I cannot access my stop button while data aquistion is going on

Question

I have written an application that aquires data from a GPS reciever over a TCP connection and displays it on a graph in "real-time" and it works except that I cannot access my STOP button to stop the connection while it is working. I think I need to do something with multithreading and have done some multithread programming before to do background processing but cannot seem to get a handle on how to make this work. I am sure it must be something qite simple as it seems like something that is quite common. Any help greatly appreciated. My application started life as the Multi XY plot example program from NI.
Thanks
DEan

Answer 1

You can pass a reference to the dialog class into your thread and manipulate the graph that way. The following snippet demonstrates one way of doing this using the DAQmx C API
UINT CDAQMTExampleDlg::AcquisitionThread(void* pParam)
CDAQMTExampleDlg* pDlg = static_cast < CDAQMTExampleDlg* >(pParam);
double data[1000];
int32 read;
int error;
if( (pParam != NULL) && pDlg->IsKindOf(RUNTIME_CLASS(CDAQMTExampleDlg)))
CoInitialize(NULL);
TaskHandle taskHandle = pDlg->taskHandle;
CNiReal64Vector vectorData(1000);
double *pvectorData = (double*)vectorData;
while(!pDlg->stopTask)
DAQmxErrChk(DAQmxReadAnalogF64(taskHandle,1000,10.0,DAQmx_Val_GroupByScanNumber,pvectorData,1000,&read,NULL));
pDlg->m_Graph.PlotY(vectorData);
Error:
DAQmxStopTask(taskHandle);
DAQmxClearTask(taskHandle);
pDlg->stopTask = true;
CoUninitialize();
return 0;
void CDAQMTExampleDlg:nBnClickedButton1()
int error;
DAQmxErrChk(DAQmxCreateTask("",&taskHandle));
DAQmxErrChk(DAQmxCreateAIVoltageChan(taskHandle,"Dev1/ai0","",DAQmx_Val_Cfg_Default,-10.0,10.0,DAQmx_Val_Volts,NULL));
DAQmxErrChk(DAQmxCfgSampClkTiming(taskHandle,"",10000.0,DAQmx_Val_Rising,DAQmx_Val_ContSamps,1000));
DAQmxErrChk(DAQmxStartTask(taskHandle));
stopTask = false;
AfxBeginThread(AcquisitionThread, this);
return;
Error:
char errBuff[2048]={'\0'};
if( DAQmxFailed(error) )
DAQmxGetExtendedErrorInfo(errBuff,2048);
if( taskHandle!=0 ) {
/*/ DAQmx Stop Code
DAQmxStopTask(taskHandle);
DAQmxClearTask(taskHandle);
if( DAQmxFailed(error) )
printf("DAQmx Error: %s\n",errBuff);
void CDAQMTExampleDlg:nBnClickedButton2()
stopTask = true;
I also wanted to make sure that you know that if you have Measurement Studio 7.x, you have access to a C++ DAQmx API. You can find examples of that API under ..\MeasurementStudio\VCNET\Examples\DAQmx. The C++ API provides asynchronous methods, which means that aquisition worker threads are automatically created for you in the background instead of you having to create them yourself. Plus the C++ DAQmx API provides error handling via exceptions and is designed for use with MFC.
I hope this helps
Bilal Durrani
NI