Instructions needed to create,abort and close this kind of thread properly

Question

2.20/5 (3 votes)

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I have a dialog box with a Save button.

When that button is clicked, I should populate excel with data from database.

To do that, I have modified code from here: http://support.microsoft.com/kb/216686[^], which suites my purposes .

The problem is that this operation is lengthy and doesn’t allow user to use GUI until it is finished.

That is why I have decided to put Excel automation in a thread function, and to launch thread when Save button is clicked.

Earlier, I have posted here on CP, questions about multithreading, but have decided to stay away from it, since I am inexperienced.

I have decided to hide the Save button while thread is running, to prevent other threads being spawned with multiple clicks on a Save button.

This will ensure that there will be only one thread running, and will avoid thread synchronization.

I have created thread function which does the Excel automation as above, and it works.

The problem is that zombie process can appear ( Excel doesn’t close ) if the user interrupts thread execution by closing the dialog box, or closing the program.

That is my problem:

How to close dialog box/main window and to abort thread properly, so the zombie process does not appear ?

I work on Windows XP using MS Visual C++ and pure Win32 API ( no MFC ).

NOTE:

Here is what I have so far:

My thread function:

C++

struct ThreadData
{
	HWND hDlg; // handle of a dialog box
	int PrimaryKey; // so I can query database
};

DWORD WINAPI MyThread( LPVOID lp )
{
	ThreadData* td = (ThreadData*)lp;

	wchar_t query[100];

	swprintf_s( query, 100, 
		L"select * from MyTable where Primary_Key = %d ;",
		td->PrimaryKey);

	HRESULT hr;

	try
	{
		CoInitialize(NULL);

		hr = // open database and perform query;

		// populate Excel as described in article

		// perform cleanup

		return 0;
	}
	catch( _comm_error & ) // no need to display textual messages , just cleanup
	{
		// do cleanup

		return 1;
	}
}

Dialog box snippet:

C++

INT_PTR CALLBACK DialogBoxProcedure(HWND hwnd, UINT Message, WPARAM wParam, LPARAM lParam)
{
	static ThreadData td;

	HANDLE threadHandle = NULL;

	static int primaryKey; // we get this value from combo box

	 switch(Message)
	 {
	 case WM_COMMAND:
		{
		    switch(LOWORD(wParam))
		    {
		     case IDCANCEL:
				
                        if( threadHandle )
		        {
			    WaitForSingleObject( threadHandle, INFINITE );

			    CloseHandle( threadHandle );
		        }

			DestroyWindow(hwnd);

			break;

		    case IDC_BUTTON1:
			{
				ShowWindow( GetDlgItem( hwnd, IDC_SAVE ), 
                                           SW_HIDE ); 

				td.hwnd = hwnd;
				td.PrimaryKey = primaryKey;

				// create thread

				DWORD threadID;

				threadHandle = CreateThread( NULL,
                                                   0,
                                                   MyThread,
                                                   (void*)&td,
                                                   0,
                                                   &threadID );
								
				if( !threadHandle )
				{
				    MessageBox( hwnd, L"Error", L"Error",
                                               MB_ICONERROR );

				    DestroyWindow(hwnd);
			        }
						
			}
			break;
		    }
		}
	// other messages
	}
// other stuff in dialog procedure
}

IMPORTANT UPDATE:

Code for Excel generation is too big, but since I have a similar thread that populates predefined Word document, here is the link to that thread procedure, since the abort mechanism is practically the same:

http://pastebin.ca/2441960[^]

Posted 1-Sep-13 9:40am

AlwaysLearningNewStuff

Updated 1-Sep-13 18:43pm

v4

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2 solutions

Solution 1

I would create a (manual reset) event (see http://www.youtube.com/watch?v=d9AsH5i5nJ8[^]), before creating the thread, but which the thread can also access, which I would "set" when wanting to terminate the thread early (e.g. when cancel is clicked - before the wait for single object; or in the close event, etc). Then in the thread itself I would be regularly checking that the event is not set, at suitable regular intervals (again using wait for single object on the event, but with 0 time) and if it is set, exit the thread gracefully, if possible performing any "rollback" that is needed.

Is that enough help to get you started?

(You could also use a Boolean instead of an event, since setting and checking a Boolean is a single operation so should be effectively thread safe, but an event, which are also designed to be thread safe, fit more nicely into the threading model. E.g. you can use the wait to create a "periodic thread" - a job that loops but where the thread effectively "sleeps" for a time - or you could have jobs that wait on more than on event or object using the wait for multiple objects function, etc, etc).

Regards,
Ian.

Posted 1-Sep-13 13:48pm

Ian A Davidson

Comments

AlwaysLearningNewStuff 1-Sep-13 20:00pm

Thank you so much for your answer.

I did something similar, and have provided a link at the bottom of my question to the similar code. Excel function is big, but this thread function is smaller and has similar abort mechanism.

If you can take a look at it, and comment it I will greatly appreciate it.

Hopefully we will "speak" soon.

P.S. Recheck the link, currently it links to a cartoon! :))

Regards.

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pasztorpisti · Accepted Answer · 2013-09-01T18:42:00

Solution 2

You can not close/terminate a thread especially if it contains blocking function calls. You have to send a signal to the thread and then you have to wait for it to exit gracefully. You can not finish/kill a thread properly from another thread without causing possibly serious damage to the process state and without leaks.

"Earlier, I have posted here on CP, questions about multithreading, but have decided to stay away from it, since I am inexperienced."
And you are still inexperienced and you will remain inexperienced by asking the same question again and again. Multithreading is not roses all the way and sometimes there are no very good solutions. You have to wait your blocking function calls on your worker thread to finish. Period. No way to kill a thread instantly, sometimes a thread is terminating for seconds depending on the function calls it performs and the period in which it checks your cancel/exit request flag. In case of some function calls there are platform/api specific tricks that can radically speed up thread termination but since we have no clue about the code inside your try block we can not help in that.

BTW, previously you got fairly good answers to your question (with step-by-step instructions) and the answers are the same to this very same question: Delaying WM_CLOSE so I can allow threads to exit gracefully[^]

You won't get better answers. If you have no clue about multithreading but you want to write multithreading applications then learn multithreading.

Posted 1-Sep-13 18:42pm

pasztorpisti

Comments

AlwaysLearningNewStuff 2-Sep-13 0:54am

Code is linked at the bottom of my question.

As for "multithreading" part, I have meant that I just want to spawn one thread on button press and then disable button, so I do not have to handle the case when I have a user clicking multiple times on a button, and thus spawns multiple threads.

As for my earlier questions, I didn't want to rewrite/edit them, so I have posted a new one.

Why? Because reading through those questions I have concluded that I have no sufficient control over my threads, so I wanted to resolve that problem once and for all.

I thank you for trying to help me, but at this point I think that I really need someone to browse through my code, in order to suggest me the improvements. That is why I have linked my code at the bottom of the question.

I need CODE SNIPPET, theory doesn't help. I believe that my problem can be easilly solved by adding two event objects, but I do not know how to implement them in this case. All other tutorials have main window + some threads, I have yet to find a tutorial that has main window + dialog box + thread. That is why I needed to ask this question, to clarify things via few code snippets.

I did most of the stuff you described, and it works, but I need this final piece of the puzzle by providing those who want to help my code.

Regards.

[no name] 2-Sep-13 1:06am

What you are asking is way outside the scope of this forum. I concur with pasztorpisti that a great deal has been done by members here to assist you already. It's over to you now.

AlwaysLearningNewStuff 2-Sep-13 1:14am

I agree, that is why I have linked my code. I believe I am close, but at this point only posting a full code can help people to show me where am I doing things wrong.

[no name] 2-Sep-13 1:16am

Please take a break and read the answers/comments carefully. Your response bears no relationship to what I have said. This is not a forum for posting reams of code for others to analyse.

AlwaysLearningNewStuff 2-Sep-13 1:22am

All right.

Regards.

pasztorpisti 2-Sep-13 3:49am

To be honest I have no time to read and analyze your code and to read the documentation of the function calls you perform in it. Generally when its about terminating a thread the problem is always interrupting the long blocking function calls of the thread. Sometimes you simply can't do anything with a blocking call if it doesn't have an "async" version or an accompanying cancel call. For example in case of sockets you can read data with blocking sockets but in this case the recv() function doesn't return until the first byte on the connection arrives, but the same recv() operation can be performed in several other (async) ways with different APIs that are (gracefully) interruptable. If you call long blocking functions on your thread and they don't return for a long time and there are no alternative, cancellable function calls to do the same then you have to wait for that function call to finish. Period. Don't wait for a magician on this or on another forum to solve a long blocking call cancellation for you because it can't be solved. All you can do in such an evil case is checking a "cancel flag" between these blocking calls in order to minimize the response time of a thread cancellation.

EDIT: From the answers you got in the previous question you can put together code on your main/gui thread that doesn't freeze the gui while your thread is "cancelling" or finishing its job. As a zero version you should solve this "waiting" stuff on the gui side, putting in better thread cancellation is OK later. In your zero version you can let the thread to do its job without any actual cancel operation. You can put the cancel button to the gui but all it does is switching the gui into "cancelling" state. Your biggest problem here is that your program performs a blocking wait on the main thread instead of doing what I have described in my answer for your previous question. You want to solve this with a magic bullet that "kills" your worker thread instantly. This won't work.

AlwaysLearningNewStuff 2-Sep-13 4:49am

I just do not know how to implement Event object in the whole thing.

I think that I just need global event object that my main window can wait in WM_CLOSE.

That event should be signaled by thread when its job is finished, so main window can close.

This is how I have pictured it, it seems simple, and I believe it can work, but I can't implement it because I do not know how.
I do not seek a "magic bullet", I just didn't want to "clog" the previous thread, since it had 6 good answers, lots of comments, and the question itself was lengthy.

Regards.

pasztorpisti 2-Sep-13 5:18am

Reading your code and your comments I simply can't believe that you have ever read my answers in your previous question. There I never mentioned waiting for anything in cancel button press events or WM_CLOSE events. Maybe someone told you that the event magic bullet works all the time but it isn't true at all. Sometimes using a properly synchronized bool flag is enough depending on the scenario. Some async windows api calls can be cancelled immediately using event object but the number of such API calls is small. If you read my answers you can implement correct gui side behavior (without gui freeze) without implementing any cancel operations just by waiting for the thread to finish. You can put in cancel checks later if it is possible or if it makes sense.

AlwaysLearningNewStuff 24-Sep-13 14:41pm

It seems that everything works fine now. I just need to handle main window's WM_CLOSE properly, and everything will be fine.

In the link you have posted, I have accepted your solution.

I have updated my progress in my comment to Mr.Mori's answer.

As an experienced developer, perhaps you can take a quick look at it, to verify if I did things right this time ?

Thank you ( +5 from me ).

Regards.