Monitoring I/O
| Chapter 24. Timeouts, I/O and Idle Functions |

## Monitoring I/O

A nifty feature of Glib (one of the libraries underlying gtkmm) is the ability to have it check for data on a file descriptor for you. This is especially useful for networking applications. The following method is used to do this:

    sigc::connection Glib::SignalIO::connect(const sigc::slot<bool(Glib::IOCondition)>& slot,
                                     Glib::PollFD::fd_t fd, Glib::IOCondition condition,
                                     int priority = Glib::PRIORITY_DEFAULT);

The first argument is a slot you wish to have called when the specified event (see argument 3) occurs on the file descriptor you specify using argument two. Argument three may be one or more (using `|`) of:

- Glib::IOCondition::IO_IN - Call your method when there is data ready for reading on your file descriptor.

- Glib::IOCondition::IO_OUT - Call your method when the file descriptor is ready for writing.

- Glib::IOCondition::IO_PRI - Call your method when the file descriptor has urgent data to be read.

- Glib::IOCondition::IO_ERR - Call your method when an error has occurred on the file descriptor.

- Glib::IOCondition::IO_HUP - Call your method when hung up (the connection has been broken usually for pipes and sockets).

The return value is a `sigc::connection` that may be used to stop monitoring this file descriptor using its `disconnect()` method. The `slot` signal handler should be declared as follows:

    bool input_callback(Glib::IOCondition condition);

where `condition` is as specified above. As usual the slot is created with `sigc::mem_fun()` (for a member method of an object), or `sigc::ptr_fun()` (for a function). A lambda expression can be used, if you don't need the automatic disconnection that `sigc::mem_fun()` provides when the object goes out of scope.

A little example follows. To use the example just execute it from a terminal; it doesn't create a window. It will create a pipe named `testfifo` in the current directory. Then start another shell and execute `echo "Hello" > testfifo`. The example will print each line you enter until you execute `echo "Q" > testfifo`.

Source Code

File: `main.cc` (For use with gtkmm 4)

    #include <gtkmm/application.h>
    #include <glibmm/main.h>
    #include <glibmm/iochannel.h>
    #include <fcntl.h>
    #include <iostream>

    #include <unistd.h> //The SUN Forte compiler puts F_OK here.

    //The SUN Forte compiler needs these for mkfifo:
    #include <sys/types.h>
    #include <sys/stat.h>

    Glib::RefPtr<Gtk::Application> app;

    int read_fd;
    Glib::RefPtr<Glib::IOChannel> iochannel;

    /*
      send to the fifo with:
      echo "Hello" > testfifo

      quit the program with:
      echo "Q" > testfifo
    */

    // this will be our signal handler for read operations
    // it will print out the message sent to the fifo
    // and quit the program if the message was 'Q'.
    bool MyCallback(Glib::IOCondition io_condition)
    {
      if ((io_condition & Glib::IOCondition::IO_IN) != Glib::IOCondition::IO_IN)
      {
        std::cerr << "Invalid fifo response" << std::endl;
      }
      else
      {
       Glib::ustring buf;

       iochannel->read_line(buf);
       std::cout << buf;
       if (buf == "Q\n")
         app->quit();
      }
      return true;
    }

    int main(int argc, char *argv[])
    {
      app = Gtk::Application::create("org.gtkmm.example");

      if (access("testfifo", F_OK) == -1)
      {
        // fifo doesn't exist - create it
        #ifndef DONT_HAVE_MKFIFO
        if (mkfifo("testfifo", 0666) != 0)
        {
          std::cerr << "error creating fifo" << std::endl;
          return -1;
        }
        #else
          std::cerr << "error creating fifo: This platform does not have mkfifo()"
              << std::endl;
        #endif //DONT_HAVE_MKFIFO
      }

      // Although we will only read from the fifo, we open it in read/write mode.
      // Due to a peculiarity with the poll() system call, used deep down in glib,
      // this small program will use all available CPU time, if the fifo is opened
      // as O_RDONLY. See a discussion on the gtkmm-list, e.g.
      // https://mail.gnome.org/archives/gtkmm-list/2015-September/msg00034.html
      // and the link from there to stackoverflow.
      read_fd = open("testfifo", O_RDWR);
      if (read_fd == -1)
      {
        std::cerr << "error opening fifo" << std::endl;
        return -1;
      }

      // connect the signal handler
      Glib::signal_io().connect(
        [](Glib::IOCondition io_condition){ return MyCallback(io_condition); },
        read_fd, Glib::IOCondition::IO_IN);

      // Creates a iochannel from the file descriptor
      iochannel = Glib::IOChannel::create_from_fd(read_fd);

      // and last but not least - run the application main loop
      app->hold(); // keep the application running without a window
      app->run(argc, argv);

      // now remove the temporary fifo
      if(unlink("testfifo"))
        std::cerr << "error removing fifo" << std::endl;

      return 0;
    }

Chapter 24. Timeouts, I/O and Idle Functions |  | Idle Functions