## Getting Started with GTK+

GTK+ is a widget toolkit. Each user interface created by GTK+ consists of widgets. This is implemented in C using GObject, an object-oriented framework for C. Widgets are organized in a hierachy. The window widget is the main container. The user interface is then built by adding buttons, drop-down menus, input fields, and other widgets to the window. If you are creating complex user interfaces it is recommended to use GtkBuilder and its GTK-specific markup description language, instead of assembling the interface manually. You can also use a visual user interface editor, like Glade.

GTK+ is event-driven. The toolkit listens for events such as a click on a button, and passes the event to your application.

This chapter 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 a simple signal-based Gtk application. This program will create an empty 200 × 200 pixel window.

Create a new file with the following content named `example-0.c.`

    #include <gtk/gtk.h>

    static void
    activate (GtkApplication* app,
              gpointer        user_data)
    {
      GtkWidget *window;

      window = gtk_application_window_new (app);
      gtk_window_set_title (GTK_WINDOW (window), "Window");
      gtk_window_set_default_size (GTK_WINDOW (window), 200, 200);
      gtk_widget_show_all (window);
    }

    int
    main (int    argc,
          char **argv)
    {
      GtkApplication *app;
      int status;

      app = gtk_application_new ("org.gtk.example", 0);
      g_signal_connect (app, "activate", G_CALLBACK (activate), NULL);
      status = g_application_run (G_APPLICATION (app), argc, argv);
      g_object_unref (app);

      return status;
    }

You can compile the program above with GCC using:

`gcc `pkg-config --cflags gtk+-3.0` -o example-0 example-0.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.

In a GTK+ application, the purpose of the `main()` function is to create a GtkApplication object and run it. In this example a GtkApplication pointer named `app` is called and then initialized using `gtk_application_new()`.

When creating a GtkApplication you need to pick an application identifier (a name) and input to `gtk_application_new()` as parameter. For this example `org.gtk.example` is used but for choosing an identifier for your application see this guide. Lastly `gtk_application_new()` takes a GApplicationFlags as input for your application, if your application would have special needs.

Next the activate signal is connected to the `activate()` function above the `main()` functions. The `activate` signal will be sent when your application is launched with `g_application_run()` on the line below. The `gtk_application_run()` also 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.

Within g_application_run the `activate()` signal is sent and we then proceed into the `activate`() function of the application. Inside the `activate()` function we want to construct our GTK window, so that a window is shown when the application is launched. The call to `gtk_application_window_new()` will create a new GtkWindow and store it inside the `window` pointer. The window will have a frame, a title bar, and window controls depending on the platform.

A window title is set using `gtk_window_set_title()`. This function takes a GtkWindow* pointer and a string as input. As our `window` pointer is a GtkWidget pointer, we need to cast it to GtkWindow*. But instead of casting `window` via `(GtkWindow*)`, `window` can be cast using the macro ``GTK_WINDOW()``. ``GTK_WINDOW()`` will check if the pointer is an instance of the GtkWindow class, before casting, and emit a warning if the check fails. More information about this convention can be found here.

Finally the window size is set using gtk_window_set_default_size and the window is then shown by GTK via `gtk_widget_show_all()`.

When you exit the window, by for example pressing the X, the `g_application_run()` in the main loop returns with a number which is saved inside an integer named "status". Afterwards, the GtkApplication object is freed from memory with `g_object_unref()`. Finally the status integer is returned and the GTK application exits.

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.

Example 1. Hello World in GTK+

Create a new file with the following content named example-1.c.

    #include <gtk/gtk.h>

    static void
    print_hello (GtkWidget *widget,
                 gpointer   data)
    {
      g_print ("Hello World\n");
    }

    static void
    activate (GtkApplication *app,
              gpointer        user_data)
    {
      GtkWidget *window;
      GtkWidget *button;
      GtkWidget *button_box;

      window = gtk_application_window_new (app);
      gtk_window_set_title (GTK_WINDOW (window), "Window");
      gtk_window_set_default_size (GTK_WINDOW (window), 200, 200);

      button_box = gtk_button_box_new (GTK_ORIENTATION_HORIZONTAL);
      gtk_container_add (GTK_CONTAINER (window), button_box);

      button = gtk_button_new_with_label ("Hello World");
      g_signal_connect (button, "clicked", G_CALLBACK (print_hello), NULL);
      g_signal_connect_swapped (button, "clicked", G_CALLBACK (gtk_widget_destroy), window);
      gtk_container_add (GTK_CONTAINER (button_box), button);

      gtk_widget_show_all (window);
    }

    int
    main (int    argc,
          char **argv)
    {
      GtkApplication *app;
      int status;

      app = gtk_application_new ("org.gtk.example", 0);
      g_signal_connect (app, "activate", G_CALLBACK (activate), NULL);
      status = g_application_run (G_APPLICATION (app), argc, argv);
      g_object_unref (app);

      return status;
    }

You can compile the program above with GCC using:

`gcc `pkg-config --cflags gtk+-3.0` -o example-1 example-1.c `pkg-config --libs gtk+-3.0``

As seen above, example-1.c builds further upon example-0.c by adding a button to our window, with the label "Hello World". Two new GtkWidget pointers are declared to accomplish this, `button` and `button_box`. The button_box variable is created to store a GtkButtonBox which is GTK+'s way of controlling the size and layout of buttons. The GtkButtonBox is created and assigned to `gtk_button_box_new()` which takes a GtkOrientation enum as parameter. The buttons which this box will contain can either be stored horizontally or vertically but this does not matter in this particular case as we are dealing with only one button. After initializing button_box with horizontal orientation, the code adds the button_box widget to the window widget using `gtk_container_add()`.

Next the `button` variable is initialized in similar manner. `gtk_button_new_with_label()` is called which returns a GtkButton to be stored inside `button`. Afterwards `button` is added to our `button_box`. Using g_signal_connect the button is connected to a function in our app called `print_hello()`, so that when the button is clicked, GTK will call this function. As the `print_hello()` function does not use any data as input, NULL is passed to it. `print_hello()` calls `g_print()` with the string "Hello World" which will print Hello World in a terminal if the GTK application was started from one.

After connecting `print_hello()`, another signal is connected to the "clicked" state of the button using `g_signal_connect_swapped()`. This functions is similar to a `g_signal_connect()` with the difference lying in how the callback function is treated. `g_signal_connect_swapped()` allow you to specify what the callback function should take as parameter by letting you pass it as data. In this case the function being called back is `gtk_widget_destroy()` and the `window` pointer is passed to it. This has the effect that when the button is clicked, the whole GTK window is destroyed. In contrast if a normal `g_signal_connect()` were used to connect the "clicked" signal with `gtk_widget_destroy()`, then the `button` itself would have been destroyed, not the window. More information about creating buttons can be found here.

The rest of the code in example-1.c is identical to example-0.c. Next section will elaborate further on how to add several GtkWidgets to your GTK application.