Monitoring I/O
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## Monitoring I/O

A nifty feature of GDK (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 Gtk::Main::input.connect(const SlotType& sd, int source,
                                        GdkInputCondition condition);

The first argument is a slot (SlotType is a typedef to a sigc::slot<>) you wish to have called when then 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:

- GDK_INPUT_READ - Call your method when there is data ready for reading on your file descriptor.

- GDK_INPUT_WRITE - Call your method when the file descriptor is ready for writing.

- GDK_INPUT_EXCEPTION - Call your method when an exception happened on the file descriptor.

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

    void input_callback(int source, GdkInputCondition condition);

where `source` and `condition` are 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 little (and somewhat dirty) example follows. To use the example just execute it from a terminal; it doesn't create a window. It will create a pipe named `testpipe` in the current directory. Then start another shell and execute `cat >testpipe`. The example will print each line you enter until you type `quit`.

Source location: examples/input/input.cc

    #include <gtkmm/main.h>
    #include <fcntl.h>
    #include <unistd.h>
    #include <sys/types.h>
    #include <sys/stat.h>
    #include <fstream>
    #include <iostream>
    #include <memory>

    using std::istream;

    using std::auto_ptr;

    auto_ptr<istream> input;

    // this will be our signal handler for read operations
    // there is not much to say. just read a string,
    // print it and quit the application if the string was quit
    void MyCallback(int, GdkInputCondition) {
      Gtk::string dummy;
      do {
        (*input) >> dummy;
        cout << dummy << endl;
        if(dummy == "quit") Gtk::Main::quit();
      } while(input->fail());
    }

    int main (int argc, char *argv[])
    {
      // the usual Gtk::Main object
      Gtk::Main app(argc, argv);

      // create a fifo for testing purposes
      if (mkfifo("testfifo",0666) != 0) {
        cerr << "error creating fifo" << endl;
        return -1;
      }

      // open the fifo
      input=new ifstream("testfifo");

    //    int fd = open("testfifo", 0);
    //    if (fd == -1) {
    //      cerr << "error opening fifo" << endl;
    //      return -1;
    //    }

      // assign the fifo's filedescriptor to our ifstream object
      //This sucks; it will only ever work with libstdc++-v3, as
      //  both istream::__filebuf_type and the basic_filebuf contructor
      //  that takes an fd are libstdc++-v3 specific.
      //input=new istream(new ifstream::__filebuf_type(fd,"testfifo"));

      // connect the signal handler
      app.input.connect(sigc::ptr_fun(MyCallback), fd, GDK_INPUT_READ);

      // and last but not least - run the application main loop
      app.run();

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

      return 0;
    }

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