QWaitCondition(3qt)QWaitCondition(3qt)NAME
QWaitCondition - Allows waiting/waking for conditions
between threads
#include <qthread.h>
Inherits Qt.
Public Members
QWaitCondition ()
virtual ~QWaitCondition ()
bool wait ( unsigned long time = ULONG_MAX )
bool wait ( QMutex * mutex, unsigned long time = ULONG_MAX
)
void wakeOne ()
void wakeAll ()
DESCRIPTION
The QWaitCondition class allows waiting/waking for
conditions between threads
QWaitConditions allow a thread to tell other threads that
some sort of condition has been met; one or many threads
can block waiting for a QWaitCondition to set a condition
with wakeOne() or wakeAll. Use wakeOne() to wake one
randomly-selected event or wakeAll() to wake them all. For
example, say we have three tasks that should be performed
every time the user presses a key; each task could be
split into a thread, each of which would have a run() body
like so:
QWaitCondition key_pressed;
while(1) {
key_pressed.wait(); // This is a QWaitCondition global variable
// Key was pressed, do something interesting
do_something();
}
A fourth thread would read key presses and wake the other
three threads up every time it receives one, like so:
QWaitCondition key_pressed;
while(1) {
getchar();
// Causes any thread in key_pressed.wait() to return from
// that method and continue processing
key_pressed.wakeAll();
}
Note that the order the three threads are woken up in is
undefined, and that if some or all of the threads are
still in do_something() when the key is pressed, they
won't be woken up (since they're not waiting on the
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QWaitCondition(3qt)QWaitCondition(3qt)
condition variable) and so the task will not be performed
for that key press. This can be avoided by, for example,
doing something like this:
QMutex mymutex;
QWaitCondition key_pressed;
int mycount=0;
// Worker thread code
while(1) {
key_pressed.wait(); // This is a QWaitCondition global variable
mymutex.lock();
mycount++;
mymutex.unlock();
do_something();
mymutex.lock();
mycount--;
mymutex.unlock();
}
// Key reading thread code
while(1) {
getchar();
mymutex.lock();
// Sleep until there are no busy worker threads
while(count>0) {
mymutex.unlock();
sleep(1);
mymutex.lock();
}
mymutex.unlock();
key_pressed.wakeAll();
}
The mutexes are necessary because the results if two
threads attempt to change the value of the same variable
simultaneously are unpredictable.
MEMBER FUNCTION DOCUMENTATIONQWaitCondition::QWaitCondition ()
Constructs a new event signalling object.
QWaitCondition::~QWaitCondition () [virtual]
Deletes the event signalling object.
bool QWaitCondition::wait ( QMutex * mutex, unsigned long time =
ULONG_MAX )
Release the locked mutex and wait on the thread event
object. The mutex must be initially locked by the calling
thread. If mutex is not in a locked state, this function
returns immediately. The mutex will be unlocked, and the
thread calling will block until one of 2 conditions is
met:
Another thread signals it using wakeOne() or wakeAll().
This function will return TRUE in this case.
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QWaitCondition(3qt)QWaitCondition(3qt)
time milliseconds has elapsed. If time is ULONG_MAX
(default argument), then the wait will never timeout (the
event must signalled). This function will return FALSE if
the wait timed out.
The mutex will be returned to the same locked state. This
function is provided to allow the atomic transition from
the locked state to the wait state.
See also wakeOne() and wakeAll().
bool QWaitCondition::wait ( unsigned long time = ULONG_MAX )
Wait on the thread event object. The thread calling this
will block until one of 2 conditions is met:
Another thread signals it using wakeOne() or wakeAll().
This function will return TRUE in this case.
time milliseconds has elapsed. If time is ULONG_MAX
(default argument), then the wait will never timeout (the
event must signalled). This function will return FALSE if
the wait timed out.
See also wakeOne() and wakeAll().
void QWaitCondition::wakeAll ()
This wakes all threads waiting on the QWaitCondition. The
order in which the threads are woken up depends on the
operating system's scheduling policies, and cannot be
controlled or predicted.
See also wakeOne().
void QWaitCondition::wakeOne ()
This wakes one thread waiting on the QWaitCondition. The
thread that woken up depends on the operating system's
scheduling policies, and cannot be controlled or
predicted.
See also wakeAll().
SEE ALSO
http://doc.trolltech.com/qwaitcondition.html
http://www.trolltech.com/faq/tech.html
COPYRIGHT
Copyright 1992-2001 Trolltech AS,
http://www.trolltech.com. See the license file included
in the distribution for a complete license statement.
AUTHOR
Generated automatically from the source code.
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QWaitCondition(3qt)QWaitCondition(3qt)BUGS
If you find a bug in Qt, please report it as described in
http://doc.trolltech.com/bughowto.html. Good bug reports
make our job much simpler. Thank you.
In case of content or formattting problems with this
manual page, please report them to qt-bugs@trolltech.com.
Please include the name of the manual page
(qwaitcondition.3qt) and the Qt version (2.3.1).
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