How to access to framebuffer to use linuxfb

[Firat]

Hello, I have a problem accessing the framebuffer. My QT application needs the linuxfb platform to run correctly, but for that I need to be able to access the host’s framebuffer.
When I run my application I get the following errors

Then in balena HOST terminal I do an ls from /dev/fb* and I got the error

I get the exact same error if I run the ls command in my app container.
My application deployment looks like this

  qt-secure-entry:
    image: x.x.x.x
    privileged: true
    devices:
      - "/dev/fb0:/dev/fb0"
    environment:
      - DISPLAY=host.docker.internal:0
      - DBUS_SYSTEM_BUS_ADDRESS=unix:path=/host/run/dbus/system_bus_socket

Thank You

[alexgg]

Hi Firat, could you please provide us with details about the device type you are using and the balenaOS version? Do you see the framebuffer device in the hostOS?

[Firat]

Hi @alexgg ,
I work on a raspberry pi4 with balenaOS host version 2.115.1+rev3. How can I do to see the framebuffer device in the hostOS?
To provide you with a bit more information, my project involves reading input from a keyboard plugged into my raspberry pi. I go through the linuxfb platform because my application has no graphical interface and therefore no x11

[alanb128]

Hello @firat, if you don’t need the FB for another purpose besides reading the keyboard input, have you looked into reading the raw keyboard events with something like evtest? https://raspberry-projects.com/pi/programming-in-c/keyboard-programming-in-c/reading-raw-keyboard-input
(or evdev in Python: Introduction — Python-evdev )
I used evdev to directly read a bar code scanner (keyboard wedge) on a Pi 4 with good results.

[Firat]

Hello @alanb128 ,
thank you for your answer, it is indeed simpler, here is an example of a solution in c++ for people who will have the same problem.

#include <iostream>
#include <fstream>
#include <cstring>
#include <linux/input.h>

int main() {
    std::ifstream input("/dev/input/eventX", std::ios::binary); // Replace X with the event number of the keyboard

    if (!input) {
        std::cerr << "Failed to open the input file." << std::endl;
        return 1;
    }

    struct input_event ev;

    while (input.read(reinterpret_cast<char*>(&ev), sizeof(struct input_event))) {
        if (ev.type == EV_KEY && ev.value == 1) {
            std::cout << "Key code: " << ev.code << std::endl;
            std::cout << "Value: " << ev.value << std::endl;
            std::cout << "Timestamp: " << ev.time.tv_sec << "." << ev.time.tv_usec << std::endl;
            std::cout << std::endl;
        }
    }

    input.close();

    return 0;
}