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<H2><A NAME="s2">2. Getting Started</A></H2>
<P>The first thing to do, of course, is download the GTK source and
install it. You can always get the latest version from ftp.gtk.org in
/pub/gtk. You can also view other sources of GTK information on
<A HREF="http://www.gtk.org/">http://www.gtk.org/</A>. GTK
uses GNU autoconf for configuration. Once untar'd, type ./configure
--help to see a list of options.
<P>The GTK source distribution also contains the complete source to all
of the examples used in this tutorial, along with Makefiles to aid
compilation.
<P>To begin our introduction to GTK, we'll start with the simplest
program possible. This program will create a 200x200 pixel window and
has no way of exiting except to be killed by using the shell.
<P>
<BLOCKQUOTE><CODE>
<PRE>
/* example-start base base.c */
#include <gtk/gtk.h>
int main( int argc,
char *argv[] )
{
GtkWidget *window;
gtk_init (&argc, &argv);
window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
gtk_widget_show (window);
gtk_main ();
return(0);
}
/* example-end */
</PRE>
</CODE></BLOCKQUOTE>
<P>You can compile the above program with gcc using:
<BLOCKQUOTE><CODE>
<PRE>
gcc base.c -o base `gtk-config --cflags --libs`
</PRE>
</CODE></BLOCKQUOTE>
<P>The meaning of the unusual compilation options is explained below in
<A HREF="#sec_compiling">Compiling Hello World</A>.
<P>All programs will of course include gtk/gtk.h which declares the
variables, functions, structures, etc. that will be used in your GTK
application.
<P>The next line:
<P>
<BLOCKQUOTE><CODE>
<PRE>
gtk_init (&argc, &argv);
</PRE>
</CODE></BLOCKQUOTE>
<P>calls the function gtk_init(gint *argc, gchar ***argv) which will be
called in all GTK applications. This sets up a few things for us such
as the default visual and color map and then proceeds to call
gdk_init(gint *argc, gchar ***argv). This function initializes the
library for use, sets up default signal handlers, and checks the
arguments passed to your application on the command line, looking for
one of the following:
<P>
<UL>
<LI> <CODE>--gtk-module</CODE></LI>
<LI> <CODE>--g-fatal-warnings</CODE></LI>
<LI> <CODE>--gtk-debug</CODE></LI>
<LI> <CODE>--gtk-no-debug</CODE></LI>
<LI> <CODE>--gdk-debug</CODE></LI>
<LI> <CODE>--gdk-no-debug</CODE></LI>
<LI> <CODE>--display</CODE></LI>
<LI> <CODE>--sync</CODE></LI>
<LI> <CODE>--no-xshm</CODE></LI>
<LI> <CODE>--name</CODE></LI>
<LI> <CODE>--class</CODE></LI>
</UL>
<P>It removes these from the argument list, leaving anything it does not
recognize for your application to parse or ignore. This creates a set
of standard arguments accepted by all GTK applications.
<P>The next two lines of code create and display a window.
<P>
<BLOCKQUOTE><CODE>
<PRE>
window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
gtk_widget_show (window);
</PRE>
</CODE></BLOCKQUOTE>
<P>The <CODE>GTK_WINDOW_TOPLEVEL</CODE> argument specifies that we want the
window to undergo window manager decoration and placement. Rather than
create a window of 0x0 size, a window without children is set to
200x200 by default so you can still manipulate it.
<P>The gtk_widget_show() function lets GTK know that we are done setting
the attributes of this widget, and that it can display it.
<P>The last line enters the GTK main processing loop.
<P>
<BLOCKQUOTE><CODE>
<PRE>
gtk_main ();
</PRE>
</CODE></BLOCKQUOTE>
<P>gtk_main() is another call you will see in every GTK application.
When control reaches this point, GTK will sleep waiting for X events
(such as button or key presses), timeouts, or file IO notifications to
occur. In our simple example, however, events are ignored.
<P>
<H2><A NAME="ss2.1">2.1 Hello World in GTK</A>
</H2>
<P>Now for a program with a widget (a button). It's the classic
hello world a la GTK.
<P>
<BLOCKQUOTE><CODE>
<PRE>
/* example-start helloworld helloworld.c */
#include <gtk/gtk.h>
/* This is a callback function. The data arguments are ignored
* in this example. More on callbacks below. */
void hello( GtkWidget *widget,
gpointer data )
{
g_print ("Hello World\n");
}
gint delete_event( GtkWidget *widget,
GdkEvent *event,
gpointer data )
{
/* If you return FALSE in the "delete_event" signal handler,
* GTK will emit the "destroy" signal. Returning TRUE means
* you don't want the window to be destroyed.
* This is useful for popping up 'are you sure you want to quit?'
* type dialogs. */
g_print ("delete event occurred\n");
/* Change TRUE to FALSE and the main window will be destroyed with
* a "delete_event". */
return(TRUE);
}
/* Another callback */
void destroy( GtkWidget *widget,
gpointer data )
{
gtk_main_quit();
}
int main( int argc,
char *argv[] )
{
/* GtkWidget is the storage type for widgets */
GtkWidget *window;
GtkWidget *button;
/* This is called in all GTK applications. Arguments are parsed
* from the command line and are returned to the application. */
gtk_init(&argc, &argv);
/* create a new window */
window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
/* When the window is given the "delete_event" signal (this is given
* by the window manager, usually by the "close" option, or on the
* titlebar), we ask it to call the delete_event () function
* as defined above. The data passed to the callback
* function is NULL and is ignored in the callback function. */
gtk_signal_connect (GTK_OBJECT (window), "delete_event",
GTK_SIGNAL_FUNC (delete_event), NULL);
/* Here we connect the "destroy" event to a signal handler.
* This event occurs when we call gtk_widget_destroy() on the window,
* or if we return FALSE in the "delete_event" callback. */
gtk_signal_connect (GTK_OBJECT (window), "destroy",
GTK_SIGNAL_FUNC (destroy), NULL);
/* Sets the border width of the window. */
gtk_container_set_border_width (GTK_CONTAINER (window), 10);
/* Creates a new button with the label "Hello World". */
button = gtk_button_new_with_label ("Hello World");
/* When the button receives the "clicked" signal, it will call the
* function hello() passing it NULL as its argument. The hello()
* function is defined above. */
gtk_signal_connect (GTK_OBJECT (button), "clicked",
GTK_SIGNAL_FUNC (hello), NULL);
/* This will cause the window to be destroyed by calling
* gtk_widget_destroy(window) when "clicked". Again, the destroy
* signal could come from here, or the window manager. */
gtk_signal_connect_object (GTK_OBJECT (button), "clicked",
GTK_SIGNAL_FUNC (gtk_widget_destroy),
GTK_OBJECT (window));
/* This packs the button into the window (a gtk container). */
gtk_container_add (GTK_CONTAINER (window), button);
/* The final step is to display this newly created widget. */
gtk_widget_show (button);
/* and the window */
gtk_widget_show (window);
/* All GTK applications must have a gtk_main(). Control ends here
* and waits for an event to occur (like a key press or
* mouse event). */
gtk_main ();
return(0);
}
/* example-end */
</PRE>
</CODE></BLOCKQUOTE>
<P>
<H2><A NAME="sec_compiling"></A> <A NAME="ss2.2">2.2 Compiling Hello World </A>
</H2>
<P>To compile use:
<P>
<BLOCKQUOTE><CODE>
<PRE>
gcc -Wall -g helloworld.c -o helloworld `gtk-config --cflags` \
`gtk-config --libs`
</PRE>
</CODE></BLOCKQUOTE>
<P>This uses the program <CODE>gtk-config</CODE>, which comes with GTK. This
program "knows" what compiler switches are needed to compile programs
that use GTK. <CODE>gtk-config --cflags</CODE> will output a list of include
directories for the compiler to look in, and <CODE>gtk-config --libs</CODE>
will output the list of libraries for the compiler to link with and
the directories to find them in. In the above example they could have
been combined into a single instance, such as
<CODE>`gtk-config --cflags --libs`</CODE>.
<P>Note that the type of single quote used in the compile command above
is significant.
<P>The libraries that are usually linked in are:
<UL>
<LI>The GTK library (-lgtk), the widget library, based on top of GDK.</LI>
<LI>The GDK library (-lgdk), the Xlib wrapper.</LI>
<LI>The gmodule library (-lgmodule), which is used to load run time
extensions.</LI>
<LI>The GLib library (-lglib), containing miscellaneous functions;
only g_print() is used in this particular example. GTK is built on top
of glib so you will always require this library. See the section on
<A HREF="gtk_tut-20.html#sec_glib">GLib</A> for details.</LI>
<LI>The Xlib library (-lX11) which is used by GDK.</LI>
<LI>The Xext library (-lXext). This contains code for shared memory
pixmaps and other X extensions.</LI>
<LI>The math library (-lm). This is used by GTK for various purposes.</LI>
</UL>
<P>
<H2><A NAME="ss2.3">2.3 Theory of Signals and Callbacks</A>
</H2>
<P>Before we look in detail at <EM>helloworld</EM>, we'll discuss signals
and callbacks. GTK is an event driven toolkit, which means it will
sleep in gtk_main until an event occurs and control is passed to the
appropriate function.
<P>This passing of control is done using the idea of "signals". (Note
that these signals are not the same as the Unix system signals, and
are not implemented using them, although the terminology is almost
identical.) When an event occurs, such as the press of a mouse button,
the appropriate signal will be "emitted" by the widget that was
pressed. This is how GTK does most of its useful work. There are
signals that all widgets inherit, such as "destroy", and there are
signals that are widget specific, such as "toggled" on a toggle
button.
<P>To make a button perform an action, we set up a signal handler to
catch these signals and call the appropriate function. This is done by
using a function such as:
<P>
<BLOCKQUOTE><CODE>
<PRE>
gint gtk_signal_connect( GtkObject *object,
gchar *name,
GtkSignalFunc func,
gpointer func_data );
</PRE>
</CODE></BLOCKQUOTE>
<P>where the first argument is the widget which will be emitting the
signal, and the second the name of the signal you wish to catch. The
third is the function you wish to be called when it is caught, and the
fourth, the data you wish to have passed to this function.
<P>The function specified in the third argument is called a "callback
function", and should generally be of the form
<P>
<BLOCKQUOTE><CODE>
<PRE>
void callback_func( GtkWidget *widget,
gpointer callback_data );
</PRE>
</CODE></BLOCKQUOTE>
<P>where the first argument will be a pointer to the widget that emitted
the signal, and the second a pointer to the data given as the last
argument to the gtk_signal_connect() function as shown above.
<P>Note that the above form for a signal callback function declaration is
only a general guide, as some widget specific signals generate
different calling parameters. For example, the CList "select_row"
signal provides both row and column parameters.
<P>Another call used in the <EM>helloworld</EM> example, is:
<P>
<BLOCKQUOTE><CODE>
<PRE>
gint gtk_signal_connect_object( GtkObject *object,
gchar *name,
GtkSignalFunc func,
GtkObject *slot_object );
</PRE>
</CODE></BLOCKQUOTE>
<P>gtk_signal_connect_object() is the same as gtk_signal_connect() except
that the callback function only uses one argument, a pointer to a GTK
object. So when using this function to connect signals, the callback
should be of the form
<P>
<BLOCKQUOTE><CODE>
<PRE>
void callback_func( GtkObject *object );
</PRE>
</CODE></BLOCKQUOTE>
<P>where the object is usually a widget. We usually don't setup callbacks
for gtk_signal_connect_object however. They are usually used to call a
GTK function that accepts a single widget or object as an argument, as
is the case in our <EM>helloworld</EM> example.
<P>The purpose of having two functions to connect signals is simply to
allow the callbacks to have a different number of arguments. Many
functions in the GTK library accept only a single GtkWidget pointer as
an argument, so you want to use the gtk_signal_connect_object() for
these, whereas for your functions, you may need to have additional
data supplied to the callbacks.
<P>
<H2><A NAME="ss2.4">2.4 Events</A>
</H2>
<P>In addition to the signal mechanism described above, there is a set
of <EM>events</EM> that reflect the X event mechanism. Callbacks may
also be attached to these events. These events are:
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