my project is a dialog base and activeX container that have event .
when i call a function of activex and wait to fire it's event (with WaitForSingleObject), the application stops and the event doesn't happen until time-out, after time out it will happen.
void COcxTesDlg::OnBnClickedButton1()
{
hEvent = CreateEvent(NULL, TRUE, FALSE, L"testEvent");
ocxObj.ocxFunction();
DWORD res = ::WaitForSingleObject(hEvent, 10000);
if(res != WAIT_OBJECT_0)
AfxMessageBox(L"Time Out");
else
AfxMessageBox(L"before Time Out");
//--- to do some works after ocx event
//---
}
void COcxTesDlg::ocxEventFunc()
{
SetEvent(hEvent);
}
i always see 'Time Out' MessageBox, but if i don't call the WaitForSingleObject() the ocxEventFunc() happens sooner than 1000ms
i want to do some works after event without waiting a lot of time.
please help me.
Of course it does: you are asking your program to wait, and it waits.
Based on your description of what you are trying to do, your design is fundamentally broken: You are inside a message handler, in your main thread and you call WaitForSingleObject. That suspends execution of that thread, which handles all the user interface - and your program if you have only one thread - until the event is signaled or until the timeout expires.
You should rethink your design. Without knowing more it's not easy to suggest alternatives. One possibility is this:
Make a note of the start time (e.g. call GetTickCountEx and store the result).
Loop while start_time - current_time < timeout value. Inside the loop call WaitForSingleObject with 0 for the timeout. That call will not block and will return immediately: if it returns WAIT_OBJECT_0 then the OCX has completed. If it returns WAIT_TIMEOUT it means the OCX hasn't completed. You can then do whatever work you want.
You already have a function that is called when the event happens. Use it.
void COcxTesDlg::OnBnClickedButton1()
{
ocxObj.ocxFunction();
}
void COcxTesDlg::ocxEventFunc()
{
//--- to do some works after ocx event
//---
}
Related
I am trying to ask a question with reference to the question Glib: Calling a iterative loop function
I am actually doing GStreamer and trying to use Glib library function in my application as much as possible. The program require some system event/response before performing some action in response to some user input
Design of flow
User input the option and application take it as user_input is asserted to be true
Application need install a callback (called it callback_A) -- which wait out for buffer flowing in that point of application
Callback-A will be called whenever buffer passes through a point.
In callback A, Application to wait for some particular condition (ie a key-frame buffer passing through) at a point. If a key frame buffer pass through, it will then install a second callback at some point downstream, callback-B. An EOS event is send out through the pipeline. Otherwise wait for next time a buffer pass through
In callback B, it will wait for the event (EOS) to arrive and determine further action. If everything is completed, set task_completed to be true
function return to main while loop. The blocking (task_completed) is released and the application will report the task completed to UI
Problem'
Currently I faced a problem of the callback not completing their task (takes a long time) before it went to being blocked by task completed (and being blocked thereafter)
Question
In Glib, is a callback within the same memory space(or thread) as its caller?
In Glib, how do I overcome the problem of being blocked? Is there some methods to ensure that the task are being run before time up and control is returned to the caller
Will a gthread help? Putting the two call-back as a separate thread since they need to wait for some events to happen
This may be too much. Any alternatives, example polling instead of callback in this case.
Code
Here is my pseudocode
gbool user_input;
gbool task_completed = false;
static void
callback_B(GstPad *pad,
GstPadProbeInfo *info,
gpointer udata)
{
//// wait for some events--- call it event B
GstEvent *event = GST_PAD_PROBE_INFO_EVENT (info);
if (GST_EVENT_TYPE (event) != GST_EVENT_EOS)
return GST_PAD_PROBE_OK;
/// do something
/// ......
task_completed =true;
return GST_PAD_PROBE_REMOVE;
}
static void
callback_A( GstPad *pad,
GstBuffer * buffer,
gpointer udata)
{
//// wait for some event call it event A
if( !GST_BUFFER_FLAG_IS_SET(buffer, GST_BUFFER_FLAG_DELTA_UNIT))
{
/// install callback-B to determine some condition
gst_pad_add_probe ( pad,
GST_PAD_PROBE_TYPE_BLOCK,
(GSourceFunc)callback_B,
//NULL,
NULL,
NULL);
GstPad* padB = gst_pad_get_peer (pad);
gst_pad_send_event(padB, gst_event_new_eos());
}
else
{
return GST_PAD_PROBE_REMOVE;
}
}
gboolean
check_cmd_session(NULL )
{
if(user_input)
{
// ........ some other actions *****************
/// initialize task_complete to be false
task_completed = false;
//// install callback_A
gst_pad_add_probe(padA,
GST_PAD_PROBE_TYPE_BUFFER,
callback_A,
NULL,
NULL);
while(!task_completed)
g_usleep(10000);
/// notify UI of changes done
notify_UI();
}
}
I believe I have a race condition in this code sample but am unsure how to mitigate it.
My scenario is that XAsync() always runs on the UI thread. Within XAsync(), I set m_importantMemberVariable and then start a timer; there's a 1 second delay before the timer fires.
My concern is the timer's tick event calls a method on m_importantMemberVariable. However, in the 1 second interval between starting the timer and Tick firing, XAsync() could be called again and overwrite m_importantMemberVariable.
Code example:
task<void> BobViewModel::XAsync()
{
return create_task(CreateSomethingAsync())
.then([this](SomethingAsync^ aThing)
{
this->m_importantMemberVariable = aThing;
OnPropertyChanged("ImportantMemberVariable");
// Timer has 1 second delay.
this->m_myDispatcherTimer->Start();
}, task_continuation_context::use_current())
.then([activity](task<void> result)
{
// more continuations...
});
}
void BobViewModel::OnTimerTick(Object^, Object^)
{
// Stopping the timer and detaching the event handler
// so timer only fires once.
m_myDispatcherTimer->Stop();
m_myDispatcherTimer->Tick -= m_impressionTimerToken;
m_myDispatcherTimer = { 0 };
// * Possible race condition *
m_importantMemberVariable->DoImportantThing();
}
Question: Assuming I'm correct about a race condition, is there a way to mitigate it?
My understanding is the tick event would fire on the UI thread so synchronization primitives won't help (as the UI thread would already have access).
All your operations are on the UI thread, so they've already been serialized (synchronized) for you. A simple flag will suffice:
bool m_busy; // set to false in constructor
task<void> BobViewModel::XAsync()
{
if (m_busy)
return;
m_busy = true;
// the rest of your code...
}
void BobViewModel::OnTimerTick(Object^, Object^)
{
m_busy = false;
// the rest of your code...
}
Just make sure you handle any exceptions such that you set m_busy back to false if something goes horribly wrong.
The answer to this question suggests using compare_exchange_strong with a std::atomic to ensure only one thread executes a function at a time. The problems with that approach, for this question, are:
1. The DispatcherTimer Tick event fires outside of the task continuation block, and can fire after the continuation completes.
2. A constraint on this problem is for the timer to only fire once.
Some alternative solutions are:
Use compare_exchange_strong but replace DispatcherTimer with create_delayed_task
Assuming the work doesn't have to happen on the UI thread, you can use create_delayed_task to delay work within a task continuation.
task<void>
BobViewModel::UseImportantVariableAsync(
Object^ importantVariable
)
{
return create_delayed_task(
std::chrono::milliseconds(1000),
[importantVariable]()
{
importantMemberVariable->DoImportantThing();
});
}
Then, from the task continuation, simply:
return UseImportantVariableAsync(m_importantMemberVariable);
Use a lambda for the DispatcherTimer's Tick event and capture 'aThing' from the question's example (instead of referencing the member variable in the handler). To only fire the timer once, assign the DispathcerTimer.Tick handler within a std::call_once block so only the first caller gets to do it.
Is it possible to stop a Thread by its associated QFuture Object ?
Currently i've been starting a video capturing process like this.
this->cameraThreadRepresentation = QtConcurrent::run(this,&MainWindow::startLiveCapturing);
Inside the startLiveCapturing-Method an infinite loop is running that captures images and displays them. So if the user wants to stop that process he should simply press a button and that operation stops.
But it seems that i can not stop this thread by calling the cancel method like this ?
this->cameraThreadRepresentation.cancel();
What i am doing wrong and how can i stop that thread or operation.
From the documentation of QtConcurrent::run:
Note that the QFuture returned by QtConcurrent::run() does not support canceling, pausing, or progress reporting. The QFuture returned can only be used to query for the running/finished status and the return value of the function.
What you could do is have a button press set a boolean flag in your main window and build your infinite loop like this:
_aborted = false;
forever // Qt syntax for "while( 1 )"
{
if( _aborted ) return;
// do your actual work here
}
Why don't you create a boolean flag that you can test inside your capturing loop and when it is set, it jumps out and the thread exits?
Something like:
MainWindow::onCancelClick() // a slot
{
QMutexLocker locker(&cancelMutex);
stopCapturing = true;
}
And then for your threaded function:
MainWindow::startLiveCapturing()
{
forever
{
...
QMutexLocker locker(&cancelMutex);
if (stopCapturing) break;
}
}
as Qt Document said, you can not use cancel() function fir QConcurrent::run() but you can cancel tasks by this answer :
https://stackoverflow.com/a/16729619/14906306
I am learning about threading and multithreading..so i just created a small application in which i will update
the progressbar and a static text using threading.I vl get two inputs from the user, start and end values
for how long the loop should rotate.I have 2threads in my application.
Thread1- to update the progressbar(according to the loop) the static text which will show the count(loop count).
Thread2 - to update the another static text which will just diplay a name
Basically if the user clicks start, the progressbar steps up and at the same time filecount and the name are displayed parallely.
There's is another operation where if the user clicks pause it(thread) has to suspend until the user clicks resume.
The problem is,the above will not work(will not suspend and resume) for both thread..but works for a singlw thread.
Please check the code to get an idea and reply me what can done!
on button click start
void CThreadingEx3Dlg::OnBnClickedStart()
{
m_ProgressBar.SetRange(start,end);
myThread1 = AfxBeginThread((AFX_THREADPROC)MyThreadFunction1,this);
myThread2 = AfxBeginThread((AFX_THREADPROC)MyThreadFunction2,this);
}
thread1
UINT MyThreadFunction1(LPARAM lparam)
{
CThreadingEx3Dlg* pthis = (CThreadingEx3Dlg*)lparam;
for(int intvalue =pthis->start;intvalue<=pthis->end; ++intvalue)
{
pthis->SendMessage(WM_MY_THREAD_MESSAGE1,intvalue);
}
return 0;
}
thread1 function
LRESULT CThreadingEx3Dlg::OnThreadMessage1(WPARAM wparam,LPARAM lparam)
{
int nProgress= (int)wparam;
m_ProgressBar.SetPos(nProgress);
CString strStatus;
strStatus.Format(L"Thread1:Processing item: %d", nProgress);
m_Static.SetWindowText(strStatus);
Sleep(100);
return 0;
}
thread2
UINT MyThreadFunction2(LPARAM lparam)
{
CThreadingEx3Dlg* pthis = (CThreadingEx3Dlg*)lparam;
for(int i =pthis->start;i<=pthis->end;i++)
{
pthis->SendMessage(WM_MY_THREAD_MESSAGE2,i);
}
return 0;
}
thread2 function
LRESULT CThreadingEx3Dlg::OnThreadMessage2(WPARAM wparam,LPARAM lparam)
{
m_Static1.GetDlgItem(IDC_STATIC6);
m_Static1.SetWindowTextW(L"Thread2 Running");
Sleep(100);
m_Static1.SetWindowTextW(L"");
Sleep(100);
return TRUE;
}
void CThreadingEx3Dlg::OnBnClickedPause()
{
// TODO: Add your control notification handler code here
if(!m_Track)
{
m_Track = TRUE;
GetDlgItem(IDCANCEL)->SetWindowTextW(L"Resume");
myThread1->SuspendThread();
WaitForSingleObject(myThread1->m_hThread,INFINITE);
myThread2->SuspendThread();
m_Static.SetWindowTextW(L"Paused..");
}
else
{
m_Track = FALSE;
GetDlgItem(IDCANCEL)->SetWindowTextW(L"Pause");
myThread1->ResumeThread();
myThread2->ResumeThread();
/*myEventHandler.SetEvent();
WaitForSingleObject(myThread1->m_hThread,INFINITE);*/
}
}
I thought I should summarize some of the discussion in the comments into an answer.
In Windows programming, you should never try to manipulate a GUI control from a background thread, as doing so can cause your program to deadlock . This means only the main thread should ever touch elements of the GUI. (Technically, what matters is which thread created the control, but it's not common to create controls in background threads).
This requirement is detailed in Joe Newcomer's article on worker threads (see "Worker Threads and the GUI II: Don't Touch the GUI").
You are using SendMessage in your thread procedures. This causes the appropriate message handler for the target control to be invoked, but in the thread that called SendMessage. In your case, that means the background threads run the message handlers and therefore update the progress bar and label.
The alternative is to use PostMessage. This causes the message to be added to a queue to be processed by the main thread's message loop. When the main thread gets to run, it processes the messages in the order they were added to the queue, calling the message handlers itself. Since the main thread owns the windows, it is safe for it to update the controls.
You should also beware that SuspendThread and ResumeThread are tricky to get right. You might want to read this section of Joe Newcomer's article, which describes some of the dangers.
Tasks like this are often better achieved by using a timer. This is a mechanism for having the operating system notify your program when a particular amount of time has passed. You could implement this with a timer as below:
BEGIN_MESSAGE_MAP(CThreadingEx3Dlg, CDialog)
ON_WM_DESTROY()
ON_WM_TIMER()
END_MESSAGE_MAP()
void CThreadingEx3Dlg::OnTimer(UINT_PTR nTimerID)
{
static int progress = 0;
if (nTimerID == 1)
{
m_ProgressBar.SetPos(progress);
CString strStatus;
strStatus.Format(_T("Processing item: %d"), progress);
m_Static.SetWindowText(strStatus);
progress++;
if (progress > end) // If we've reached the end of the updates.
KillTimer(1);
}
}
BOOL CThreadingEx3Dlg::OnInitDialog()
{
// ... initialize controls, etc, as necessary.
SetTimer(1, 100, 0);
}
void CThreadingEx3Dlg::OnDestroy()
{
KillTimer(1);
}
If you want both updates handled at the same time, they can use the same timer. If they need to happen at different times (such as one at a 100 ms interval and another at a 150 ms interval) then you can call SetTimer twice with different IDs. To pause the action, call KillTimer. To resume it, call SetTimer again.
Multi-threading and message queuing is quite a complex game. When you SendMessage from ThreadA to the same thread then it just calls the message handler. If you do it from ThreadA to another thread (ThreadB) then it gets more complicated. ThreadA then posts a message to the ThreadB's message queue and waits on a signal to say that ThreadB has finished processing the message and sent the return value. This raises an instant problem. If ThreadB is not pumping messages then you have a deadlock as the message in ThreadB's will never get "dispatched". This also raises an EVEN bigger problem. If ThreadB's message needs to send a message to a control created in ThreadA then you have a massive architectural problem. As ThreadA is currently suspended waiting for ThreadB to return and ThreadB is suspended waiting for ThreadA to return. Nothing will happen ... They will both sit suspended.
Thats about it really. Its pretty easy as long as you bear these issues in mind. ie It absoloutely IS possible despite what the others have said.
In general though your threading is pretty pointless because you straight away send a message to the main thread to do some processing. Why bother starting the threads in the first place. You may as well not bother because the threads will just sit there waiting for the main thread to return.
Why do you "WaitForSingleObject" anyway when you suspend the first thread? Why not just suspend them both.
All round, though, you aren't giving enough information about what you are doing to say exactly whats going on. What happens when you click pause, for example?
Windows will not operate properly when more than one thread interacts with the GUI. You'll need to reorganize your program so that does not happen.
I want to create a thread for some db writes that should not block the ui in case the db is not there. For synchronizing with the main thread, I'd like to use windows messages. The main thread sends the data to be written to the writer thread.
Sending is no problem, since CreateThread returns the handle of the newly created thread. I thought about creating a standard windows event loop for processing the messages. But how do I get a window procedure as a target for DispatchMessage without a window?
Standard windows event loop (from MSDN):
while( (bRet = GetMessage( &msg, NULL, 0, 0 )) != 0)
{
if (bRet == -1)
{
// handle the error and possibly exit
}
else
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
}
Why windows messages? Because they are fast (windows relies on them) and thread-safe. This case is also special as there is no need for the second thread to read any data. It just has to recieve data, write it to the DB and then wait for the next data to arrive. But that's just what the standard event loop does. GetMessage waits for the data, then the data is processed and everything starts again. There's even a defined signal for terminating the thread that is well understood - WM_QUIT.
Other synchronizing constructs block one of the threads every now and then (critical section, semaphore, mutex). As for the events mentioned in the comment - I don't know them.
It might seem contrary to common sense, but for messages that don't have windows, it's actually better to create a hidden window with your window proc than to manually filter the results of GetMessage() in a message pump.
The fact that you have an HWND means that as long as the right thread has a message pump going, the message is going to get routed somewhere. Consider that many functions, even internal Win32 ones, have their own message pumps (for example MessageBox()). And the code for MessageBox() isn't going to know to invoke your custom code after its GetMessage(), unless there's a window handle and window proc that DispatchMessage() will know about.
By creating a hidden window, you're covered by any message pump running in your thread, even if it isn't written by you.
EDIT: but don't just take my word for it, check these articles from Microsoft's Raymond Chen.
Thread messages are eaten by modal loops
Why do messages posted by PostThreadMessage disappear?
Why isn't there a SendThreadMessage function?
NOTE: Refer this code only when you don't need any sort of UI-related or some COM-related code. Other than such corner cases, this code works correctly: especially good for pure computation-bounded worker thread.
DispathMessage and TranslateMessage are not necessary if the thread is not having a window. So, simply just ignore it. HWND is nothing to do with your scenario. You don't actually need to create any Window at all. Note that that two *Message functions are needed to handle Windows-UI-related message such as WM_KEYDOWN and WM_PAINT.
I also prefer Windows Messages to synchronize and communicate between threads by using PostThreadMessage and GetMessage, or PeekMessage. I wanted to cut and paste from my code, but I'll just briefly sketch the idea.
#define WM_MY_THREAD_MESSAGE_X (WM_USER + 100)
#define WM_MY_THREAD_MESSAGE_Y (WM_USER + 100)
// Worker Thread: No Window in this thread
unsigned int CALLBACK WorkerThread(void* data)
{
// Get the master thread's ID
DWORD master_tid = ...;
while( (bRet = GetMessage( &msg, NULL, 0, 0 )) != 0)
{
if (bRet == -1)
{
// handle the error and possibly exit
}
else
{
if (msg.message == WM_MY_THREAD_MESSAGE_X)
{
// Do your task
// If you want to response,
PostThreadMessage(master_tid, WM_MY_THREAD_MESSAGE_X, ... ...);
}
//...
if (msg.message == WM_QUIT)
break;
}
}
return 0;
}
// In the Master Thread
//
// Spawn the worker thread
CreateThread( ... WorkerThread ... &worker_tid);
// Send message to worker thread
PostThreadMessage(worker_tid, WM_MY_THREAD_MESSAGE_X, ... ...);
// If you want the worker thread to quit
PostQuitMessage(worker_tid);
// If you want to receive message from the worker thread, it's simple
// You just need to write a message handler for WM_MY_THREAD_MESSAGE_X
LRESULT OnMyThreadMessage(WPARAM, LPARAM)
{
...
}
I'm a bit afraid that this is what you wanted. But, the code, I think, is very easy to understand. In general, a thread is created without having message queue. But, once Window-message related function is called, then the message queue for the thread is initialized. Please note that again no Window is necessary to post/receive Window messages.
You don't need a window procedure in your thread unless the thread has actual windows to manage. Once the thread has called Peek/GetMessage(), it already has the same message that a window procedure would receive, and thus can act on it immediately. Dispatching the message is only necessary when actual windows are involved. It is a good idea to dispatch any messages that you do not care about, in case other objects used by your thread have their own windows internally (ActiveX/COM does, for instance). For example:
while( (bRet = GetMessage(&msg, NULL, 0, 0)) != 0 )
{
if (bRet == -1)
{
// handle the error and possibly exit
}
else
{
switch( msg.message )
{
case ...: // process a message
...
break;
case ...: // process a message
...
break;
default: // everything else
TranslateMessage(&msg);
DispatchMessage(&msg);
break;
}
}
}