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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<title>
The Main Loop
</title>
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<font color="#000000" size="2">GTK+ / Gnome Application
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<div class="SECT1">
<h1 class="SECT1">
<a name="SEC-MAINLOOP">The Main Loop</a>
</h1>
<p>
The GTK+ main loop's primary role is to listen for events
on a file descriptor connected to the X server, and forward
them to widgets. <a href="sec-gdkevent.html#SEC-GTKEVENTS">
the section called <i>Receiving GDK Events in GTK+</i> in
the chapter called <i>GDK Basics</i></a> describes the main
loop's event handling in more detail. This section explains
the main loop in general terms, and describes how to add
new functionality to the main loop: callbacks to be invoked
when the loop is idle, at a specified interval, when a file
descriptor is ready for reading or writing, and when the
main loop exits.
</p>
<div class="SECT2">
<h2 class="SECT2">
<a name="Z60">Main Loop Basics</a>
</h2>
<p>
The main loop is primarily implemented by glib, which has
a generic main loop abstraction. GTK+ attaches the glib
main loop to GDK's X server connection, and presents a
convenient interface (the glib loop is slightly
lower-level than the GTK+ loop). The core GTK+ main loop
interface is shown in <a href=
"sec-mainloop.html#FL-MAINLOOP">Figure 28</a>.
</p>
<p>
<tt class="FUNCTION">gtk_main()</tt> runs the main loop.
<tt class="FUNCTION">gtk_main()</tt> will not return
until <tt class="FUNCTION">gtk_main_quit()</tt> is
called. <tt class="FUNCTION">gtk_main()</tt> can be
called recursively; each call to <tt class="FUNCTION">
gtk_main_quit()</tt> exits one instance of <tt class=
"FUNCTION">gtk_main()</tt>. <tt class="FUNCTION">
gtk_main_level()</tt> returns the level of recursion;
that is, it returns 0 if no <tt class="FUNCTION">
gtk_main()</tt> is on the stack, 1 if one <tt class=
"FUNCTION">gtk_main()</tt> is running, etc.
</p>
<p>
All instances of <tt class="FUNCTION">gtk_main()</tt> are
functionally identical; they are all watching the same
connection to the X server and working from the same
event queue. <tt class="FUNCTION">gtk_main()</tt>
instances are used to block, halting a function's flow of
control until some conditions are met. All GTK+ programs
use this technique to keep <tt class="FUNCTION">
main()</tt> from exiting while the application is
running. The <tt class="FUNCTION">gnome_dialog_run()</tt>
function (see <a href="sec-modaldialogs.html">the section
called <i>Modal Dialogs</i> in the chapter called <i>User
Communication: Dialogs</i></a>) uses a recursive main
loop, so it doesn't return until the user clicks a dialog
button.
</p>
<p>
Sometimes you want to process a few events, without
handing the flow of control to <tt class="FUNCTION">
gtk_main()</tt>. You can perform a single iteration of
the main loop by calling <tt class="FUNCTION">
gtk_main_iteration()</tt>. This might process a single
event, for example; it depends on what tasks are pending.
You can check whether any events need to be processed by
calling the <tt class="FUNCTION">
gtk_events_pending()</tt> predicate. Together, these two
functions allow you to temporarily return control to
GTK+, so the GUI can "catch up." For example, during a
long computation, you will want to display a progress
bar; you must allow the GTK+ main loop to run
periodically, so GTK+ can redraw the progress bar. Use
this code:
</p>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="PROGRAMLISTING">
while (gtk_events_pending())
gtk_main_iteration();
</pre>
</td>
</tr>
</table>
<div class="FIGURE">
<a name="FL-MAINLOOP"></a>
<div class="FUNCSYNOPSIS">
<a name="FL-MAINLOOP.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <gtk/gtkmain.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">gtk_main</tt></code>(void);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">gtk_main_quit</tt></code>(void);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">
gtk_main_iteration</tt></code>(void);</code>
</p>
<p>
<code><code class="FUNCDEF">gint <tt class=
"FUNCTION">
gtk_events_pending</tt></code>(void);</code>
</p>
<p>
<code><code class="FUNCDEF">guint <tt class=
"FUNCTION">gtk_main_level</tt></code>(void);</code>
</p>
</div>
<p>
<b>Figure 28. Main Loop</b>
</p>
</div>
</div>
<div class="SECT2">
<h2 class="SECT2">
<a name="Z61">Quit Functions</a>
</h2>
<p>
A <i class="FIRSTTERM">quit function</i> is a callback to
be invoked when <tt class="FUNCTION">gtk_main_quit()</tt>
is called. In other words, the callback runs just before
<tt class="FUNCTION">gtk_main()</tt> returns. The
callback should be a <span class="STRUCTNAME">
GtkFunction</span>, defined as follows:
</p>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="PROGRAMLISTING">
typedef gint (*GtkFunction) (gpointer data);
</pre>
</td>
</tr>
</table>
<p>
Quit functions are added with <tt class="FUNCTION">
gtk_quit_add()</tt> (<a href=
"sec-mainloop.html#FL-QUITFUNCS">Figure 29</a>). When
adding a quit function, you must specify a main loop
level, as returned by <tt class="FUNCTION">
gtk_main_level()</tt>. The second and third arguments
specify a callback and callback data.
</p>
<p>
The callback's return value indicates whether the
callback should be invoked again. As long as the callback
returns <span class="STRUCTNAME">TRUE</span>, it will be
repeatedly invoked. As soon as it returns <span class=
"STRUCTNAME">FALSE</span>, it is disconnected. When all
quit functions have returned <span class="STRUCTNAME">
FALSE</span>, <tt class="FUNCTION">gtk_main()</tt> can
return.
</p>
<p>
<tt class="FUNCTION">gtk_quit_add()</tt> returns an ID
number that can be used to remove the quit function with
<tt class="FUNCTION">gtk_quit_remove()</tt>. You can also
remove a quit function by passing its callback data to
<tt class="FUNCTION">gtk_quit_remove_by_data()</tt>.
</p>
<div class="FIGURE">
<a name="FL-QUITFUNCS"></a>
<div class="FUNCSYNOPSIS">
<a name="FL-QUITFUNCS.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <gtk/gtkmain.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF">guint <tt class=
"FUNCTION">gtk_quit_add</tt></code>(guint <tt class=
"PARAMETER"><i>main_level</i></tt>, GtkFunction <tt
class="PARAMETER"><i>function</i></tt>, gpointer <tt
class="PARAMETER"><i>data</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">gtk_quit_remove</tt></code>(guint <tt
class="PARAMETER"><i>
quit_handler_id</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">
gtk_quit_remove_by_data</tt></code>(gpointer <tt
class="PARAMETER"><i>data</i></tt>);</code>
</p>
</div>
<p>
<b>Figure 29. Quit Functions</b>
</p>
</div>
</div>
<div class="SECT2">
<h2 class="SECT2">
<a name="Z62">Timeout Functions</a>
</h2>
<p>
<i class="FIRSTTERM">Timeout functions</i> are connected
and disconnected exactly as quit functions are; the
expected callback is the same. <tt class="FUNCTION">
gtk_timeout_add()</tt> expects an <span class=
"STRUCTNAME">interval</span> argument; the callback is
invoked every <span class="STRUCTNAME">interval</span>
milliseconds. If the callback ever returns <span class=
"STRUCTNAME">FALSE</span>, it is removed from the list of
timeout functions, just as if you'd called <tt class=
"FUNCTION">gtk_timeout_remove()</tt>. It is not safe to
call <tt class="FUNCTION">gtk_timeout_remove()</tt> from
within a timeout function; this modifies the timeout list
while GTK+ is iterating over it, causing a crash.
Instead, return <span class="STRUCTNAME">FALSE</span> to
remove a function.
</p>
<div class="FIGURE">
<a name="FL-TIMEOUTFUNCS"></a>
<div class="FUNCSYNOPSIS">
<a name="FL-TIMEOUTFUNCS.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <gtk/gtkmain.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF">guint <tt class=
"FUNCTION">gtk_timeout_add</tt></code>(guint32 <tt
class="PARAMETER"><i>interval</i></tt>, GtkFunction
<tt class="PARAMETER"><i>function</i></tt>, gpointer
<tt class="PARAMETER"><i>data</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">gtk_timeout_remove</tt></code>(guint <tt
class="PARAMETER"><i>
timeout_handler_id</i></tt>);</code>
</p>
</div>
<p>
<b>Figure 30. Timeout Functions</b>
</p>
</div>
</div>
<div class="SECT2">
<h2 class="SECT2">
<a name="Z63">Idle Functions</a>
</h2>
<p>
<i class="FIRSTTERM">Idle functions</i> run continuously
while the GTK+ main loop has nothing else to do. Idle
functions run only when the event queue is empty and the
main loop would normally sit idly, waiting for something
to happen. As long as they return <span class=
"STRUCTNAME">TRUE</span> they are invoked over and over;
when they return <span class="STRUCTNAME">FALSE</span>,
they are removed, just as if <tt class="FUNCTION">
gtk_idle_remove()</tt> had been called.
</p>
<p>
The idle function API, shown in <a href=
"sec-mainloop.html#FL-IDLEFUNCS">Figure 31</a>, is
identical to the timeout and quit function APIs. Again,
<tt class="FUNCTION">gtk_idle_remove()</tt> should not be
called from within an idle function, because it will
corrupt GTK+'s idle function list. Return <span class=
"STRUCTNAME">FALSE</span> to remove the idle function.
</p>
<p>
Idle functions are mostly useful to queue "one-shot"
code, which is run after all events have been handled.
Relatively expensive operations such as GTK+ size
negotiation and <tt class="CLASSNAME">GnomeCanvas</tt>
repaints take place in idle functions that return <span
class="STRUCTNAME">FALSE</span>. This ensures that
expensive operations are performed only once, even though
multiple consecutive events independently request the
recalculation.
</p>
<p>
The GTK+ main loop contains a simple scheduler; idle
functions actually have priorities assigned to them, just
as UNIX processes do. You can assign a non-default
priority to your idle functions, but it's a complicated
topic outside the scope of this book.
</p>
<div class="FIGURE">
<a name="FL-IDLEFUNCS"></a>
<div class="FUNCSYNOPSIS">
<a name="FL-IDLEFUNCS.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <gtk/gtkmain.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF">guint <tt class=
"FUNCTION">gtk_idle_add</tt></code>(GtkFunction <tt
class="PARAMETER"><i>function</i></tt>, gpointer <tt
class="PARAMETER"><i>data</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">gtk_idle_remove</tt></code>(guint <tt
class="PARAMETER"><i>
idle_handler_id</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">
gtk_idle_remove_by_data</tt></code>(gpointer <tt
class="PARAMETER"><i>data</i></tt>);</code>
</p>
</div>
<p>
<b>Figure 31. Idle Functions</b>
</p>
</div>
</div>
<div class="SECT2">
<h2 class="SECT2">
<a name="Z64">Input Functions</a>
</h2>
<p>
<i class="FIRSTTERM">Input functions</i> are handled on
the GDK level. They are invoked when a given file
descriptor is ready for reading or writing. They're
especially useful for networked applications.
</p>
<p>
To add an input function, you specify the file descriptor
to monitor, the state you want to wait for (ready for
reading or writing), and a callback/data pair. <a href=
"sec-mainloop.html#FL-INPUTFUNCS">Figure 32</a> shows the
API. Functions can be removed using the tag returned by
<tt class="FUNCTION">gdk_input_add()</tt>. Unlike quit,
timeout, and idle functions, it should be safe to call
<tt class="FUNCTION">gdk_input_remove()</tt> from inside
the input function; GTK+ will not be in the midst of
iterating over the list of input functions.
</p>
<p>
To specify the condition(s) to wait for, use the <span
class="STRUCTNAME">GdkInputCondition</span> flags: <span
class="STRUCTNAME">GDK_INPUT_READ</span>, <span class=
"STRUCTNAME">GDK_INPUT_WRITE</span>, and <span class=
"STRUCTNAME">GDK_INPUT_EXCEPTION</span>. You can OR one
or more flags together. These correspond to the three
file descriptor sets passed to the <tt class="FUNCTION">
select()</tt> system call; consult a good UNIX
programming book for details. If any condition is met,
the input function is invoked.
</p>
<p>
The callback should look like this:
</p>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="PROGRAMLISTING">
typedef void (*GdkInputFunction) (gpointer data,
gint source_fd,
GdkInputCondition condition);
</pre>
</td>
</tr>
</table>
<p>
It receives your callback data, the file descriptor being
watched, and the conditions that were met (possibly a
subset of those you were watching for).
</p>
<div class="FIGURE">
<a name="FL-INPUTFUNCS"></a>
<div class="FUNCSYNOPSIS">
<a name="FL-INPUTFUNCS.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <gdk/gdk.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF">gint <tt class=
"FUNCTION">gdk_input_add</tt></code>(gint <tt class=
"PARAMETER"><i>source_fd</i></tt>, GdkInputCondition
<tt class="PARAMETER"><i>condition</i></tt>,
GdkInputFunction <tt class="PARAMETER"><i>
function</i></tt>, gpointer <tt class="PARAMETER"><i>
data</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">gdk_input_remove</tt></code>(gint <tt
class="PARAMETER"><i>tag</i></tt>);</code>
</p>
</div>
<p>
<b>Figure 32. Input Functions</b>
</p>
</div>
</div>
</div>
<div class="NAVFOOTER">
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