

   106     \sa QReadWriteLock()

   107 */

   108 

   109 /*!

   110     Constructs a QReadWriteLock object in NonRecursive mode.

   111 

   112     \sa lockForRead(), lockForWrite()

   113 */

   114 QReadWriteLock::QReadWriteLock()

   115     :d(new QReadWriteLockPrivate(NonRecursive))

   116 { }

   117 

   118 /*!

   119     \since 4.4

   120 

   121     Constructs a QReadWriteLock object in the given \a recursionMode.

   122 

   123     \sa lockForRead(), lockForWrite(), RecursionMode

   124 */

   125 QReadWriteLock::QReadWriteLock(RecursionMode recursionMode)

   126     : d(new QReadWriteLockPrivate(recursionMode))

   127 { }

   128 

   129 /*!

   130     Destroys the QReadWriteLock object.

   131 

   132     \warning Destroying a read-write lock that is in use may result

   133     in undefined behavior.

   134 */

   135 QReadWriteLock::~QReadWriteLock()

   136 {

   137     delete d;

   138 }

   139 

   140 /*!

   141     Locks the lock for reading. This function will block the current

   142     thread if any thread (including the current) has locked for

   143     writing.

   144 

   145     \sa unlock() lockForWrite() tryLockForRead()

   146 */

   147 void QReadWriteLock::lockForRead()

   148 {

   149     QMutexLocker lock(&d->mutex);

   150 

   151     Qt::HANDLE self = 0;

   152     if (d->recursive) {

   153         self = QThread::currentThreadId();

   154 

   155         QHash<Qt::HANDLE, int>::iterator it = d->currentReaders.find(self);

   156         if (it != d->currentReaders.end()) {

   157             ++it.value();

   158             ++d->accessCount;

   159             Q_ASSERT_X(d->accessCount > 0, "QReadWriteLock::lockForRead()",

   160                        "Overflow in lock counter");

   161             return;

   162         }

   163     }

   164 

   165     while (d->accessCount < 0 || d->waitingWriters) {

   166         ++d->waitingReaders;

   167         d->readerWait.wait(&d->mutex);

   168         --d->waitingReaders;

   169     }

   170     if (d->recursive)

   171         d->currentReaders.insert(self, 1);

   172 

   173     ++d->accessCount;

   174     Q_ASSERT_X(d->accessCount > 0, "QReadWriteLock::lockForRead()", "Overflow in lock counter");

   175 }

   176 

   177 /*!

   178     Attempts to lock for reading. If the lock was obtained, this

   179     function returns true, otherwise it returns false instead of

   180     waiting for the lock to become available, i.e. it does not block.

   181 

   182     The lock attempt will fail if another thread has locked for

   183     writing.

   184 

   185     If the lock was obtained, the lock must be unlocked with unlock()

   186     before another thread can successfully lock it.

   187 

   188     \sa unlock() lockForRead()

   189 */

   190 bool QReadWriteLock::tryLockForRead()

   191 {

   192     QMutexLocker lock(&d->mutex);

   193 

   194     Qt::HANDLE self = 0;

   195     if (d->recursive) {

   196         self = QThread::currentThreadId();

   197 

   198         QHash<Qt::HANDLE, int>::iterator it = d->currentReaders.find(self);

   199         if (it != d->currentReaders.end()) {

   200             ++it.value();

   201             ++d->accessCount;

   202             Q_ASSERT_X(d->accessCount > 0, "QReadWriteLock::tryLockForRead()",

   203                        "Overflow in lock counter");

   204             return true;

   205         }

   206     }

   207 

   208     if (d->accessCount < 0)

   209         return false;

   210     if (d->recursive)

   211         d->currentReaders.insert(self, 1);

   212 

   213     ++d->accessCount;

   214     Q_ASSERT_X(d->accessCount > 0, "QReadWriteLock::tryLockForRead()", "Overflow in lock counter");

   215 

   216     return true;

   217 }

   218 

   219 /*! \overload

   220 

   221     Attempts to lock for reading. This function returns true if the

   222     lock was obtained; otherwise it returns false. If another thread

   223     has locked for writing, this function will wait for at most \a

   224     timeout milliseconds for the lock to become available.

   225 

   226     Note: Passing a negative number as the \a timeout is equivalent to

   227     calling lockForRead(), i.e. this function will wait forever until

   228     lock can be locked for reading when \a timeout is negative.

   229 

   230     If the lock was obtained, the lock must be unlocked with unlock()

   231     before another thread can successfully lock it.

   232 

   233     \sa unlock() lockForRead()

   234 */

   235 bool QReadWriteLock::tryLockForRead(int timeout)

   236 {

   237     QMutexLocker lock(&d->mutex);

   238 

   239     Qt::HANDLE self = 0;

   240     if (d->recursive) {

   241         self = QThread::currentThreadId();

   242 

   243         QHash<Qt::HANDLE, int>::iterator it = d->currentReaders.find(self);

   244         if (it != d->currentReaders.end()) {

   245             ++it.value();

   246             ++d->accessCount;

   247             Q_ASSERT_X(d->accessCount > 0, "QReadWriteLock::tryLockForRead()",

   248                        "Overflow in lock counter");

   249             return true;

   250         }

   251     }

   252 

   253     while (d->accessCount < 0 || d->waitingWriters) {

   254         ++d->waitingReaders;

   255         bool success = d->readerWait.wait(&d->mutex, timeout < 0 ? ULONG_MAX : timeout);

   256         --d->waitingReaders;

   257         if (!success)

   258             return false;

   259     }

   260     if (d->recursive)

   261         d->currentReaders.insert(self, 1);

   262 

   263     ++d->accessCount;

   264     Q_ASSERT_X(d->accessCount > 0, "QReadWriteLock::tryLockForRead()", "Overflow in lock counter");

   265 

   266     return true;

   267 }

   268 

   269  /*!

   270     Locks the lock for writing. This function will block the current

   271     thread if another thread has locked for reading or writing.

   272 

   273     \sa unlock() lockForRead() tryLockForWrite()

   274  */

   275 void QReadWriteLock::lockForWrite()

   276 {

   277     QMutexLocker lock(&d->mutex);

   278 

   279     Qt::HANDLE self = 0;

   280     if (d->recursive) {

   281         self = QThread::currentThreadId();

   282 

   283         if (d->currentWriter == self) {

   284             --d->accessCount;

   285             Q_ASSERT_X(d->accessCount < 0, "QReadWriteLock::lockForWrite()",

   286                        "Overflow in lock counter");

   287             return;

   288         }

   289     }

   290 

   291     while (d->accessCount != 0) {

   292         ++d->waitingWriters;

   293         d->writerWait.wait(&d->mutex);

   294         --d->waitingWriters;

   295     }

   296     if (d->recursive)

   297         d->currentWriter = self;

   298 

   299     --d->accessCount;

   300     Q_ASSERT_X(d->accessCount < 0, "QReadWriteLock::lockForWrite()", "Overflow in lock counter");

   301 }

   302 

   303 /*!

   304     Attempts to lock for writing. If the lock was obtained, this

   305     function returns true; otherwise, it returns false immediately.

   306 

   307     The lock attempt will fail if another thread has locked for

   308     reading or writing.

   309 

   310     If the lock was obtained, the lock must be unlocked with unlock()

   311     before another thread can successfully lock it.

   312 

   313     \sa unlock() lockForWrite()

   314 */

   315 bool QReadWriteLock::tryLockForWrite()

   316 {

   317     QMutexLocker lock(&d->mutex);

   318 

   319     Qt::HANDLE self = 0;

   320     if (d->recursive) {

   321         self = QThread::currentThreadId();

   322 

   323         if (d->currentWriter == self) {

   324             --d->accessCount;

   325             Q_ASSERT_X(d->accessCount < 0, "QReadWriteLock::lockForWrite()",

   326                        "Overflow in lock counter");

   327             return true;

   328         }

   329     }

   330 

   331     if (d->accessCount != 0)

   332         return false;

   333     if (d->recursive)

   334         d->currentWriter = self;

   335 

   336     --d->accessCount;

   337     Q_ASSERT_X(d->accessCount < 0, "QReadWriteLock::tryLockForWrite()",

   338                "Overflow in lock counter");

   339 

   340     return true;

   341 }

   342 

   343 /*! \overload

   344 

   345     Attempts to lock for writing. This function returns true if the

   346     lock was obtained; otherwise it returns false. If another thread

   347     has locked for reading or writing, this function will wait for at

   348     most \a timeout milliseconds for the lock to become available.

   349 

   350     Note: Passing a negative number as the \a timeout is equivalent to

   351     calling lockForWrite(), i.e. this function will wait forever until

   352     lock can be locked for writing when \a timeout is negative.

   353 

   354     If the lock was obtained, the lock must be unlocked with unlock()

   355     before another thread can successfully lock it.

   356 

   357     \sa unlock() lockForWrite()

   358 */

   359 bool QReadWriteLock::tryLockForWrite(int timeout)

   360 {

   361     QMutexLocker lock(&d->mutex);

   362 

   363     Qt::HANDLE self = 0;

   364     if (d->recursive) {

   365         self = QThread::currentThreadId();

   366 

   367         if (d->currentWriter == self) {

   368             --d->accessCount;

   369             Q_ASSERT_X(d->accessCount < 0, "QReadWriteLock::lockForWrite()",

   370                        "Overflow in lock counter");

   371             return true;

   372         }

   373     }

   374 

   375     while (d->accessCount != 0) {

   376         ++d->waitingWriters;

   377         bool success = d->writerWait.wait(&d->mutex, timeout < 0 ? ULONG_MAX : timeout);

   378         --d->waitingWriters;

   379 

   380         if (!success)

   381             return false;

   382     }

   383     if (d->recursive)

   384         d->currentWriter = self;

   385 

   386     --d->accessCount;

   387     Q_ASSERT_X(d->accessCount < 0, "QReadWriteLock::tryLockForWrite()",

   388                "Overflow in lock counter");

   389 

   390     return true;

   391 }

   392 

   393 /*!

   394     Unlocks the lock.

   395 

   396     Attempting to unlock a lock that is not locked is an error, and will result

   397     in program termination.

   398 

   399     \sa lockForRead() lockForWrite() tryLockForRead() tryLockForWrite()

   400 */

   401 void QReadWriteLock::unlock()

   402 {

   403     QMutexLocker lock(&d->mutex);

   404 

   405     Q_ASSERT_X(d->accessCount != 0, "QReadWriteLock::unlock()", "Cannot unlock an unlocked lock");

   406 

   407     bool unlocked = false;

   408     if (d->accessCount > 0) {

   409         // releasing a read lock

   410         if (d->recursive) {

   411             Qt::HANDLE self = QThread::currentThreadId();

   412             QHash<Qt::HANDLE, int>::iterator it = d->currentReaders.find(self);

   413             if (it != d->currentReaders.end()) {

   414                 if (--it.value() <= 0)

   415                     d->currentReaders.erase(it);

   416             }

   417         }

   418 

   419         unlocked = --d->accessCount == 0;

   420     } else if (d->accessCount < 0 && ++d->accessCount == 0) {

   421         // released a write lock

   422         unlocked = true;

   423         d->currentWriter = 0;

   424     }

   425 

   426     if (unlocked) {

   427         if (d->waitingWriters) {

   428             d->writerWait.wakeOne();

   429         } else if (d->waitingReaders) {

   430             d->readerWait.wakeAll();

   431         }

   432     }

   433 }

   434 

   435 /*!

   436     \class QReadLocker

   437     \brief The QReadLocker class is a convenience class that

   438     simplifies locking and unlocking read-write locks for read access.

   439 

   440     \threadsafe

   441 

   442     \ingroup thread

   443 

   444     The purpose of QReadLocker (and QWriteLocker) is to simplify

   445     QReadWriteLock locking and unlocking. Locking and unlocking

   446     statements or in exception handling code is error-prone and

   447     difficult to debug. QReadLocker can be used in such situations

   448     to ensure that the state of the lock is always well-defined.

   449 

   450     Here's an example that uses QReadLocker to lock and unlock a

   451     read-write lock for reading:

   452 

   453     \snippet doc/src/snippets/code/src_corelib_thread_qreadwritelock.cpp 1

   454 

   455     It is equivalent to the following code:

   456 

   457     \snippet doc/src/snippets/code/src_corelib_thread_qreadwritelock.cpp 2

   458 

   459     The QMutexLocker documentation shows examples where the use of a

   460     locker object greatly simplifies programming.

   461 

   462     \sa QWriteLocker, QReadWriteLock

   463 */

   464 

   465 /*!

   466     \fn QReadLocker::QReadLocker(QReadWriteLock *lock)

   467 

   468     Constructs a QReadLocker and locks \a lock for reading. The lock

   469     will be unlocked when the QReadLocker is destroyed. If \c lock is

   470     zero, QReadLocker does nothing.

   471 

   472     \sa QReadWriteLock::lockForRead()

   473 */

   474 

   475 /*!

   476     \fn QReadLocker::~QReadLocker()

   477 

   478     Destroys the QReadLocker and unlocks the lock that was passed to

   479     the constructor.

   480 

   481     \sa QReadWriteLock::unlock()

   482 */

   483 

   484 /*!

   485     \fn void QReadLocker::unlock()

   486 

   487     Unlocks the lock associated with this locker.

   488 

   489     \sa QReadWriteLock::unlock()

   490 */

   491 

   492 /*!

   493     \fn void QReadLocker::relock()

   494 

   495     Relocks an unlocked lock.

   496 

   497     \sa unlock()

   498 */

   499 

   500 /*!

   501     \fn QReadWriteLock *QReadLocker::readWriteLock() const

   502 

   503     Returns a pointer to the read-write lock that was passed

   504     to the constructor.

   505 */

   506 

   507 /*!

   508     \class QWriteLocker

   509     \brief The QWriteLocker class is a convenience class that

   510     simplifies locking and unlocking read-write locks for write access.

   511 

   512     \threadsafe

   513 

   514     \ingroup thread

   515 

   516     The purpose of QWriteLocker (and QReadLocker is to simplify

   517     QReadWriteLock locking and unlocking. Locking and unlocking

   518     statements or in exception handling code is error-prone and

   519     difficult to debug. QWriteLocker can be used in such situations

   520     to ensure that the state of the lock is always well-defined.

   521 

   522     Here's an example that uses QWriteLocker to lock and unlock a

   523     read-write lock for writing:

   524 

   525     \snippet doc/src/snippets/code/src_corelib_thread_qreadwritelock.cpp 3

   526 

   527     It is equivalent to the following code:

   528 

   529     \snippet doc/src/snippets/code/src_corelib_thread_qreadwritelock.cpp 4

   530 

   531     The QMutexLocker documentation shows examples where the use of a

   532     locker object greatly simplifies programming.

   533 

   534     \sa QReadLocker, QReadWriteLock

   535 */

   536 

   537 /*!

   538     \fn QWriteLocker::QWriteLocker(QReadWriteLock *lock)

   539 

   540     Constructs a QWriteLocker and locks \a lock for writing. The lock

   541     will be unlocked when the QWriteLocker is destroyed. If \c lock is

   542     zero, QWriteLocker does nothing.

   543 

   544     \sa QReadWriteLock::lockForWrite()

   545 */

   546 

   547 /*!

   548     \fn QWriteLocker::~QWriteLocker()

   549 

   550     Destroys the QWriteLocker and unlocks the lock that was passed to

   551     the constructor.

   552 

   553     \sa QReadWriteLock::unlock()

   554 */

   555 

   556 /*!

   557     \fn void QWriteLocker::unlock()

   558 

   559     Unlocks the lock associated with this locker.

   560 

   561     \sa QReadWriteLock::unlock()

   562 */

   563 

   564 /*!

   565     \fn void QWriteLocker::relock()

   566 

   567     Relocks an unlocked lock.

   568 

   569     \sa unlock()

   570 */

   571 

   572 /*!

   573     \fn QReadWriteLock *QWriteLocker::readWriteLock() const

   574 

   575     Returns a pointer to the read-write lock that was passed

   576     to the constructor.

   577 */

   578 

   579 QT_END_NAMESPACE

   580 

   581 #endif // QT_NO_THREAD