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:

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(sigc::ptr_fun(MyCallback), 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;
}