QSystemReadWriteLock Class Reference
[QtBaseModule]

Algorithm

The QSystemReadWriteLock class uses Linux kernel semaphores to synchronize access between readers and writers. Two semaphores ReadCount and WriteCount are used to allow either multiple concurrent readers or a single exclusive writer. When writers are waiting, any new readers must wait until all writers complete. That is, writers can starve readers.

The following semaphore conditions determine reader and writer operations.

See also QSystemMutex.

Member Function Documentation

QSystemReadWriteLock::QSystemReadWriteLock ( unsigned int id, bool owner )

Construct a system read write lock from the provided id. If owner is true, the instance will create the system resources required for the lock and will remove them when it is destructed.

QSystemReadWriteLock::~QSystemReadWriteLock ()

Destroy the lock instance. If owner was specified in the QSystemReadWriteLock constructor, all the system resources used by this lock will also be removed and further use of the lock by other threads or processes will fail.

unsigned int QSystemReadWriteLock::id () const

Return the id of lock as passed to the constructor.

bool QSystemReadWriteLock::isNull () const

Return true if the lock is invalid.

bool QSystemReadWriteLock::lockForRead ( int milliSec )

Attempt to acquire the read lock. This method will return true if the lock was successfully acquired, false otherwise.

Aquisition of the read lock may fail if:

The timeout milliSec, in milliseconds, expired.

If the caller wants to poll the lock in a non-blocking way, it should specify a timeout of 0. If the caller would prefer to block until the lock is acquired it should specify a timeout of -1.

Currently, only systems that support the semtimedop(2) system call can perform non-blocking, or time blocked calls. All other systems will block indefinately until the lock is acquired, regardless of the milliSec value.

The QSystemReadWriteLock instance does not refer to a valid lock.

Callers can check for an invalid lock using the isNull() method.

bool QSystemReadWriteLock::lockForWrite ( int milliSec )

Attempt to acquire the write lock. This method will return true if the lock was successfully acquired, false otherwise.

Aquisition of the write lock may fail if:

The timeout milliSec, in milliseconds, expired.

If the caller wants to poll the lock in a non-blocking way, it should specify a timeout of 0. If the caller would prefer to block until the lock is acquired it should specify a timeout of -1.

Currently, only systems that support the semtimedop(2) system call can perform non-blocking, or time blocked calls. All other systems will block indefinately until the lock is acquired, regardless of the milliSec value.

The QSystemReadWriteLock instance does not refer to a valid lock.

Callers can check for an invalid lock using the isNull() method.

void QSystemReadWriteLock::unlock ()

Release the lock.