C ++多线程使应用程序崩溃

Question

I am trying to get 2 threads running, one takes a screenshot and changes the global image variable, the other one reads the image and detects objects. Everything works fine when im using a std::mutex to lock the image like the following:

Thread1

void mHandler()
{
    while (true) {
      mat_img_mutex.lock();
      mat_img = hwnd2mat(); //screenshot is taken and saved into global mat_img
      mat_img_mutex.unlock();
      Sleep(3);
   }
}

Thread2

mat_img_mutex.lock();
result_vec = detector.detect(mat_img); //image detection
mat_img_mutex.unlock();

So far so good, but this doesn't really help me because its blocking each others thread execution so it doesn't have any performance advantage over just using one thread.

What i tried: Thread1, taking the screenshot function out of the mutex lock so only the variable definition happens in the mutex. This CRASHES the program though and i have no idea why.

void mHandler()
{
    while (true) {
      cv::Mat img_copy = hwnd2mat(); //heavy screenshot function out of the mutex
      mat_img_mutex.lock();
      mat_img = img_copy //screenshot is taken and saved into global mat_img
      mat_img_mutex.unlock();
      Sleep(3);
   }
}

Thread2, i also tried taking the detection function out of the mutex with the same idea which resulted in a crash as well which is weird as i thought the copy instructor of c++ inst referencing the variable what so ever so it shouldn't have any problems with simultaneous read/writes...

Answer 1

cv::Mat uses reference counting. Make sure to get a copy of the data. Use

mat_img = img_copy.clone()

to get a deep copy of the image.