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How can a C program poll for user input while simultaneously performing other actions in a Linux environment?



The 2019 Stack Overflow Developer Survey Results Are In
Announcing the arrival of Valued Associate #679: Cesar Manara
Planned maintenance scheduled April 17/18, 2019 at 00:00UTC (8:00pm US/Eastern)
The Ask Question Wizard is Live!
Data science time! April 2019 and salary with experienceHow do I prompt for Yes/No/Cancel input in a Linux shell script?How can I use grep to show just filenames on Linux?reading input with fgets(goes into endless loop)C - how to handle user input in a while loopHow to interrupt loop/process using terminal input in C++ on a Linux applicationDealing with user input on linuxHow to scan for input while looping (C Program)How to use fgets() to control the execution of while loop through user input in c?Stopping Linux console from echoing input during program executionHow can I poll keyboard input in c?



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6















Background:



I'm a relatively inexperienced developer trying to write software to interface with a PCI motion controller. I'm using C (compiled with gcc) on Ubuntu Linux 18.04.



The program I'm writing needs to regularly check for unsolicited status messages sent by the motion controller (approx. once per second) and display any messages it finds on a terminal screen (for which I'm using the ncurses library).



What I have:



Right now, to do this, I'm calling a function that checks for unsolicited messages in a while loop. The code is roughly akin to:



while (1)

// check for messages from PCI and store them in a traffic buffer
checkForMessages(PCIconnection, trafficBuffer);

// output the traffic buffer to the screen
printf("%s", trafficBuffer);



What I need:



I need the user to be prompted for input in a way that allows them to end the loop. For example, the user could input end causing the loop to stop and the program to continue.



The problem:



I'm not aware of a way to achieve this without putting fgets inside the while loop, causing the program to stop and wait for the user to input something on every loop iteration.



I've looked for a solution, but I haven't been able to find discussion on how to achieve the functionality I need. Opening a new thread or process seems like a step in the right direction?



I'm open to completely restructuring my code if what I'm currently doing is poor practice.



Thank you for any help!










share|improve this question







New contributor




josephsturm is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.


























    6















    Background:



    I'm a relatively inexperienced developer trying to write software to interface with a PCI motion controller. I'm using C (compiled with gcc) on Ubuntu Linux 18.04.



    The program I'm writing needs to regularly check for unsolicited status messages sent by the motion controller (approx. once per second) and display any messages it finds on a terminal screen (for which I'm using the ncurses library).



    What I have:



    Right now, to do this, I'm calling a function that checks for unsolicited messages in a while loop. The code is roughly akin to:



    while (1)

    // check for messages from PCI and store them in a traffic buffer
    checkForMessages(PCIconnection, trafficBuffer);

    // output the traffic buffer to the screen
    printf("%s", trafficBuffer);



    What I need:



    I need the user to be prompted for input in a way that allows them to end the loop. For example, the user could input end causing the loop to stop and the program to continue.



    The problem:



    I'm not aware of a way to achieve this without putting fgets inside the while loop, causing the program to stop and wait for the user to input something on every loop iteration.



    I've looked for a solution, but I haven't been able to find discussion on how to achieve the functionality I need. Opening a new thread or process seems like a step in the right direction?



    I'm open to completely restructuring my code if what I'm currently doing is poor practice.



    Thank you for any help!










    share|improve this question







    New contributor




    josephsturm is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
    Check out our Code of Conduct.






















      6












      6








      6








      Background:



      I'm a relatively inexperienced developer trying to write software to interface with a PCI motion controller. I'm using C (compiled with gcc) on Ubuntu Linux 18.04.



      The program I'm writing needs to regularly check for unsolicited status messages sent by the motion controller (approx. once per second) and display any messages it finds on a terminal screen (for which I'm using the ncurses library).



      What I have:



      Right now, to do this, I'm calling a function that checks for unsolicited messages in a while loop. The code is roughly akin to:



      while (1)

      // check for messages from PCI and store them in a traffic buffer
      checkForMessages(PCIconnection, trafficBuffer);

      // output the traffic buffer to the screen
      printf("%s", trafficBuffer);



      What I need:



      I need the user to be prompted for input in a way that allows them to end the loop. For example, the user could input end causing the loop to stop and the program to continue.



      The problem:



      I'm not aware of a way to achieve this without putting fgets inside the while loop, causing the program to stop and wait for the user to input something on every loop iteration.



      I've looked for a solution, but I haven't been able to find discussion on how to achieve the functionality I need. Opening a new thread or process seems like a step in the right direction?



      I'm open to completely restructuring my code if what I'm currently doing is poor practice.



      Thank you for any help!










      share|improve this question







      New contributor




      josephsturm is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.












      Background:



      I'm a relatively inexperienced developer trying to write software to interface with a PCI motion controller. I'm using C (compiled with gcc) on Ubuntu Linux 18.04.



      The program I'm writing needs to regularly check for unsolicited status messages sent by the motion controller (approx. once per second) and display any messages it finds on a terminal screen (for which I'm using the ncurses library).



      What I have:



      Right now, to do this, I'm calling a function that checks for unsolicited messages in a while loop. The code is roughly akin to:



      while (1)

      // check for messages from PCI and store them in a traffic buffer
      checkForMessages(PCIconnection, trafficBuffer);

      // output the traffic buffer to the screen
      printf("%s", trafficBuffer);



      What I need:



      I need the user to be prompted for input in a way that allows them to end the loop. For example, the user could input end causing the loop to stop and the program to continue.



      The problem:



      I'm not aware of a way to achieve this without putting fgets inside the while loop, causing the program to stop and wait for the user to input something on every loop iteration.



      I've looked for a solution, but I haven't been able to find discussion on how to achieve the functionality I need. Opening a new thread or process seems like a step in the right direction?



      I'm open to completely restructuring my code if what I'm currently doing is poor practice.



      Thank you for any help!







      c linux






      share|improve this question







      New contributor




      josephsturm is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.











      share|improve this question







      New contributor




      josephsturm is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.









      share|improve this question




      share|improve this question






      New contributor




      josephsturm is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.









      asked 7 hours ago









      josephsturmjosephsturm

      334




      334




      New contributor




      josephsturm is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.





      New contributor





      josephsturm is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.






      josephsturm is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.






















          3 Answers
          3






          active

          oldest

          votes


















          6














          Your task requires an event loop based on select or epoll. One event it would wait for is user input - when STDIN_FILENO becomes ready for read. Another is the 1-second periodic timer when you need to poll the controller.



          There are quite a few libraries that implement an event loop for you so that you can focus on what events you need to handle and how. libevent is one of the oldest, feature rich and popular.






          share|improve this answer























          • Thank you for answering! This is definitely a valid answer to the question I asked, so I'm accepting it. After a lot more reading, I also discovered that there's an answer built into ncurses itself. Calling the function timeout and passing it 0 makes getch (an ncurses input function) non-blocking, which also solved my problem.

            – josephsturm
            1 hour ago



















          3














          Simple answer is multi-threading, where you have thread deployed to wait for user input, while loop continues on. So have this:



          char flag = 1;

          while (flag)
          // run the loop

          // if thing happens deploy the thread which will ask user for input




          I have not done threading in a while, I think this page would be better than me trying to explain it to you:
          https://randu.org/tutorials/threads/






          share|improve this answer


















          • 1





            But see 5 Big Fat Reasons Why Mutexes Suck Big Time for a few considerations on multi-threading.

            – David C. Rankin
            6 hours ago











          • tnx will read this article

            – Gox
            6 hours ago











          • Yes, I'd didn't have much of a feel one way or the other about where and what kind of pitfalls were involved in multithreading until that article was posted on the accu-general mailing list accu-general@accu.org. It makes some very good points on the inherent inability to validate multithreaded code.

            – David C. Rankin
            5 hours ago


















          2














          I believe that the "Unix" way would be not to ask for user input, but to react to a user signal. For example, when the user presses Ctrl-C, the currently running process receives SIGINT.



          An example how to properly use SIGINT to interrupt a loop can be found here. Copying it into the answer in case the link gets stale:



          #include <stdlib.h>
          #include <signal.h>
          #include <stdio.h>
          #include <string.h>
          #include <unistd.h>

          static volatile sig_atomic_t got_signal = 0;

          static void my_sig_handler(int signo)

          got_signal = 1;


          int main()

          struct sigaction sa;

          memset(&sa, 0, sizeof(struct sigaction));
          sa.sa_handler = &my_sig_handler;
          if (sigaction(SIGINT, &sa, NULL) == -1)
          perror("sigaction");
          return EXIT_FAILURE;


          for (;;)
          if (got_signal)
          got_signal = 0;
          printf("Received interrupt signal!n");

          printf("Doing useful stuff...n");
          sleep(1); /* Sleep is not only useful, it is essential! */

          return EXIT_SUCCESS;



          (in your case it would be a good idea to put break; into the if block or to use while(!got_signal))






          share|improve this answer























            Your Answer






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            3 Answers
            3






            active

            oldest

            votes








            3 Answers
            3






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

            votes









            6














            Your task requires an event loop based on select or epoll. One event it would wait for is user input - when STDIN_FILENO becomes ready for read. Another is the 1-second periodic timer when you need to poll the controller.



            There are quite a few libraries that implement an event loop for you so that you can focus on what events you need to handle and how. libevent is one of the oldest, feature rich and popular.






            share|improve this answer























            • Thank you for answering! This is definitely a valid answer to the question I asked, so I'm accepting it. After a lot more reading, I also discovered that there's an answer built into ncurses itself. Calling the function timeout and passing it 0 makes getch (an ncurses input function) non-blocking, which also solved my problem.

              – josephsturm
              1 hour ago
















            6














            Your task requires an event loop based on select or epoll. One event it would wait for is user input - when STDIN_FILENO becomes ready for read. Another is the 1-second periodic timer when you need to poll the controller.



            There are quite a few libraries that implement an event loop for you so that you can focus on what events you need to handle and how. libevent is one of the oldest, feature rich and popular.






            share|improve this answer























            • Thank you for answering! This is definitely a valid answer to the question I asked, so I'm accepting it. After a lot more reading, I also discovered that there's an answer built into ncurses itself. Calling the function timeout and passing it 0 makes getch (an ncurses input function) non-blocking, which also solved my problem.

              – josephsturm
              1 hour ago














            6












            6








            6







            Your task requires an event loop based on select or epoll. One event it would wait for is user input - when STDIN_FILENO becomes ready for read. Another is the 1-second periodic timer when you need to poll the controller.



            There are quite a few libraries that implement an event loop for you so that you can focus on what events you need to handle and how. libevent is one of the oldest, feature rich and popular.






            share|improve this answer













            Your task requires an event loop based on select or epoll. One event it would wait for is user input - when STDIN_FILENO becomes ready for read. Another is the 1-second periodic timer when you need to poll the controller.



            There are quite a few libraries that implement an event loop for you so that you can focus on what events you need to handle and how. libevent is one of the oldest, feature rich and popular.







            share|improve this answer












            share|improve this answer



            share|improve this answer










            answered 7 hours ago









            Maxim EgorushkinMaxim Egorushkin

            90.2k11104192




            90.2k11104192












            • Thank you for answering! This is definitely a valid answer to the question I asked, so I'm accepting it. After a lot more reading, I also discovered that there's an answer built into ncurses itself. Calling the function timeout and passing it 0 makes getch (an ncurses input function) non-blocking, which also solved my problem.

              – josephsturm
              1 hour ago


















            • Thank you for answering! This is definitely a valid answer to the question I asked, so I'm accepting it. After a lot more reading, I also discovered that there's an answer built into ncurses itself. Calling the function timeout and passing it 0 makes getch (an ncurses input function) non-blocking, which also solved my problem.

              – josephsturm
              1 hour ago

















            Thank you for answering! This is definitely a valid answer to the question I asked, so I'm accepting it. After a lot more reading, I also discovered that there's an answer built into ncurses itself. Calling the function timeout and passing it 0 makes getch (an ncurses input function) non-blocking, which also solved my problem.

            – josephsturm
            1 hour ago






            Thank you for answering! This is definitely a valid answer to the question I asked, so I'm accepting it. After a lot more reading, I also discovered that there's an answer built into ncurses itself. Calling the function timeout and passing it 0 makes getch (an ncurses input function) non-blocking, which also solved my problem.

            – josephsturm
            1 hour ago














            3














            Simple answer is multi-threading, where you have thread deployed to wait for user input, while loop continues on. So have this:



            char flag = 1;

            while (flag)
            // run the loop

            // if thing happens deploy the thread which will ask user for input




            I have not done threading in a while, I think this page would be better than me trying to explain it to you:
            https://randu.org/tutorials/threads/






            share|improve this answer


















            • 1





              But see 5 Big Fat Reasons Why Mutexes Suck Big Time for a few considerations on multi-threading.

              – David C. Rankin
              6 hours ago











            • tnx will read this article

              – Gox
              6 hours ago











            • Yes, I'd didn't have much of a feel one way or the other about where and what kind of pitfalls were involved in multithreading until that article was posted on the accu-general mailing list accu-general@accu.org. It makes some very good points on the inherent inability to validate multithreaded code.

              – David C. Rankin
              5 hours ago















            3














            Simple answer is multi-threading, where you have thread deployed to wait for user input, while loop continues on. So have this:



            char flag = 1;

            while (flag)
            // run the loop

            // if thing happens deploy the thread which will ask user for input




            I have not done threading in a while, I think this page would be better than me trying to explain it to you:
            https://randu.org/tutorials/threads/






            share|improve this answer


















            • 1





              But see 5 Big Fat Reasons Why Mutexes Suck Big Time for a few considerations on multi-threading.

              – David C. Rankin
              6 hours ago











            • tnx will read this article

              – Gox
              6 hours ago











            • Yes, I'd didn't have much of a feel one way or the other about where and what kind of pitfalls were involved in multithreading until that article was posted on the accu-general mailing list accu-general@accu.org. It makes some very good points on the inherent inability to validate multithreaded code.

              – David C. Rankin
              5 hours ago













            3












            3








            3







            Simple answer is multi-threading, where you have thread deployed to wait for user input, while loop continues on. So have this:



            char flag = 1;

            while (flag)
            // run the loop

            // if thing happens deploy the thread which will ask user for input




            I have not done threading in a while, I think this page would be better than me trying to explain it to you:
            https://randu.org/tutorials/threads/






            share|improve this answer













            Simple answer is multi-threading, where you have thread deployed to wait for user input, while loop continues on. So have this:



            char flag = 1;

            while (flag)
            // run the loop

            // if thing happens deploy the thread which will ask user for input




            I have not done threading in a while, I think this page would be better than me trying to explain it to you:
            https://randu.org/tutorials/threads/







            share|improve this answer












            share|improve this answer



            share|improve this answer










            answered 7 hours ago









            GoxGox

            2,42121129




            2,42121129







            • 1





              But see 5 Big Fat Reasons Why Mutexes Suck Big Time for a few considerations on multi-threading.

              – David C. Rankin
              6 hours ago











            • tnx will read this article

              – Gox
              6 hours ago











            • Yes, I'd didn't have much of a feel one way or the other about where and what kind of pitfalls were involved in multithreading until that article was posted on the accu-general mailing list accu-general@accu.org. It makes some very good points on the inherent inability to validate multithreaded code.

              – David C. Rankin
              5 hours ago












            • 1





              But see 5 Big Fat Reasons Why Mutexes Suck Big Time for a few considerations on multi-threading.

              – David C. Rankin
              6 hours ago











            • tnx will read this article

              – Gox
              6 hours ago











            • Yes, I'd didn't have much of a feel one way or the other about where and what kind of pitfalls were involved in multithreading until that article was posted on the accu-general mailing list accu-general@accu.org. It makes some very good points on the inherent inability to validate multithreaded code.

              – David C. Rankin
              5 hours ago







            1




            1





            But see 5 Big Fat Reasons Why Mutexes Suck Big Time for a few considerations on multi-threading.

            – David C. Rankin
            6 hours ago





            But see 5 Big Fat Reasons Why Mutexes Suck Big Time for a few considerations on multi-threading.

            – David C. Rankin
            6 hours ago













            tnx will read this article

            – Gox
            6 hours ago





            tnx will read this article

            – Gox
            6 hours ago













            Yes, I'd didn't have much of a feel one way or the other about where and what kind of pitfalls were involved in multithreading until that article was posted on the accu-general mailing list accu-general@accu.org. It makes some very good points on the inherent inability to validate multithreaded code.

            – David C. Rankin
            5 hours ago





            Yes, I'd didn't have much of a feel one way or the other about where and what kind of pitfalls were involved in multithreading until that article was posted on the accu-general mailing list accu-general@accu.org. It makes some very good points on the inherent inability to validate multithreaded code.

            – David C. Rankin
            5 hours ago











            2














            I believe that the "Unix" way would be not to ask for user input, but to react to a user signal. For example, when the user presses Ctrl-C, the currently running process receives SIGINT.



            An example how to properly use SIGINT to interrupt a loop can be found here. Copying it into the answer in case the link gets stale:



            #include <stdlib.h>
            #include <signal.h>
            #include <stdio.h>
            #include <string.h>
            #include <unistd.h>

            static volatile sig_atomic_t got_signal = 0;

            static void my_sig_handler(int signo)

            got_signal = 1;


            int main()

            struct sigaction sa;

            memset(&sa, 0, sizeof(struct sigaction));
            sa.sa_handler = &my_sig_handler;
            if (sigaction(SIGINT, &sa, NULL) == -1)
            perror("sigaction");
            return EXIT_FAILURE;


            for (;;)
            if (got_signal)
            got_signal = 0;
            printf("Received interrupt signal!n");

            printf("Doing useful stuff...n");
            sleep(1); /* Sleep is not only useful, it is essential! */

            return EXIT_SUCCESS;



            (in your case it would be a good idea to put break; into the if block or to use while(!got_signal))






            share|improve this answer



























              2














              I believe that the "Unix" way would be not to ask for user input, but to react to a user signal. For example, when the user presses Ctrl-C, the currently running process receives SIGINT.



              An example how to properly use SIGINT to interrupt a loop can be found here. Copying it into the answer in case the link gets stale:



              #include <stdlib.h>
              #include <signal.h>
              #include <stdio.h>
              #include <string.h>
              #include <unistd.h>

              static volatile sig_atomic_t got_signal = 0;

              static void my_sig_handler(int signo)

              got_signal = 1;


              int main()

              struct sigaction sa;

              memset(&sa, 0, sizeof(struct sigaction));
              sa.sa_handler = &my_sig_handler;
              if (sigaction(SIGINT, &sa, NULL) == -1)
              perror("sigaction");
              return EXIT_FAILURE;


              for (;;)
              if (got_signal)
              got_signal = 0;
              printf("Received interrupt signal!n");

              printf("Doing useful stuff...n");
              sleep(1); /* Sleep is not only useful, it is essential! */

              return EXIT_SUCCESS;



              (in your case it would be a good idea to put break; into the if block or to use while(!got_signal))






              share|improve this answer

























                2












                2








                2







                I believe that the "Unix" way would be not to ask for user input, but to react to a user signal. For example, when the user presses Ctrl-C, the currently running process receives SIGINT.



                An example how to properly use SIGINT to interrupt a loop can be found here. Copying it into the answer in case the link gets stale:



                #include <stdlib.h>
                #include <signal.h>
                #include <stdio.h>
                #include <string.h>
                #include <unistd.h>

                static volatile sig_atomic_t got_signal = 0;

                static void my_sig_handler(int signo)

                got_signal = 1;


                int main()

                struct sigaction sa;

                memset(&sa, 0, sizeof(struct sigaction));
                sa.sa_handler = &my_sig_handler;
                if (sigaction(SIGINT, &sa, NULL) == -1)
                perror("sigaction");
                return EXIT_FAILURE;


                for (;;)
                if (got_signal)
                got_signal = 0;
                printf("Received interrupt signal!n");

                printf("Doing useful stuff...n");
                sleep(1); /* Sleep is not only useful, it is essential! */

                return EXIT_SUCCESS;



                (in your case it would be a good idea to put break; into the if block or to use while(!got_signal))






                share|improve this answer













                I believe that the "Unix" way would be not to ask for user input, but to react to a user signal. For example, when the user presses Ctrl-C, the currently running process receives SIGINT.



                An example how to properly use SIGINT to interrupt a loop can be found here. Copying it into the answer in case the link gets stale:



                #include <stdlib.h>
                #include <signal.h>
                #include <stdio.h>
                #include <string.h>
                #include <unistd.h>

                static volatile sig_atomic_t got_signal = 0;

                static void my_sig_handler(int signo)

                got_signal = 1;


                int main()

                struct sigaction sa;

                memset(&sa, 0, sizeof(struct sigaction));
                sa.sa_handler = &my_sig_handler;
                if (sigaction(SIGINT, &sa, NULL) == -1)
                perror("sigaction");
                return EXIT_FAILURE;


                for (;;)
                if (got_signal)
                got_signal = 0;
                printf("Received interrupt signal!n");

                printf("Doing useful stuff...n");
                sleep(1); /* Sleep is not only useful, it is essential! */

                return EXIT_SUCCESS;



                (in your case it would be a good idea to put break; into the if block or to use while(!got_signal))







                share|improve this answer












                share|improve this answer



                share|improve this answer










                answered 6 hours ago









                Kit.Kit.

                71069




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