# Getting Started with GTK+

This chapter is contains some tutorial information to get you started with GTK+ programming. It assumes that you have GTK+, its dependencies and a C compiler installed and ready to use. If you need to build GTK+ itself first, refer to the Compiling the GTK+ libraries section in this reference.

## Basics

To begin our introduction to GTK, we'll start with the simplest program possible. This program will create an empty 200x200 pixel window:

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You can compile the program above with GCC using:

          gcc `pkg-config --cflags gtk+-3.0` -o window-default window-default.c `pkg-config --libs gtk+-3.0`

For more information on how to compile a GTK+ application, please refer to the Compiling GTK+ Applications section in this reference.

All GTK+ applications will, of course, include `gtk/gtk.h`, which declares functions, types and macros required by GTK+ applications.

Even if GTK+ installs multiple header files, only the top-level `gtk/gtk.h` header can be directly included by third party code. The compiler will abort with an error if any other header is directly included.

We then proceed into the `main`() function of the application, and we declare a `window` variable as a pointer of type #GtkWidget.

The following line will call gtk_init(), which is the initialization function for GTK+; this function will set up GTK+, the type system, the connection to the windowing environment, etc. The gtk_init() takes as arguments the pointers to the command line arguments counter and string array; this allows GTK+ to parse specific command line arguments that control the behavior of GTK+ itself. The parsed arguments will be removed from the array, leaving the unrecognized ones for your application to parse.

For more information on which command line arguments GTK+ recognizes, please refer to the Running GTK+ Applications section in this reference.

The call to gtk_window_new() will create a new #GtkWindow and store it inside the `window` variable. The type of the window is %GTK_WINDOW_TOPLEVEL, which means that the #GtkWindow will be managed by the windowing system: it will have a frame, a title bar and window controls, depending on the platform.

In order to terminate the application when the #GtkWindow is destroyed, we connect the #GtkWidget::destroy signal to the gtk_main_quit() function. This function will terminate the GTK+ main loop started by calling gtk_main() later. The #GtkWidget::destroy signal is emitted when a widget is destroyed, either by explicitly calling gtk_widget_destroy() or when the widget is unparented. Top-level #GtkWindows are also destroyed when the Close window control button is clicked.

#GtkWidgets are hidden by default. By calling gtk_widget_show() on a #GtkWidget we are asking GTK+ to set the visibility attribute so that it can be displayed. All this work is done after the main loop has been started.

The last line of interest is the call to gtk_main(). This function will start the GTK+ main loop and will block the control flow of the main() until the gtk_main_quit() function is called.

While the program is running, GTK+ is receiving events. These are typically input events caused by the user interacting with your program, but also things like messages from the window manager or other applications. GTK+ processes these and as a result, signals may be emitted on your widgets. Connecting handlers for these signals is how you normally make your program do something in response to user input.

The following example is slightly more complex, and tries to showcase some of the capabilities of GTK+.

In the long tradition of programming languages and libraries, it is called Hello, World.

### Hello World in GTK+

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## Packing

When creating an application, you'll want to put more than one widget inside a window. Our first helloworld example only used one widget so we could simply use a gtk_container_add() call to "pack" the widget into the window. But when you want to put more than one widget into a window, it it becomes important to control how each widget is positioned and sized. This is where packing comes in.

GTK+ comes with a large variety of layout containers whose purpose it is to control the layout of the child widgets that are added to them. See for an overview.

The following example shows how the GtkGrid container lets you arrange several buttons:

### Packing buttons

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## Drawing

Many widgets, like buttons, do all their drawing themselves. You just tell them the label you want to see, and they figure out what font to use, draw the button outline and focus rectangle, etc. Sometimes, it is necessary to do some custom drawing. In that case, a #GtkDrawingArea might be the right widget to use. It offers a canvas on which you can draw by connecting to the #GtkWidget::draw signal.

The contents of a widget often need to be partially or fully redrawn, e.g. when another window is moved and uncovers part of the widget, or when tie window containing it is resized. It is also possible to explicitly cause part or all of the widget to be redrawn, by calling gtk_widget_queue_draw() or its variants. GTK+ takes care of most of the details by providing a ready-to-use cairo context to the ::draw signal handler.

The following example shows a ::draw signal handler. It is a bit more complicated than the previous examples, since it also demonstrates input event handling by means of ::button-press and ::motion-notify handlers.

### Drawing in response to input

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## Building interfaces

When construcing a more complicated user interface, with dozens or hundreds of widgets, doing all the setup work in C code is cumbersome, and making changes becomes next to impossible.

Thankfully, GTK+ supports the separation of user interface layout from your business logic, by using UI descriptions in an XML format that can be parsed by the #GtkBuilder class.

### Packing buttons with GtkBuilder

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The builder.ui file looks like this:

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Note that GtkBuilder can also be used to construct objects that are not widgets, such as tree models, adjustments, etc. That is the reason the method we use here is called gtk_builder_get_object() and returns a GObject* instead of a GtkWidget*.

Normally, you would pass a full path to gtk_builder_add_from_file() to make the execution of your program independent of the current directory. A common location to install UI descriptions and similar data is `/usr/share/appname`.

It is also possible to embed the UI description in the source code as a string and use gtk_builder_add_from_string() to load it. But keeping the UI description in a separate file has several advantages: It is then possible to make minor adjustments to the UI without recompiling your program, and, more importantly, graphical UI editors such as glade can load the file and allow you to create and modify your UI by point-and-click.