C / C++ / MFC

19-Nov-02 10:23

Hello everyone,

I have a question about multithreading and syncing data. I have two threads A and B; A feeds data to a cache in B then waits until B is finished with its cache. Example in Pseudo code:
The Mutexes were taken out for code clarity.
A thread:
{
for (I=0; I < 5; ++I)
B->loadData(i);

B->Wait(); // Now wait for thread B to finish processing data.
...
}

B Thread
{
Handle dwWait;
Handle dwRun;
Vector<int> vI;
ThreadFunc()
{
while (1)
{
WORD wdCheck = WaitForSingleObject(dwRun, INFINITE);

switch (wdCheck)
{
case: 0
{
for (int I = 0; I < vI.size(); ++I)
...
SetEvent(dwWait); // Now vector is empty send event.
}
case: 1
{
}
}
}

void Wait()
{
WaitForSingleObject(dwWait, INFINITE);
}

void loadData(int I)
{
vI.push_back(I);
SetEvent(dwRun);
}
}

My question is, why doesn’t thread A always wait for Thread B to finish before continuing on.

Thanks in advance.
Ken

Joaquín M López Muñoz19-Nov-02 11:42

19-Nov-02 11:42

The key is in the piece of pseucode:

for (I=0; I < 5; ++I) 
B->loadData(i);

This feeds the cache with 5 ints, signalling dwRun after inserting each int. So, imagine the following (possible) pattern of execution:

A feeds the 1st int.
A sets dwRun.
The system yields controls to B.
B processes the cache (containing only one int) and sets dwWait.
The system yields controls to A.
A loads the remaining four ints, setting dwRun after each insertion.
A Waits and exits immediately, as dwWait is already set. There's still some data in B's cache.

See the problem now?

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo

Madmaximus20-Nov-02 2:11

20-Nov-02 2:11

Thanks for your help and time, Joaquín Big Grin | :-D

I see, so the event is already set by the time the wait function is called by A. Thus that is why Thread A does not always wait for Thread B. So how would one go about fixing this dilemma? Perhaps by putting another mutex around the wait function call? Or maybe use a while loop in the Wait function that checks if the vector is empty? Are there any examples of something like this that I could refer to?

Example:
void Wait() // While loop checking size of vector.
{
while (vI.size() != 0)
WaitForSingleObject(dwWait, INFINITE);
} // But this does not seem like a very clean solution.

Thanks again for your help.
Ken

Joaquín M López Muñoz20-Nov-02 2:30

20-Nov-02 2:30

The easiest solution is probably to split LoadData into two functions, say LoadData and Run, the first feeding the cache and the second setting dwRun, and call Run only when finished pushing the data.

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo

Madmaximus21-Nov-02 4:18

21-Nov-02 4:18

Thanks again. Cool | :cool:

Another possible solution that I came up with, which is to reset dwWait in the Wait function. This seems to work??? Smile | :)

But I'm wondering if there is an even cleaner way to solve this? Confused | :confused:

Example:
void Wait()
{
if (vI.size() != 0)
{
ResetEvent(dwWait); // This will make sure to wait for event.
WaitForSingleObject(dwWait, INFINITE);
}
}

Thanks for all of the help. Big Grin | :-D

Ken

Joaquín M López Muñoz21-Nov-02 7:50

21-Nov-02 7:50

Watch out, this solution you propose is broken! The possibility of deadlock is minimal, but this is even worse, as chances are you don't notice anything till it's too late and the app has been released. Counterexample:

A feeds the data.
A checks vI.size()!=0. This evaluates to true (B has not been given access by the system yet).
Thread switch: B begins executing, process the whole data and sets dwWait.
Thread switch: A resets dwWait and wait for completion --for ever.

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo

Madmaximus22-Nov-02 9:03

Canceling the closing of a modal dialog

22-Nov-02 9:03

Hi again,

Ok, here is my latest attempt to resolve this.

Handle mMutex;
mMutex = CreatMutex(...);
ThreadFunc()
{
while (1)
{
WORD wdCheck = WaitForSingleObject(dwRun, INFINITE);
switch (wdCheck)
{
case: 0
{
WaitForSingleObject(mMutex, INFINITE); // New code here.

for (int I = 0; I < vI.size(); ++I)
...

ReleaseMutex(mMutex); // New code here.
SetEvent(dwWait); // Now vector is empty send event.
}
case: 1
{
}
}
}

void Wait()
{
WaitForSingleObject(mMutex, INFINITE); //New code here.

if (vI.size() != 0)
{
ResetEvent(dwWait); // This will make sure to wait for event.
ReleaseMutex(mMutex); // New code here.

WaitForSingleObject(dwWait, INFINITE);
}

ReleaseMutex(mMutex); // New code here.
}

Wooweee, I think I got it right this time.

Again thanks for all of your help. Cool | :cool:

Ken

Ken Goguen19-Nov-02 9:11

Ken Goguen

19-Nov-02 9:11

Is there a way to prevent a modal dialog from closing when a user initiates the WM_CLOSE?

I'm handling WM_CLOSE message using my own OnClose() where I display a message box with YESNOCANCEL. If the user selects CANCEL I want to return them to the current dialog, unharmed. I tried just returning before executing CDialog::OnClose(), but it still closes and ends the application.

I've read many suggestions within the forum and nothing seems to do what I want, all result in the dialog closing and app ending.

-kg

Ken Goguen

RobJones19-Nov-02 9:19

RobJones

19-Nov-02 9:19

This is done Handling the WM_CLOSE..

void CTestDlg::OnClose() 
{
	if(MessageBox("Close?","Really?",MB_YESNOCANCEL)== IDYES)
	{
		CDialog::OnClose();
	}
	else
	{
		// Don't Close
	}
}

Ken Goguen19-Nov-02 9:30

Ken Goguen

19-Nov-02 9:30

Like I said, that's what I am doing and it still closes...

void CMyDlg::OnClose()
{
if (curFileDirty) {
int answer=MessageBox( "Changes have been made. Would you like to Save them?", "Warning", MB_YESNOCANCEL | MB_ICONQUESTION );

if (answer==IDYES) {
if (currentPrjFilePath=="")
OnFileSaveAsConsolidation();
else
OnFileSaveConsolidation();
finalClose();
CDialog::OnClose();
}

if (answer==IDNO) {
finalClose();
CDialog::OnClose();
}
}
}

Ken Goguen

Joaquín M López Muñoz19-Nov-02 9:55

19-Nov-02 9:55

Ummm... Override also OnOK and OnCancel to do-nothing functions and see what happens.

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo