threading

**laasunde** · January 23rd, 2004, 06:20 PM

Hello,

I've created an mfc application which at the moment listen to a specific port using csocket and output the data into a text file. This process is run in a separate thread (a worker thread I belive). The program works fine.

To upgrade the program I thought about adding a text field with a cstring object attached to it. My question is then, how can I update the textfield with the data that my listen-thread receive ? Can I post\send the data or a pointer from one thread to another ? What about synchronising ?

Links to related articles or tutorials would be much appreciated.

Cheers

**Andreas Masur** · January 23rd, 2004, 07:18 PM

Take a look at the following FAQ...

**kuphryn** · January 24th, 2004, 12:37 PM

One solution is messages.

Kuphryn

**laasunde** · January 24th, 2004, 03:14 PM

The below code compiles and links ok but I get an 'Unhandled exception at 0x00.... access violation" at the :

ostmessage line.

Code:

struct threadData
{
	Driver *d;
	HWND *hwnd;
};

UINT StartThread( LPVOID pParam )
{
	threadData * structData = (threadData*) pParam;
	Driver *obj = structData->d;
	HWND *hwnd  = structData->hwnd;
	if (obj == 0)
		return -1;

	int status = obj->start(hwnd);
	return 0;
}

LRESULT CReceiverDlg::OnUpdateControl(WPARAM wParam, LPARAM lParam)
{
  // TODO - add code
  return 0;
}

BEGIN_MESSAGE_MAP(CReceiverDlg, CDialog)
	ON_WM_PAINT()
	ON_WM_QUERYDRAGICON()
	//}}AFX_MSG_MAP
	ON_MESSAGE(WM_UPDATE_CONTROL, OnUpdateControl)
END_MESSAGE_MAP()

BOOL CReceiverDlg::OnInitDialog()
{
	// more code...
	Driver *drv = new Driver();
	threadData *obj = new threadData();
	obj->d = drv;
	obj->hwnd = (HWND*)GetSafeHwnd(); 
	AfxBeginThread(StartThread, (LPVOID) drv);

	return TRUE;  
}

int Driver::start(HWND * hwnd)
{
	CSocket sock;
	char buf[1024];
	CString addr;
	UINT port;
	if (sock.Create(5000, SOCK_DGRAM, "10.0.0.10") == 0)
		return -1;

	while(1)
	{
		int received_bytes = sock.ReceiveFrom((void*) buf, 1024, addr, port);
		if (received_bytes == SOCKET_ERROR)
		{
			out << "Recevefrom failed" << endl;
			return -1;
		}
	         ::PostMessage(*hwnd, WM_UPDATE_CONTROL, 0, 0);
	}
	sock.Close();
	return 0;
}

How can I post the buf data back to the primary thread ?

**Andreas Masur** · January 24th, 2004, 05:36 PM

Your problem is the following line

Code:

obj->hwnd = (HWND*)GetSafeHwnd();

'GetSafeHwnd()' returns a 'HWND' and not a pointer. You treat the returned handle as a pointer to a handle (by casting it). While dereferencing it in the thread function you of course call for trouble...

Change your structure to accept a HWND...

Code:

struct threadData
{
  Driver *d;
  HWND hwnd;
};

Then assign it accordingly...

Code:

obj->hwnd = GetSafeHwnd();

The thread function needs to delete the passed data structure before it terminates otherwise you will have memory leaks...

Code:

UINT StartThread( LPVOID pParam )
{
  threadData * structData = (threadData*) pParam;
  Driver *obj = structData->d;
  HWND hwnd  = structData->hwnd;
  if (obj == 0)
    return -1;

  int status = obj->start(hwnd);

  // Delete passed data
  delete structData;

  return 0;
}

Finally your 'start()' function...

Code:

int Driver::start(HWND hwnd)
{
  CSocket sock;
  char buf&#091;1024&#093;;
  CString addr;
  UINT port;

  if (sock.Create(5000, SOCK_DGRAM, "10.0.0.10") == 0)
    return -1;

  while(1)
  {
    int received_bytes = sock.ReceiveFrom((void*) buf, 1024, addr, port);

    if (received_bytes == SOCKET_ERROR)
    {
      out << "Recevefrom failed" << endl;
      return -1;
    }

    ::PostMessage(hwnd, WM_UPDATE_CONTROL, 0, 0);
  }

  sock.Close();
  return 0;
}

**abc_coder** · January 24th, 2004, 07:08 PM

Originally posted by Andreas Masur

The thread function needs to delete the passed data structure before it terminates otherwise you will have memory leaks...

that´s a bad policy - better leave the memory management for the threadData completely to the dialog object:

add a threadData member
initialize it inside the onInitDialog() function
pass its address to the thread (no new/delete needed)

otherwise if you happen to pass the same pointer to more than one thread, your solution leads to multiple deletes.

next, to pass the data back to the dialog, you need a common buffer and a synchronization object to serialize access to it (say, 2 more members for the threadData struct, a char[] and a mutex, and have them passed forward to the Driver::start() function as well)

**Andreas Masur** · January 25th, 2004, 02:51 AM

Originally posted by abc_coder
that´s a bad policy - better leave the memory management for the threadData completely to the dialog object:

add a threadData member
initialize it inside the onInitDialog() function
pass its address to the thread (no new/delete needed)

otherwise if you happen to pass the same pointer to more than one thread, your solution leads to multiple deletes.

Well...I do not want to start a big discussion about how to do these things right. Nevertheless your solution lacks from some of the mentioned points as well and requires more handling then you have described.

Since the thread data are a member of your class, you need to make sure that the thread will be terminated before your class will be destroyed. Otherwise your thread will deal with a dangling pointer.

Another point is the number of threads. As long as each thread gets the same data you can simply use one structure. However, as soon as the data needs to vary for each thread you will simply fill up your class with n member variables.

Originally posted by abc_coder
next, to pass the data back to the dialog, you need a common buffer and a synchronization object to serialize access to it (say, 2 more members for the threadData struct, a char[] and a mutex, and have them passed forward to the Driver::start() function as well)

This might be a different approach while dealing with one thread, nevertheless, Windows is an event-driven operating system...so, why not make use of it?

Furthermore, this approach is not practible while dealing with several threads...first of all the processing time would decrease since every thread has to wait until the previous result would have been processed, and adding several kernel objects adds another overhead which is not necessary...

**laasunde** · January 25th, 2004, 07:39 AM

Andreas Masur, thanks for your reply. I've made the updates you adviced. The code now both compiles\links and runs just fine without any error messages.

The problem however is that OnUpdateControl() doesnt seem to be executed when :

ostmessage is executed.

Just for testing purpose my OnUpdateControl lools like this :

Code:

LRESULT CReceiverDlg::OnUpdateControl(WPARAM wParam, LPARAM lParam)
{
  GetDlgItem(IDC_EDIT1)->SetWindowText("OnUpdateControl");
  return 0;
}

Let me know if you need more information.

**Andreas Masur** · January 25th, 2004, 07:47 AM

Did you define all necessary entries in the message map? Maybe you can post the definition of the message and the mapping of the message to the function in the message map.

Other than that, I do not see any reason why it should not work...

**laasunde** · January 25th, 2004, 08:05 AM

Thanks for your prompt reply.

Code:

#define WM_UPDATE_CONTROL    WM_USER + 0x10

BEGIN_MESSAGE_MAP(CReceiverDlg, CDialog)
	ON_WM_PAINT()
	ON_WM_QUERYDRAGICON()
	//}}AFX_MSG_MAP
	ON_MESSAGE(WM_UPDATE_CONTROL, OnUpdateControl)
	ON_BN_CLICKED(IDOK, OnBnClickedOk)
END_MESSAGE_MAP()

Do you need anymore code ?

**Andreas Masur** · January 25th, 2004, 08:37 AM

Well...looks okay except that you should change 'WM_USER' to 'WM_APP' (it was from the FAQ, however, I have changed the FAQ accordingly...).

Can you probably post your project here (without the debug and release directories)?

**laasunde** · January 25th, 2004, 09:08 AM

Here is my project files.

Thanks for your help man.

**Andreas Masur** · January 25th, 2004, 09:54 AM

After getting through some compiler errors and warnings (you did not send the 'res' directory), it is running okay with the following changes:

Remove '#define WM_UPDATE_CONTROL WM_USER + 0x10' from Driver.h
Change #include "fstream.h" to #include"fstream" in 'Driver.cpp' and put it a the top
Add #include "iostream" in 'Driver.cpp' and put it a the top as well
Add #include "ReceiverDlg.h" in 'Driver.cpp'
Change 'ofstream out("file.txt");' to 'std::ofstream out("file.txt");' in Driver.cpp
Change 'endl' to 'std::endl' in Driver.cpp
Add #include "resource.h" in 'ReceiverDlg.h'

Change in to the following in 'OnInitDialog()' of 'ReceiverDlg.cpp':

Code:

threadData *obj = new threadData();
  
obj->d = new Driver();
obj->hwnd = GetSafeHwnd(); 
  
AfxBeginThread(StartThread, (LPVOID) obj);

Change the function 'StartThread' in 'ReceiverDlg.cpp' as follows:

Code:

UINT StartThread(LPVOID pParam)
{
  threadData * structData = (threadData*) pParam;
  Driver *obj = structData->d;
  HWND hwnd  = structData->hwnd;
  if (obj == 0)
    return -1;

  int status = obj->start(hwnd);
  delete structData->d;
  delete structData;
  return 0;
}

**laasunde** · January 27th, 2004, 10:49 AM

Andreas Masur, thank you sir.

Your help is much appreciated

**Andreas Masur** · January 27th, 2004, 11:08 AM

Originally posted by laasunde
Andreas Masur, thank you sir.

Your help is much appreciated

You are welcome...

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