Pass Document data to thread

My application contains RS232 communication. The user should be able to pick which Com port to use. The seriall interface is controlled by a separate thread. When the user pick a Com port from the list an int is stored in the Document corresponding to a Com port. This int has to be passed to the seriall com thread. I have managed to get a pointer to the Document but I get an assertion about access violation when passing the int value.

here is the assertion,
_AFX_INLINE int CString::Compare(LPCTSTR lpsz) const
{ ASSERT(AfxIsValidString(lpsz)); return _tcscmp(m_pchData, lpsz); }

here is the code used in the seriall com thread,

CWinApp* theApp = AfxGetApp();
appPos = theApp->GetFirstDocTemplatePosition();
CDocTemplate* docTemplate = theApp->
GetNextDocTemplate(appPos);
docPos = docTemplate->GetFirstDocPosition();
CGsdoc_b1Doc* pDoc = (CGsdoc_b1Doc*)docTemplat
-> GetNextDoc(docPos);
comPort = pDoc->GetComPort(); //Access violatio!!!
comStr.Format("%s%d", "Com", comPort);

There are several solutions. One solution is a pointer to a constant document member variable. Another solution is a global variable.

Kuphryn

You can not pass MFC objects into a thread. Instead of passing
the pointer to the app, pass the number in dircetly.

How do I pass the number in directely? With a global variable? I have never used that before.

This variable does not change during the lifetime of the thread, right?
If so, then you do not need a global variable.

One solution is to create a struct

struct COM_PORT_THREAD_PARAMETERS
{
unsigned int ComPort_;
//anything you want to pass in goes in here. NO MFC
}

then when it comes time to create the thread

COM_PORT_THREAD_PARAMETERS thread_param = new COM_PORT_THREAD_PARAMETERS;
and pass this pointer into the thread function.
It becomes the threads responsibility to delete the pointer
to free ther memory allocated.

It seems to work when using constant variables. Thanks guys.

The thread is created in the InitInstance function. Since the com port value used in the thread comes from the Document the thread should not be started before all the Serialisation of the Document. So I create the thread in the beginning of InitInstance in suspended mode. Then the thread is started with ResumeThread at the end of InitInstance. But when debugging inside the Thread function a UM_FINISH message is recieved meaning that the thread should be terminated.
I don't understand why.