flip and thread play Video error

edit

Hi,

I have a function(assume the function is A) that capture image from my webcam, and another function play a video (assume the function is B). Both of them can run independently well. but when I mix them by using thread mThead(B) in function A. The B function will fail. 
I have found that in function A if I execute the flip(...), the program can run into function B but it cannot play the video. If without flip(), the function B will correctly run.

Is there anyone know what happen to this? And how can I fix it?

Thank you very much.

This is some of my code:

using namespace cv;
using namespace std;

// Global variables
Mat frame; //current frame
int keyboard; //input from keyboard

void processVideo();
void playVideo();

int main()
{

    //create GUI windows
    namedWindow("Frame");
    processVideo();

    //destroy GUI windows
    destroyAllWindows();
    return EXIT_SUCCESS;
}

void playVideo(){
    VideoCapture cap("A.avi");
    Mat myframe;

    if (!cap.isOpened()) {
        cout << "Cannot open the video file on C++ API" << std::endl;
        exit(EXIT_FAILURE);
    }

    for (;;) {
        cap >> myframe;
        if (myframe.empty())
            break;
        imshow("Video Frame", myframe);
        int c = waitKey(10);
        if ((char)c == 27) { break; }
    }
    cap.release();
}

void processVideo() {
    Mat dst;

    //create the capture object
    VideoCapture cap(0);
    //open capture
    if (!cap.isOpened()){
        cout << "Open camera failed";
        exit(EXIT_FAILURE);
    }

    //read input data. ESC or 'q' for quitting
    while ((char)keyboard != 'q' && (char)keyboard != 27){
        //read the current frame
        if (!cap.read(dst)) {
            cerr << "Unable to read next frame." << endl;
            cerr << "Exiting..." << endl;
            exit(EXIT_FAILURE);
        }

        flip(dst, frame, 1);   //something image processing

        if (condition is true){
            thread mThread(playVideo);
            // wait the thread stop
            mThread.join();
        }

        //show the current frame and the fg masks
        imshow("Frame", frame);

        //get the input from the keyboard
        keyboard = waitKey(30);
    }

    //delete capture object
    cap.release();
}