Taking in user input from a different thread - c++

Question

I'm making a timer program the counts the time passed since the program started. In the background I'm also checking for keyboard input (enter/return to exit, click on the window); this is done in a separate thread I've run as detached.
It seems the second thread cannot receive the input from the main thread. When I use the keyboard or mouse, nothing happens. Also, nothing appears on the screen, just white.
std::mutex g_mutex;
std::condition_variable cv;
// check for input from the user using the window object
// sets stopProgram to true if the user wishes to exit
void poll(sf::RenderWindow& window, bool& stopProgram) {
std::unique_lock<std::mutex> lk(g_mutex);
// wait for main thread to open window
cv.wait(lk, [&] { return !stopProgram && window.isOpen(); });
sf::Event event;
while (true) {
if (window.pollEvent(event)) {
// if user wants to exit program
if (event.type == sf::Event::Closed || (event.type == sf::Event::KeyPressed &&
(event.key.code == sf::Keyboard::Return || event.key.code == sf::Keyboard::Escape))) {
window.close();
// main thread will explicitly exit the main loop
stopProgram = true;
break;
}
}
}
}
int main()
{
int hour = 0, minute = 0, second = 0;
auto text = textObject();
bool stopProgram = false;
// run a background thread that checks for input while the main program runs
std::thread(poll, std::ref(window), std::ref(stopProgram)).detach();
std::once_flag flag;
std::lock_guard<std::mutex> lk(g_mutex);
while (window.isOpen()) {
// notify once window opens
std::call_once(flag, [&] { cv.notify_one(); });
// set timestamp
text->setString(makeTimeStamp(hour, minute, second));
// if the background thread set stopProgram, end the program
if (stopProgram) break;
window.clear(sf::Color::White);
window.draw(*text);
window.display();
// update time
second = (second + 1) % MAX_SEC;
if (second == 0) minute = (minute + 1) % MAX_MIN;
if (second == 0 && minute == 0) hour = (hour + 1) % MAX_HOUR;
// sleep one second
std::this_thread::sleep_for(std::chrono::seconds(1));
}
}
Is my use of multithreading correct? If so, can only the main thread receive input and that's why it's not working?
Update: If I get rid of the while (true) and use while (window.pollEvent(event)) and move the lock_guard to just before if (stopProgram) then the text (timestamp) appears on the screen, but I still cannot process input.

Answer 1

main thread launches poll thread.
std::thread(poll, std::ref(window), std::ref(stopProgram)).detach();
main thread acquires g_mutex and never ever releases it.
std::lock_guard<std::mutex> lk(g_mutex);
poll thread waits for g_mutex to be released:
std::unique_lock<std::mutex> lk(g_mutex);
but the main thread never releases it, so the poll thread never does anything.
To fix it. Change the beginning of the main() function:
int main()
{
int hour = 0, minute = 0, second = 0;
auto text = textObject();
volatile bool stopProgram = false;
// run a background thread that checks for input while the main program runs
std::thread(poll, std::ref(window), std::ref(stopProgram)).detach();
while (!window.isOpen()) { /* busy loop */ }
{
std::lock_guard<std::mutex> lk(g_mutex);
cv.notify_all();
}
while (window.isOpen()) {
...
This SFML API makes things more difficult than other windowing frameworks that I've used. It would be sooo useful if there were a thread-safe window.pushCustomEvent() function.