Understanding OpenCV Clone

 frame=cv::imdecode(tmp,cv::IMREAD_COLOR);

This line refers to cv::Mat::operator=().

Before assigning new data, the old data is de-referenced via Mat::release.

Data held by the frame is released as described in .If you read it from a different thread using frame.clone(), you will naturally get an error.

Although it can be resolved by inserting an exclusive action, we recommend that you consider what to do with the lock range and whether there are any other exclusions besides the frame variable.

<Threads retrieving images>

 cv::Mat frame;
cv::Mutex mutex;
DWORD WINAPI ImageThread (LPVOID lpData)
{   
    while(!lpd->bEnd){

        if(mJpegData!=nullptr&mJpegSize>0){
            std::vector<unsigned char>tmp(mJpegData, mJpegData+mJpegSize);
            cv::AutoLock lock (mutex);
            frame=cv::imdecode(tmp,cv::IMREAD_COLOR);
            cv::imwrite("img_debug.bmp", frame);
        }
    }
    return 0;
}

<Threads copying images>

DWORD WINAPI ConThread (LPVOID lpData)
{
    cv::Matimg;

    while(!lpd->bEnd){
        cv::AutoLock lock (mutex);
        if(!frame.empty){
            img = frame.clone();
        }
    }
}

Am I correct in understanding that when (!frame.empty()) is done on the thread copying the image, an error may occur because there may be an undefined (irregular) timing between this if statement and "img=frame.clone()" in frame?

Access violations occur if Mat::release releases memory on a separate thread, even if frame.clone() is running as well as frame.clone().Because objects are not GC-managed in the C++ language, there is no guarantee that they will not be released because they are running.