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The QSystemReadWriteLock class provides read-write locking between processes. More...
#include <QSystemReadWriteLock>
The QSystemReadWriteLock class provides read-write locking between processes.
A read-write lock is a synchronization tool for protecting resources that can be accessed for reading and writing. This type of lock is useful if you want to allow multiple threads to have simultaneous read-only access, but as soon as one thread wants to write to the resource, all other threads must be blocked until the writing is complete.
QSystemReadWriteLock behaves much like the QReadWriteLock class, but it also works across multiple processes. In order to clean up the system resources used to coordinate cross process synchronization, one QReadWriteLock instance is designated the lock "owner". This instance is responsible for creating the required resources, and removing them when it is destroyed. The lock's owner should always be instantiated before any others.
System locks are identified by a 32-bit integer, which allows other processes to share the same lock. While the selection of this identifier is left upto the caller, under Linux it is common to use the ftok(3) function call which uses the identity of a specified file to generate such an identifier.
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.
| Operation | Condition Steps |
|---|---|
| Reader Progression | WAIT(Increment ReadCount) AND WAIT(WriteCount == 0) |
| Reader Complete | WAIT(Decrement ReadCount) |
| Writer Progression | WAIT(Increment WriteCount) |
| WAIT(ReadCount == 0) | |
| Writer Complete | WAIT(Decrement WriteCount) |
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.
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.
Return the id of lock as passed to the constructor.
Return true if the lock is invalid.
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 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.
The QSystemReadWriteLock instance does not refer to a valid lock.
Callers can check for an invalid lock using the isNull() method.
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 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.
The QSystemReadWriteLock instance does not refer to a valid lock.
Callers can check for an invalid lock using the isNull() method.
Release the lock.
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