C++ and QT thread extern variable not working

Question

I am writing a C++ program and I'm using QT for the GUI. I need to pass data from a QT thread to another, so I decided to use an external variable but it does not work. This is my code:

stream_buffer.h

#ifndef STREAM_BUFFER_H
#define STREAM_BUFFER_H

#include "opencv2/core.hpp"

extern cv::Mat frame_buffer;

#endif // STREAM_BUFFER_H

Thread1.cpp

#include "stream_buffer.h"

//Some code

cv::Mat frame_buffer;

video_stream >> frame_buffer;

//other code

Thread2.cpp

#include "stream_buffer.h

imshow("debug", frame_buffer);

During the compilation, I get the error unresolved external symbol frame_buffer in function void run in Thread2.cpp. If I define the frame_buffer variable also in Thread2.cpp it compiles but of course it does not work (since it is a local variable so it doesn't read the value from the common header file).

How can I solve this problem? Thanks