QSemaphore Class

 QSemaphore sem(5);      // sem.available() == 5

 sem.acquire(3);         // sem.available() == 2
 sem.acquire(2);         // sem.available() == 0
 sem.release(5);         // sem.available() == 5
 sem.release(5);         // sem.available() == 10

 sem.tryAcquire(1);      // sem.available() == 9, returns true
 sem.tryAcquire(250);    // sem.available() == 9, returns false

Member Function Documentation

QSemaphore::QSemaphore(int n = 0)

Creates a new semaphore and initializes the number of resources it guards to n (by default, 0).

See also release() and available().

QSemaphore::~QSemaphore()

Destroys the semaphore.

Warning: Destroying a semaphore that is in use may result in undefined behavior.

void QSemaphore::acquire(int n = 1)

Tries to acquire n resources guarded by the semaphore. If n > available(), this call will block until enough resources are available.

See also release(), available(), and tryAcquire().

int QSemaphore::available() const

Returns the number of resources currently available to the semaphore. This number can never be negative.

See also acquire() and release().

void QSemaphore::release(int n = 1)

Releases n resources guarded by the semaphore.

This function can be used to "create" resources as well. For example:

 QSemaphore sem(5);      // a semaphore that guards 5 resources
 sem.acquire(5);         // acquire all 5 resources
 sem.release(5);         // release the 5 resources
 sem.release(10);        // "create" 10 new resources

See also acquire() and available().

bool QSemaphore::tryAcquire(int n = 1)

Tries to acquire n resources guarded by the semaphore and returns true on success. If available() < n, this call immediately returns false without acquiring any resources.

Example:

 QSemaphore sem(5);      // sem.available() == 5
 sem.tryAcquire(250);    // sem.available() == 5, returns false
 sem.tryAcquire(3);      // sem.available() == 2, returns true

See also acquire().

bool QSemaphore::tryAcquire(int n, int timeout)

Tries to acquire n resources guarded by the semaphore and returns true on success. If available() < n, this call will wait for at most timeout milliseconds for resources to become available.

Note: Passing a negative number as the timeout is equivalent to calling acquire(), i.e. this function will wait forever for resources to become available if timeout is negative.

Example:

 QSemaphore sem(5);            // sem.available() == 5
 sem.tryAcquire(250, 1000);    // sem.available() == 5, waits 1000 milliseconds and returns false
 sem.tryAcquire(3, 30000);     // sem.available() == 2, returns true without waiting

See also acquire().