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Non blocking console reading.



  • I have a console application

    int main(int argc, char *argv[])
    {
        QCoreApplication a(argc, argv);
    
        SYS_ParamInit(sys_params_file);
    
        while (1)
        {
             //is blocking!!!
             ch_ptr = fgets(cons_str, 256, stdin);
    
             //also blocking!!!
            chr = static_cast<char>(fgetchar());
            
                DoSomeStuff();
         
                SCRIPT_Run();
        }
        return 0;
    }
    

    Among the other tasks I have to read a user input from a console. But all methods (fgets, fgetchar) are blocking - it stops and waits for input.
    How can I read the console input in non blocking way?



  • @jenya7
    This is going to be hard (and may depend on what platform you are on). Approaches:
    https://gist.github.com/gjorquera/2576569
    https://stackoverflow.com/questions/7543313/how-to-handle-keypress-events-in-a-qt-console-application?rq=1

    Before going any further: why are you writing a QCoreApplication/console input program in the first place? And what sort of thing is it going to be doing while awaiting a non-blocking console input?



  • @JonB said in Non blocking console reading.:

    @jenya7
    This is going to be hard (and may depend on what platform you are on). Approaches:
    https://gist.github.com/gjorquera/2576569
    https://stackoverflow.com/questions/7543313/how-to-handle-keypress-events-in-a-qt-console-application?rq=1

    Before going any further: why are you writing a QCoreApplication/console input program in the first place? And what sort of thing is it going to be doing while awaiting a non-blocking console input?

    Actually a console input program works much faster. In some cases I don't need GUI, only user input enough. In a while(1) loop I do some tasks, like parsing script and getting data from remote sensors. Further I have to get to lower level - to operate some hardware like GPIO, UART, CAN, SPI.



  • @jenya7 said in Non blocking console reading.:

    Actually a console input program works much faster.

    A dubious statement. And if you need, or may need, a GUI for other purposes you would be better starting with a GUI.

    The way you phrase your objective, and write your code so far, makes it sound like you are not really wanting to use Qt at all. If you are going to do everything yourself, and in a C way, it is not apparent you are gaining any advantage from trying to use Qt at all.

    In any case, if you persist your way I have given you the links to see what is involved in getting non-blocking console input (as you can see, not cross-platform). So there you are.



  • @jenya7 said in Non blocking console reading.:

    Among the other tasks I have to read a user input from a console. But all methods (fgets, fgetchar) are blocking - it stops and waits for input.
    How can I read the console input in non blocking way?

    As I have already say to you: Qt is an asynchronous framework, and Qt needs a working event queue to be able to handle signals/slots.
    This means:

    • no forever loop
    • no active wait.

    Or, do not use Qt but another framework.
    This code will not enable Qt to work properly.
    You have to think asynchronous, which means work with events.

    I would suggest you to use something like this project QConsoleListener to handle console in/out on asynchronous way.



  • @KroMignon
    I see. No looping. But

    int main(int argc, char *argv[])
    {
    	QCoreApplication a(argc, argv);
    
    	SomeSetup();
    
    	return a.exec();
    }
    

    Where do I execute periodic tasks - not event driven?



  • @jenya7 said in Non blocking console reading.:

    Where do I execute periodic tasks - not event driven?

    What's wrong with QTimer?



  • @KroMignon
    I see. So time sensitive tasks I can run with QTimer and set zero delay? Actually it's quite good.



  • @jenya7 said in Non blocking console reading.:

    I see. So time sensitive tasks I can run with QTimer and set zero delay? Actually it's quite good.

    I am not sure to right understand your question.
    First, Qt is not a realtime framework.
    You can defined a QTimer instance with a given interval and so ensure a give slot will be call every XXX milliseconds.
    You can used QTimer::singleShot() to do it once.

    All this do, is emitting a signal which will be handle by the event queue, which will call the attached slot(s)/lambda(s)/functor(s) attached to it.
    That's all the magic behind Qt signals/slots.
    This is why not blocking the event queue is vital for Qt applications.



  • @KroMignon
    One more question. If I install several timers - does it runs in separate threads? Do I have to worry about cross thread collisions?

    int main(int argc, char *argv[])
    {
        QCoreApplication a(argc, argv);
    
        QTimer timer1;
        QObject::connect(&timer1, &QTimer::timeout,  Func1);
        timer1.start(1);
    
        QTimer timer2;
        QObject::connect(&timer2, &QTimer::timeout,  Func2);
        timer2.start(10);
    
        return a.exec();
    }
    
    


  • @jenya7
    No they do not run in separate threads. In Qt only one timer signal is issued at a time, the next one cannot happen until any slot(s) attached to the first one have returned. At around 10 seconds both of your timers will timeout, in undefined order.


  • Moderators

    @JonB said in Non blocking console reading.:

    In Qt only one timer signal is issued at a time, the next one cannot happen until any slot(s) attached to the first one have returned.

    Normally!
    There's a (Qt) way to break that, rather easily.



  • @J-Hilk
    That, it seems to me, is true for any Qt signals (certain code can cause another signal to be emitted while one is still being handled). However, I took the OP's question to mean if she just does some non-Qt processing in the slots.

    In any case, even if the second timer signal is emitted while the first is still being handled, I don't see that is a threading issue (again, assuming, OP is not doing her own threading stuff).



  • I've experimented a bit, just to understand deeper

    int main(int argc, char *argv[])
    {
        QCoreApplication a(argc, argv);
    
      QFuture<void> future = QtConcurrent::run(&TerminalRead);
    
      while(1)
      {
           DoSomething();
      }
    }
    
    __attribute__ ((noreturn)) void TerminalRead()
    {
        while (1)
        {
            ch_ptr = fgets(cons_str, 256, stdin);
            if (ch_ptr != nullptr)
            {
                trimmed_str =  Trim(cons_str, TRIM_BOTH);
                COMPARSER_ParseCommand(trimmed_str);
            }
        }
    }
    

    And it works quite well. I get non blocking input from console and parse it. And tasks run in the loop.
    Is using QtConcurrent good practice? Does it creates a separate thread? Is it alternative to timers?



  • @jenya7
    Yes it creates a separate thread. That is one way of using blocking calls without blocking the main thread. I didn't want to complicate with that possibility earlier (but you seem to have dived right in there!).

    Be aware: I'm not sure whether C runtime calls like fgets() are thread-safe if you use them, or other stdio, stuff in other threads. (Answer: it is not thread-safe, unless you are using some version/library which has been rewritten to be thread-safe.) You must also be careful what you do in COMPARSER_ParseCommand(), as that is running in its own thread.



  • @JonB
    With timers it works very well. I'm glad. Thank you.



  • @jenya7 said in Non blocking console reading.:

    With timers it works very well. I'm glad. Thank you.

    I don't think 'this works well'.
    It is horrible in terms of Qt code conformity.
    Please be consistent.
    If you want to use Qt for developing an application, then use it in the right way or you will only be frustrated because nothing will work as expected.

    Again and again do not use forever loops/ active waits with Qt if you want your application to work.



  • @KroMignon
    I have a discover function - it runs through all sensors and detects who is alive - I see no way but to iterate all sensors in a loop. It looks like

    discover_state = DISC_ST_START;
    
        printf("Discovering...\n");
    
         while (done == 0)
         {
            switch (discover_state)
            {
                case DISC_ST_START:
                    data[3] = static_cast<char>(MSG_OPCODE_WHO_IS);
                    data[2] = static_cast<char>(idx); //to  a slave
    
                    //reset for the next sensor
                    new_message_flag = 0;
                    timer.restart();
    
                   UDP_Send(data, sys_params.broadcast_ip, sys_params.remote_port);
    
                    discover_state = DISC_ST_WAIT;
                break;
                case DISC_ST_WAIT:
                    if (new_message_flag)
                    {
                        printf("Discovered sensor ID: %d\n", idx);
    
                        discover_state = DISC_ST_NEXT;
                    }
                    else
                    {
                        millisec = timer.elapsed();
    
                        if (millisec > SENS_RESPONSE_TIME)
                           discover_state = DISC_ST_NEXT;
                    }
                break;
                case DISC_ST_NEXT:
                    idx++;
                    if (idx >= count || idx >= 254)
                        done = 1;
                    else
                        discover_state = DISC_ST_START;
                break;
            }
        }
         printf("Done.\n");
    

    May I get rid of the loop?



  • @jenya7
    You are asking a very different question here.

    Let's go back to the original blocking/non-blocking input from console/terminal.

    There are only two choices for how to approach:

    • The single-threaded (Qt) way. Cannot use a while (true) loop, and cannot be done with a blocking call like fgets(). You would either have to pursue the links I posted, or change over to a QApplication instead of QCoreApplication.

    • The multi-threaded (not-really-Qt) way you posted. Use a thread. That can allow fgets(), and also while (true): they both block, but it's OK because they are in a thread.



  • @JonB
    Thank you. I have to process all this information. :)


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