Gtk Hello World

30 Sep 2016

Hi to everybody. At this post I will show a hello world program using gtk+-2.0. You must have gtk+-2.0 headers installed. At Fedora I had to install gtk2-devel. Our program will have only one button. When the button is pressed the “Hello world!!!” message is printed to the console.

The first thing to do is include the gtk header as below:

#include <gtk/gtk.h>

Next we do define the callback invoked when the button is pressed. The registration goes next, in the our main() :)

static void button_callback(GtkButton *button, gpointer user_data)
{
	g_print(user_data);
}

Notice the g_print. Gtk is build on top of glib. glib provide a lot of nice features but let’s focus on Gtk right now. So, for now, g_print is simply the glib’s printf. Let’s define or main:

int main(int argc, char **argv)
{
	GtkWidget *window;
	GtkWidget *button;

	gtk_init(&argc, &argv);

Every Gtk program starts this way. gtk_init() will process each Gtk’s supported argument leaving unknown arguments untouched. You can follow gtk_init() by apropriated getopt calls so that you have your own arguments together with Gtk’s arguments. Note that two GtkWidgets were declared. Both, our root window and the button are widgets. In Gtk everything that you see in screen is a widget. This design is inherited from glib too where new types (objects) inherit features from its parents, like in OOP. Now let’s create our widgets and connect some signals.

	window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
	button = gtk_button_new_with_label("Hello world");
	g_signal_connect(button, "clicked", G_CALLBACK(button_callback), "Hello world!!!\n");
	g_signal_connect(window, "destroy", G_CALLBACK(gtk_main_quit), NULL);
	gtk_container_add(GTK_CONTAINER(window), button);

First two lines create our window and button widgets. The third line connects our callback to the clicked signal. The fourth parameter is some data passed to the callback when it is invoked. It goes trought that gpointer opaque type. Each type of widget has it’s own signals. The fourth line connects the destroy signal to gtk_main_quit. gtk_main_quit function is used to exit our application. Finaly the fifth line adds the button to the window.

	gtk_window_set_position(GTK_WINDOW(window), GTK_WIN_POS_CENTER);
	gtk_window_set_title(GTK_WINDOW(window), "Hello gtk world");

These two lines setup the position and the title of our window.

	gtk_widget_show_all(window);
	gtk_main();
	return 0;
}

And finaly we finish our main() function. The first line here make all the window popup. The second, gtk_main() is the Gtk’s main loop. This call will block tills gtk_main_quit() is invoked. The main loop is resposible to wait for user input events and delivery they to the programmer’s callbacks. And last but no least (I really love to say (write) that!) we need to compile all that lines in something executable. The Gtk library is a wild beast. Happly the compiler flags are handled by pkg-config tool, so compile this example, assuming that the file is called gtk-button-helloworld.c can be done with this line:

gcc -Wall -o gtk-button-helloworld gtk-button-helloworld.c $(pkg-config --libs --cflags gtk+-2.0) 

And here is a screenshot of the program running:

The whole code can be found here

That’s it! Cheers!!!