about timing loop and GUI control

Hi all,

  I have a code with two buttons placed on the panel, one is RUN, another is STOP. Initially, STOP is disabled and when I kick RUN, the STOP will be disable. But when STOP is  kicked, the RUN will be activated.

#include "testimg.h"

//==============================================================================
//
// Title:		BECQCL_mainkick
// Purpose:		This is the CVI implementation for BEC_PAkick.vi, which will 
//              generate MOT, BEC and submit kicking if loaded
//
// Created on:	2013/08/11 by Wakun Lam
// Copyright:	All Rights Reserved by Bose-Einstein Condensation and Quantum Chaos Laboratory in Oklahoma State University  
//
//==============================================================================

#include <windows.h>
#include <utility.h>
#include "asynctmr.h"
#include <ansi_c.h>
#include <cvirte.h>		
#include <userint.h>
#include <NIDAQmx.h>
#include "toolbox.h"


static int panelHandle = 0;

static int stoprun = 0;


int main (int argc, char *argv[])
{
	int error = 0;
	
	nullChk (InitCVIRTE (0, argv, 0));
	errChk (panelHandle = LoadPanel (0, "testimg.uir", PANEL));
	
	SetCtrlAttribute(PANEL, PANEL_STOPBUTTON, ATTR_DIMMED, TRUE);
	
	errChk (DisplayPanel (panelHandle));
	errChk (RunUserInterface ());
Error:
	if (panelHandle > 0) DiscardPanel (panelHandle);
	return 0;
}


int CVICALLBACK MainPanelResponse(int panel, int event, void *callbackData, int eventData1, int eventData2)
{
  if (event == EVENT_CLOSE) QuitUserInterface (0);
  return 0;
}

int CVICALLBACK RunButtonResponse (int panel, int control, int event, void *callbackData, int eventData1, int eventData2)
{
  if (event==EVENT_COMMIT)
  {
	int N=20;
	
	stoprun = 0;
	SetCtrlAttribute(panel, PANEL_STOPBUTTON, ATTR_DIMMED, FALSE);
	SetCtrlAttribute(panel, PANEL_RUNBUTTON, ATTR_DIMMED, TRUE);
	for (int i=0; i<N; i++)
	{
	  // some heavy task
	  // here I use Sleep instead
	  Sleep(5000);
	  ProcessSystemEvents();
	  Sleep(2000);
	  if (stoprun>0) break;
	}
	SetCtrlAttribute(panel, PANEL_STOPBUTTON, ATTR_DIMMED, TRUE);
	SetCtrlAttribute(panel, PANEL_RUNBUTTON, ATTR_DIMMED, FALSE);
  }
  return 0;
}

int CVICALLBACK StopButtonResponse (int panel, int control, int event, void *callbackData, int eventData1, int eventData2)
{
  if (event==EVENT_COMMIT)
  {
	stoprun = 1;
  }
  return 0;
}

My code is designed so to repeat a operation for N times after the START button is kicked. And if STOP button is kicked before the task is finished running. Stop the current application and return false; I wrote the code but it seems doesn't work. It seems that the application cannot be interrupted before the completete of the task. I don't know what's wrong with my code. If you have any experienence with that, could you please tell me what's wrong with my cdoe? Thanks.