Create a data streaming from C++ stream and read it in LabView

Hi all.

I'm working on a project that is to connect to a tracker of movement and reading data of position and orientation of this in real time. The code to get the data is in c ++, so I decided that the best way to do it would be to create a c++ DLL that contains all the functions necessary to first connect to the device and it reads the data and use the node to call a library function to power the Labview data.

I have a problem, because, ideally, I would like a continuous flow of data from the code c ++ in Labview, and I don't know how to do this. Put the node function of library call for a while loop seems like an obvious solution, but if I do it this way I'd have to reconnect to the device whenever I get the data, which is quite a bit too slow.

So my question is, if I created c ++ function that creates a data stream, could I read that in Labview without continually having to call a function? I would rather have only to call a function once, and then read the data stream until a stop command is given.

I'm using Labview 2010, version 10.0.

Apologies if the question is badly worded, thank you very much for your help.

Dave

dr8086 wrote:

This method resembles an excellent suggestion, but I have a few questions where I don't think I understood fully.

I understand the basic principle is to use a call library function node to access a DLL that creates an instance of the device object and passes a pointer too in labview. Then a separate call library function node would pass this pointer to another DLL that could access the device object, update and read the data. This part could be in a while loop, then continue on reading the data until a stop command is given.

That's all. I'm including some skeleton for example code. I am also including the code because I don't know how you experience multi threading so I show how you can use critical sections to avoid interactions between threads so that they do not lead to questions.

// exported function to access the devices
extern "C"  __declspec(dllexport) int __stdcall init(uintptr_t *ptrOut)
{
    *ptrOut= (uintptr_t)new CDevice();
    return 0;
}

extern "C"  __declspec(dllexport) int __stdcall get_data(uintptr_t ptr, double vals[], int size)
{
    return ((CDevice*)ptr)->get_data(vals, size);
}

extern "C"  __declspec(dllexport) int __stdcall close(uintptr_t ptr, double last_vals[], int size)
{
    int r= ((CDevice*)ptr)->close();
    ((CDevice*)ptr)->get_data(last_vals, size);
    delete (CDevice*)ptr;
    return r;
}

// h file
// Represents a device
class CDevice
{
public:
    virtual ~CDevice();
    int init();
    int get_data(double vals[], int size);
    int close();

    // only called by new thread
    int ThreadProc();

private:
    CRITICAL_SECTION    rBufferSafe;    // Needed for thread saftey
    vhtTrackerEmulator *tracker;
    HANDLE              hThread;
    double              buffer[500];
    int                 buffer_used;
    bool                done;       // this HAS to be protected by critical section since 2 threads access it. Use a get/set method with critical sections inside
}

//cpp file

DWORD WINAPI DeviceProc(LPVOID lpParam)
{
    ((CDevice*)lpParam)->ThreadProc();      // Call the function to do the work
    return 0;
}

CDevice::~CDevice()
{
    DeleteCriticalSection(&rBufferSafe);
}

int CDevice::init()
{
    tracker = new vhtTrackerEmulator();
    InitializeCriticalSection(&rBufferSafe);
    buffer_used= 0;
    done= false;
    hThread = CreateThread(NULL, 0, DeviceProc, this, 0, NULL); // this thread will now be saving data to an internal buffer
    return 0;
}

int CDevice::get_data(double vals[], int size)
{
    EnterCriticalSection(&rBufferSafe);
    if (vals)   // provides a way to get the current used buffer size
    {
        memcpy(vals, buffer, min(size, buffer_used));
        int len= min(size, buffer_used);
        buffer_used= 0;                 // Whatever wasn't read is erased
    } else  // just return the buffer size
        int len= buffer_used;
    LeaveCriticalSection(&rBufferSafe);
    return len;
}

int CDevice::close()
{
    done= true;
    WaitForSingleObject(hThread, INFINITE); // handle timeouts etc.
    delete tracker;
    tracker= NULL;
    return 0;
}

int CDevice::ThreadProc()
{
    while (!bdone)
    {
        tracker->update();
        EnterCriticalSection(&rBufferSafe);
        if (buffer_used<500)
            buffer[buffer_used++]= tracker->getRawData(0);
        LeaveCriticalSection(&rBufferSafe);
        Sleep(100);
    }
    return 0;
}

dr8086 wrote:

My main concern is that the object can get out of memory or be deallocated since it would not take place to any namespace or whatever it is.

As you create the object with the new, the object expire not until the dll is unloaded or the process (LabVIEW) closes. If the object will remain valid between condition LabVIEW dll calls not a not unload the dll (who does if the screws are closed). When that happens, I don't know exactly what happens to the active objects (that is, if you forgot to call close), I guess the system recovers the memory, but the device could still be opened.

What to do to make sure that everything is closed when the dll before I could call unloads close and remove the object is whenever I create a new object in the dll that I add to a list when the dll is unloaded, if the object is still on the list, that I'm deleting it.

dr8086 wrote:

I also have a more general question of programming on the purpose of the buffer. The buffer would essentially be a large table of position values, which are stored until they can be read in the rest of the VI?

Yes, see the code example.

However, according to the frequency with which you need to collect data from the device you have this buffer at all. That is, if take you a sample on every 100ms, then you can remove all threads and buffer related functions and instead to read data from the read feature itself like this:

double CDevice::get_data()
{
    tracker->update();
    return tracker->getRawData(0);
}

Because you need only a buffer and a separate if thread you collect data at a high frequency and you can not lose any data.

Matt

Tags: NI Software

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