This is a text-only version of the following page on https://raymii.org:
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Title       : 	C++ async, threads and user input
Author      : 	Remy van Elst
Date        : 	24-04-2020
URL         : 	https://raymii.org/s/articles/Cpp_async_threads_and_user_input.html
Format      : 	Markdown/HTML
---



For an unrelated piece of code, I recently spent a few days trying to figure 
out if there was a portable, modern C++ way to handle user input with a 
timeout. If there is no input after a few seconds the program can continue
doing other things. TL;DR, there is none, since `stdin` is blocking I/O. 

`alarm`, `conio.h`, using `ncurses` or manually polling `stdin` are all way
to complex for the scope of the program. I ended up with using two `std::threads`,
one for input and one for the "other things". I did play with `std::future` and
`std::async` since that way is easier to just 'wait until this is done' as 
opposed to manually managing 'actual big boy' threads. 

This article has example code that uses `std::async` in an attempt to wait until
the user has given some input and otherwise quit after 5 seconds. It does not
work since `std::getline` is blocking. The `main()` function ends, but the
`async` function is still waiting for user input.

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At the end of the article I'll also provide the code I ended up using, with the 
two threads, one for input and one for 'other work'.

### The async code

Below is a single `test.cpp` file. It is not my actual program, but a simplified
version to show 

    #include <iostream>
    #include <string>
    #include <future>
    #include <chrono>
      
    std::string getString()
    {
      std::cout << "# getString() start\n";
      std::cout << "# getString(): Please enter a string. You have 5 seconds...\n";
      std::string input;
      std::getline(std::cin, input);
      std::cout << "# getString() end\n";
      return input;
    }
     
    int main()
    {
      std::cout << "# main() start\n";
      std::cout << "# main(): Starting std::async(getString)\n";
   
      std::future<std::string> futureString = std::async(std::launch::async, getString);

      std::cout << "# main(): Waiting 5 seconds for input...\n";

      std::chrono::system_clock::time_point five_seconds_passed
              = std::chrono::system_clock::now() + std::chrono::seconds(5);
      std::future_status status = futureString.wait_until(five_seconds_passed);

      if (status == std::future_status::ready)
      {
          auto  result = futureString.get();
          std::cout << "# main(): Input result: " << result << "\n";
      }
      else
      {
          std::cout << "# main(): Timeout reached... \n";
      }

      std::cout << "# main() end" << std::endl;
      return 0;
    }

Make sure you pass `-pthread` while compiling. In CMake:

    find_package(Threads REQUIRED)
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pthread" )

### The result

The below gif shows the program when input is given within five seconds:

![input][1]

Textual output:

    # main() start
    # main(): Starting std::async(getString)
    # main(): Waiting 5 seconds for input...
    # getString() start
    # getString(): Please enter a string. You have 5 seconds...
        ===== RETURN PRESSED ON KEYBOARD =====
    # getString() end
    # main(): Input result: 
    # main() end

    Process finished with exit code 0

The below gif shows the program when input is not given on time (within 5 sec):

![no input][2]

Textual output:

    # main() start
    # main(): Starting std::async(getString)
    # getString() start
    # getString(): Please enter a string. You have 5 seconds...
    # main(): Waiting 5 seconds for input...
    # main(): Timeout reached... 
    # main() end
       ===== RETURN PRESSED ON KEYBOARD =====
    # getString() end

    Process finished with exit code 0


As you can see, the `async` thread will stay running until the user has given
some input, then the program ends. The timeout we want is sort of available,
the main function does continue. But, stopping the user input thread does not 
happen. This, again, is because the `getline()` call is blocking. The thread
will stop after the call is complete. I did try other tricks such as putting
the terminal in nonblocking mode or manually polling with `poll()`, but those 
all were not portable (windows/linux) or involved `memcpy` and more `C` like code
than I like, as opposed to modern C++.


### Conclusion and alternative solution

I didn't get to my goal of having user input with a timeout. For the program,
it turns out that a solution with two threads, one for input and one for 'other 
work' was a better choice. There is no timeout on the user input, whenever input
is received it is handled and signaled to the main thread. Here below is a 
simplified version with a thread that is 'doing work' and one that handles
input. If there is specific input, it does a thing and quits.

Here is a GIF that shows the program:

![thread][3]

Textual output:

    Please enter a command: 
    # (3 seconds pass)
    I'm doing other work... 
    # (3 seconds pass)
    I'm doing other work...
    # user input is given:
    magic
    The answer to life, the universe and everything!

Below is the file, single file again as above:

    #include <iostream>
    #include <memory>
    #include <string>
    #include <thread>
    #include <mutex>
    #include <chrono>
    #include <atomic>

    class Example {
        std::atomic<bool> running;
        std::atomic<bool> renderedText;
        std::mutex inputMutex;
        std::mutex otherThingsMutex;
        std::thread otherThread;
        std::thread inputThread;

        void renderText() {
            if(!renderedText) {
                renderedText = true;
                std::cout << "Please enter a command: " << std::endl;
            }
        }

        static void doSomethingWithInput(const std::string& input) {
            if (input == "magic")
                std::cout << "The answer to life, the universe and everything!" << std::endl;
        }

    public:
        Example() : running(true), renderedText(false),
                    otherThread(&Example::otherThings, this),
                    inputThread(&Example::input, this)
        {
        }

        ~Example() {
            inputThread.join();
            otherThread.join();
        }

        inline void quit() {
            running = false;
        }

        void handleInput() {
            std::string input;
            std::getline(std::cin, input);
            doSomethingWithInput(input);
            quit();
        }

        static void doOtherTask() {
            std::cout << "I'm doing other work..." << std::endl;
            std::this_thread::sleep_for(std::chrono::seconds(3));
        }

        void input() {
            while (running) {
                std::lock_guard<std::mutex> locker(inputMutex);
                handleInput();
            }
        }

        void otherThings() {
            while (running) {
                std::lock_guard<std::mutex> locker(otherThingsMutex);
                renderText();
                doOtherTask();
            }
        }
    };

    int main() {
        std::unique_ptr<Example> test = std::make_unique<Example>();
        return 0;
    }




[1]: /s/inc/img/cpp-timeout-withinput.gif
[2]: /s/inc/img/cpp-timeout-without-input.gif
[3]: /s/inc/img/cpp-timeout-thread.gif



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