The purpose of a
QMutex is to protect an object, data structure or section of code so that only one thread can access it at a time (this is similar to the Java
synchronized keyword). It is usually best to use a mutex with a
QMutexLocker since this makes it easy to ensure that locking and unlocking are performed consistently.
For example, say there is a method that prints a message to the user on two lines:
number = 6 def method1(): number *= 5 number /= 4 def method2(): number *= 3 number /= 2
If these two methods are called in succession, the following happens:
# method1() number *= 5 # number is now 30 number /= 4 # number is now 7 # method2() number *= 3 # number is now 21 number /= 2 # number is now 10
If these two methods are called simultaneously from two threads then the following sequence could result:
# Thread 1 calls method1() number *= 5 # number is now 30 # Thread 2 calls method2(). # # Most likely Thread 1 has been put to sleep by the operating # system to allow Thread 2 to run. number *= 3 # number is now 90 number /= 2 # number is now 45 # Thread 1 finishes executing. number /= 4 # number is now 11, instead of 10
If we add a mutex, we should get the result we want:
mutex = QMutex() number = 6 def method1(): mutex.lock() number *= 5 number /= 4 mutex.unlock() def method2(): mutex.lock() number *= 3 number /= 2 mutex.unlock()
Then only one thread can modify
number at any given time and the result is correct. This is a trivial example, of course, but applies to any other case where things need to happen in a particular sequence.
When you call
lock() in a thread, other threads that try to call
lock() in the same place will block until the thread that got the lock calls
unlock() . A non-blocking alternative to
QMutex is optimized to be fast in the non-contended case. It will not allocate memory if there is no contention on that mutex. It is constructed and destroyed with almost no overhead, which means it is fine to have many mutexes as part of other classes.
Constructs a new mutex. The mutex is created in an unlocked state.
timeout – int
- Return type
Attempts to lock the mutex. This function returns
true if the lock was obtained; otherwise it returns
false. If another thread has locked the mutex, this function will wait for at most
timeout milliseconds for the mutex to become available.
Note: Passing a negative number as the
timeout is equivalent to calling
lock() , i.e. this function will wait forever until mutex can be locked if
timeout is negative.
If the lock was obtained, the mutex must be unlocked with
unlock() before another thread can successfully lock it.
Calling this function multiple times on the same mutex from the same thread will cause a dead-lock.
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