QSemaphore - Robust integer semaphore
All the functions in this class are thread-safe when Qt is built with thread support.</p>
QSemaphore ( int maxcount )
virtual ~QSemaphore ()
int available () const
int total () const
int operator++ ( int )
int operator-- ( int )
int operator+= ( int n )
int operator-= ( int n )
bool tryAccess ( int n )
The QSemaphore class provides a robust integer semaphore.
A QSemaphore can be used to serialize thread execution, in a similar way to a QMutex. A
semaphore differs from a mutex, in that a semaphore can be accessed by more than one
thread at a time.
For example, suppose we have an application that stores data in a large tree structure.
The application creates 10 threads (commonly called a thread pool) to perform searches on
the tree. When the application searches the tree for some piece of data, it uses one
thread per base node to do the searching. A semaphore could be used to make sure that two
threads don't try to search the same branch of the tree at the same time.
A non-computing example of a semaphore would be dining at a restuarant. A semaphore is
initialized to have a maximum count equal to the number of chairs in the restuarant. As
people arrive, they want a seat. As seats are filled, the semaphore is accessed, once per
person. As people leave, the access is released, allowing more people to enter. If a party
of 10 people want to be seated, but there are only 9 seats, those 10 people will wait, but
a party of 4 people would be seated (taking the available seats to 5, making the party of
10 people wait longer).
When a semaphore is created it is given a number which is the maximum number of concurrent
accesses it will permit. Accesses to the sempahore are gained using operator++() or
operator+=(), and released with operator--() or operator-=(). The number of accesses
allowed is retrieved with available(), and the total number with total(). Note that the
incrementing functions will block if there aren't enough available accesses. Use
tryAccess() if you want to acquire accesses without blocking.
See also Environment Classes and Threading.
MEMBER FUNCTION DOCUMENTATION
QSemaphore::QSemaphore ( int maxcount )
Creates a new semaphore. The semaphore can be concurrently accessed at most maxcount
QSemaphore::~QSemaphore () [virtual]
Destroys the semaphore.
Warning: If you destroy a semaphore that has accesses in use the resultant behavior is
int QSemaphore::available () const
Returns the number of accesses currently available to the semaphore.
int QSemaphore::operator++ ( int )
Postfix ++ operator.
Try to get access to the semaphore. If available() == 0, this call will block until it can
get access, i.e. until available() > 0.
int QSemaphore::operator+= ( int n )
Try to get access to the semaphore. If available() < n, this call will block until it can
get all the accesses it wants, i.e. until available() >= n.
int QSemaphore::operator-- ( int )
Postfix -- operator.
Release access of the semaphore. This wakes all threads waiting for access to the
int QSemaphore::operator-= ( int n )
Release n accesses to the semaphore.
int QSemaphore::total () const
Returns the total number of accesses to the semaphore.
bool QSemaphore::tryAccess ( int n )
Try to get access to the semaphore. If available() < n, this function will return FALSE
immediately. If available() >= n, this function will take n accesses and return TRUE. This
function does not block.
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Trolltech AS 2 February 2007 QSemaphore(3qt)