windows c++ thread waiting on queue data-push - c++

Question

My program is setup as follows:
There is a thread-safe queue class, one thread pushing data onto it while sitting in an infinite loop and a second thread popping data off of it while sitting in an infinite loop. I am trying to think of a way to use windows events, or some other mechanism to make the thread_1 (below), wait in the infinite while loop and only iterate when the queue depth is greater than or equal to 1.
class thread-safe_Queue
{
public:
push();
pop();
};
DWORD thread_1()
{
while(1)
{
// wait for thread-safe queue to have data on it
// pop data off
// process data
}
}
DWORD thread_2()
{
while(1)
{
// when data becomes available, push data onto thread-safe queue
}
}

Answer 1

I think this might do the trick. Derive class Event and overload the Process() function.
#include <process.h> // Along with all the normal windows includes
//*********************************************
using namespace os;
Mutex globalQueueMutex;
class QueueReader : public Event
{
public:
virtual void Process()
{
// Lock the queue
Locker l(globalQueueMutex);
// pop data off
// process data
return; // queue will automatically unlock
}
};
QueueReader myQueueReader;
//*********************************************
// The queue writer would have functions like :
void StartQueueReader()
{
Thread(QueueReader::StartEventHandler, &myQueueReader);
}
void WriteToQueue()
{
Locker l(globalQueueMutex);
// write to the queue
myQueueReader.SignalProcess(); // tell reader to wake up
}
// When want to shutdown
void Shutdown()
{
myQueueReader.SignalShutdown();
}
Here are the classes that perform the magic.
namespace os {
// **********************************************************************
/// Windows implementation to spawn a thread.
static uintptr_t Thread (void (*StartAddress)(void *), void *ArgList)
{
return _beginthread(StartAddress, 0, ArgList);
}
// **********************************************************************
/// Windows implementation of a critical section.
class Mutex
{
public:
// Initialize section on construction
Mutex() { InitializeCriticalSection( &cs_ ); }
// Delete section on destruction
~Mutex() { DeleteCriticalSection( &cs_ ); }
// Lock it
void lock() { EnterCriticalSection( &cs_ ); }
// Unlock it
void unlock() { LeaveCriticalSection( &cs_ ); }
private:
CRITICAL_SECTION cs_;
}; // class Mutex
/// Locks/Unlocks a mutex
class Locker
{
public:
// Lock the mutex on construction
Locker( Mutex& mutex ): mutex_( mutex ) { mutex_.lock(); }
// Unlock on destruction
~Locker() { mutex_.unlock(); }
private:
Mutex& mutex_;
}; // class Locker
// **********************************************************************
// Windows implementation of event handler
#define ProcessEvent hEvents[0]
#define SetTimerEvent hEvents[1]
#define ShutdownEvent hEvents[2]
/// Windows implementation of events
class Event
{
/// Flag set when shutdown is complete
bool Shutdown;
/// Max time to wait for events
DWORD Timer;
/// The three events - process, reset timer, and shutdown
HANDLE hEvents[3];
public:
/// Timeout is disabled by default and Events assigned
Event( DWORD timer = INFINITE) : Timer(timer)
{
Shutdown = false;
ProcessEvent = CreateEvent( NULL,TRUE,FALSE,NULL );
SetTimerEvent = CreateEvent( NULL,TRUE,FALSE,NULL );
ShutdownEvent = CreateEvent( NULL,TRUE,FALSE,NULL );
}
/// Close the event handles
virtual ~Event()
{
CloseHandle(ProcessEvent);
CloseHandle(SetTimerEvent);
CloseHandle(ShutdownEvent);
}
/// os::Thread calls this to start the Event handler
static void StartEventHandler(void *pMyInstance)
{ ((Event *)pMyInstance)->EventHandler(); }
/// Call here to Change/Reset the timeout timer
void ResetTimer(DWORD timer) { Timer = timer; SetEvent(SetTimerEvent); }
/// Set the signal to shutdown the worker thread processing events
void SignalShutdown() { SetEvent(ShutdownEvent); while (!Shutdown) Sleep(30);}
/// Set the signal to run the process
void SignalProcess() { SetEvent(ProcessEvent); }
protected:
/// Overload in derived class to process events with worker thread
virtual void Process(){}
/// Override to process timeout- return true to terminate thread
virtual bool Timeout(){ return true;}
/// Monitor thread events
void EventHandler()
{
DWORD WaitEvents;
while (!Shutdown)
{
// Wait here, looking to be signaled what to do next
WaitEvents = WaitForMultipleObjects(3, hEvents, FALSE, Timer);
switch (WaitEvents)
{
// Process event - process event then reset for the next one
case WAIT_OBJECT_0 + 0:
Process();
ResetEvent(ProcessEvent);
break;
// Change timer event - see ResetTimer(DWORD timer)
case WAIT_OBJECT_0 + 1:
ResetEvent(SetTimerEvent);
continue;
// Shutdown requested so exit this thread
case WAIT_OBJECT_0 + 2:
Shutdown = true;
break;
// Timed out waiting for an event
case WAIT_TIMEOUT:
Shutdown = Timeout();
break;
// Failed - should never happen
case WAIT_FAILED:
break;
default:
break;
}
}
}
};
} // namespace os

Answer 2

How about this one (I assume you are familiar with events mechanism).
1.
thread_safe_Queue::push(something)
{
// lock the queue
...
// push object
// Signal the event
SetEvent(notification);
// unlock the queue
}
2.
thread_safe_Queue::pop(something)
{
WaitForSingleObject(notification);
// lock the queue
...
// get object
// reset the event
if (queue is empty)
ResetEvent(notification);
// unlock the queue
}
3. thread_1 just tries to pop the object and handle it. As something is pushed, the event is enabled, so pop can be successfully called. Otherwise it will wait inside pop. Actually you can use other synchronizing objects like mutexes or critical sections instead of the events in this case.
UPDATE. External events:
thread 1:
void thread_1()
{
while(1)
{
WaitForSingleObject(notification);
if (!pop(object)) // pop should return if there are any objects left in queue
SetEvent(notification);
}
}
thread_2
void thread_2()
{
while(1)
{
// push the object and than signal event
ResetEvent(notification)
}
}

Answer 3

You can use named events. Each thread would call CreateEvent passing in the same name. Then use WaitForMultipleObjects to wait for either queue related event or end program event. The pop thread would wait on queue_has_data and end_program events. The push thread would wait on data_available and end_program events and set the queue_has_data event when it puts something into the queue.