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<div class="sect1" title="Monitoring I/O">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="sec-monitoring-io"></a>Monitoring I/O</h2></div></div></div>
<p>
A nifty feature of Glib (one of the libraries underlying
<span class="application">gtkmm</span>) 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:
</p>
<p>
</p>
<pre class="programlisting">
sigc::connection Glib::SignalInput::connect(const sigc::slot<bool,Glib::IOCondition>& slot,
int fd, Glib::IOCondition condition,
int priority = Glib::PRIORITY_DEFAULT);
</pre>
<p>
</p>
<p>
The first argument is a 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
<code class="literal">|</code>) of:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p>
Glib::IO_IN - Call your method when there is data ready for
reading on your file descriptor.
</p></li>
<li class="listitem"><p>
Glib::IO_OUT - Call your method when the file descriptor is
ready for writing.
</p></li>
<li class="listitem"><p>
Glib::IO_PRI - Call your method when the file descriptor has urgent data to be read.
</p></li>
<li class="listitem"><p>
Glib::IO_ERR - Call your method when an error has occurred on the file descriptor.
</p></li>
<li class="listitem"><p>
Glib::IO_HUP - Call your method when hung up (the connection has been broken usually for pipes and sockets).
</p></li>
</ul></div>
<p>
The return value is a <code class="classname">sigc::connection</code> that may be used to stop monitoring
this file descriptor using its <code class="methodname">disconnect()</code> method. The
<em class="parameter"><code>slot</code></em> signal handler should be declared as follows:
</p>
<p>
</p>
<pre class="programlisting">
bool input_callback(Glib::IOCondition condition);
</pre>
<p>
</p>
<p>
where <em class="parameter"><code>condition</code></em> is as
specified above. As usual the slot is created with
<code class="function">sigc::mem_fun()</code> (for a member method of an object.), or
<code class="function">sigc::ptr_fun()</code> (for a function).
</p>
<p>
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
<code class="literal">testfifo</code> in the current directory. Then start another shell
and execute <code class="literal">echo "Hello" > testfifo</code>. The example will
print each line you enter until you execute <code class="literal">echo "Q" >
testfifo</code>.
</p>
<p><a class="ulink" href="http://git.gnome.org/cgit/gtkmm-documentation/tree/examples/book/input/" target="_top">Source Code</a></p>
<p>File: <code class="filename">main.cc</code>
</p>
<pre class="programlisting">
#include <build/config.h>
#include <gtkmm/main.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>
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::IO_IN) == 0) {
std::cerr << "Invalid fifo response" << std::endl;
}
else {
Glib::ustring buf;
#ifdef GLIBMM_EXCEPTIONS_ENABLED
iochannel->read_line(buf);
#else
std::auto_ptr<Glib::Error> ex;
iochannel->read_line(buf, ex);
if(ex.get())
std::cerr << "Error: " << ex->what() << std::endl;
#endif //GLIBMM_EXCEPTIONS_ENABLED
std::cout << buf;
if (buf == "Q\n")
Gtk::Main::quit ();
}
return true;
}
int main(int argc, char *argv[])
{
// the usual Gtk::Main object
Gtk::Main app(argc, argv);
if (access("testfifo", F_OK) == -1) {
// fifo doesn't exit - create it
#ifdef 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 //HAVE_MKFIFO
}
read_fd = open("testfifo", O_RDONLY);
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::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.run();
// now remove the temporary fifo
if(unlink("testfifo"))
std::cerr << "error removing fifo" << std::endl;
return 0;
}
</pre>
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