multithreading with serial port

hi, i'm writing serial port listener(monitor) to my ARM microcontroler , using VISA to listen to com port  and print data in the textbox. but in the same time i want to use another GUI element and another functions.

here some code explanation:

here i'm open thread for task:

int CVICALLBACK SW_ON_OFF (int panel, int control, int event,
		void *callbackData, int eventData1, int eventData2)
{
	char *value;
	int val;
	char params[32]; 
	switch (event)
	{
	   case EVENT_COMMIT:
	   GetCtrlVal (panel, PANEL_BINARYSWITCH, &val);
	   if(val == 1)
	   {
	     CmtScheduleThreadPoolFunction(DEFAULT_THREAD_POOL_HANDLE, ThreadFunction, NULL, &mainThreadID);
	     CmtWaitForThreadPoolFunctionCompletion (DEFAULT_THREAD_POOL_HANDLE, mainThreadID, OPT_TP_PROCESS_EVENTS_WHILE_WAITING);          
	   }
	   else
	   {
		 CmtReleaseThreadPoolFunctionID (DEFAULT_THREAD_POOL_HANDLE, mainThreadID);
	        ResetTextBox (panelHandle, PANEL_TEXTBOX, "");
		ResetTextBox (panelHandle, PANEL_TEXTBOX_STATUS,"");
		executeCOMCommand("CLOSE", 1);
	    }
		break;
	}
	return 0;
}

double executeCOMCommand(char *commandString, int deviceNumber)
{

	ViSession defaultRM;  
	ViReal64 measurement = 0;
   	ViUInt32 num_bytes;
   	ViUInt32 RTCount;
	ViBoolean packetOK;
	int result, pp = 0;
	
	char dirName[4096], pathName[4096];
	
	int i, currentIndex, verbal = 0; //verbal - the returned measurement is in words
	int *deviceNumberTosend = malloc(sizeof(int));
	char adressString[1024], Databuf[1024], meas[1024], debugMSG[1024];
	 
	char dataToSend[4], err[4096], *packetdata;
	
	int status;
	
	char *command = strtok(commandString, " ");
	
	if(deviceNumber<0)
	{
		MessagePopup("Internal Error", "Window");
		return -1;
	}
		//Run coresponding thread
		*deviceNumberTosend = deviceNumber;

	
	
	//init anyway
	sprintf(adressString, "%s", UUTAddr);    
	viOpenDefaultRM (&defaultRM);
	result = viOpen(defaultRM,(ViRsrc)adressString, VI_NULL ,VI_NULL, &ssp);	
	if(result!=0)
		return -1;

	//init of ssp protocol
    viSetAttribute (ssp, VI_ATTR_TERMCHAR_EN, VI_TRUE);   //disable termination char
	viSetAttribute (ssp, VI_ATTR_TERMCHAR, 0x0A);
	viSetAttribute (ssp, VI_ATTR_ASRL_END_IN, VI_ASRL_END_LAST_BIT);
	
	viSetAttribute (ssp, VI_ATTR_ASRL_BAUD, 38400); 
	viSetAttribute (ssp, VI_ATTR_ASRL_PARITY, VI_ASRL_PAR_NONE);
	 
	viSetAttribute (ssp, VI_ATTR_TMO_VALUE, 25000); //TMO 15 seconds
	
	viFlush (ssp, VI_ASRL_IN_BUF_DISCARD);
	Sleep(1);
	
	if (!strcmp(command,"CLOSE")) //Reading     
	{
           viFlush (ssp, VI_WRITE_BUF);
	   viFlush (ssp, VI_READ_BUF);  
	   viClose(ssp);
	   return 0;
        }
	
        if (!strcmp(command,"LISTEN")) //Reading     
	{
	   status = viScanf(ssp, "%t", &Databuf);
	InsertTextBoxLine(panelHandle, PANEL_TEXTBOX, debugTextBoxLine++, Databuf); 
	//*************** Auto Scroll ***********************
	SetCtrlAttribute(panelHandle, PANEL_TEXTBOX, ATTR_FIRST_VISIBLE_LINE, debugTextBoxLine);
		//****************************************************
		return 0;
	} 
	   viFlush (ssp, VI_WRITE_BUF);
	   viFlush (ssp, VI_READ_BUF);  
	   viClose(ssp);
	   return 0;
}

 and thread function is: this function listen to com port and writes data to textbox

int CVICALLBACK ThreadFunction(void *functionData)
{
	while(!done)
	{
	    executeCOMCommand("LISTEN", 1);	
	}
	return 0;
}

 when i press quit button

int CVICALLBACK Quit (int panel, int control, int event,
		void *callbackData, int eventData1, int eventData2)
{
	switch (event)
	{
		case EVENT_COMMIT:
			done = 1;
			executeCOMCommand("CLOSE", 1);
			QuitUserInterface (0);
			break;
	}
	return 0;
}

 the program hangs and give me general protection error , it's still check the THreadFunction and do executeCOMCommand("LISTEN", 1).

do i wrote it correctly for multithreading or i must to use something else. how i can quit program in write way.

thanks