QSemaphore(3qt)QSemaphore(3qt)NAME
QSemaphore - Robust integer semaphore
SYNOPSIS
All the functions in this class are thread-safe when Qt is built with
thread support.</p>
#include <qsemaphore.h>
Public Members
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 )
DESCRIPTION
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 DOCUMENTATIONQSemaphore::QSemaphore ( int maxcount )
Creates a new semaphore. The semaphore can be concurrently accessed at
most maxcount times.
QSemaphore::~QSemaphore () [virtual]
Destroys the semaphore.
Warning: If you destroy a semaphore that has accesses in use the
resultant behavior is undefined.
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 semaphore.
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.
SEE ALSO
http://doc.trolltech.com/qsemaphore.html
http://www.trolltech.com/faq/tech.html
COPYRIGHT
Copyright 1992-2007 Trolltech ASA, http://www.trolltech.com. See the
license file included in the distribution for a complete license
statement.
AUTHOR
Generated automatically from the source code.
BUGS
If you find a bug in Qt, please report it as described in
http://doc.trolltech.com/bughowto.html. Good bug reports help us to
help you. Thank you.
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If you find errors in this manual page, please report them to qt-
bugs@trolltech.com. Please include the name of the manual page
(qsemaphore.3qt) and the Qt version (3.3.8).
Trolltech AS 2 February 2007 QSemaphore(3qt)