With GTK+ 2.x it was common to include just the header files for a few widgets that your application was using, which could lead to problems with missing definitions, etc. GTK+ 3 tightens the rules about which header files you are allowed to include directly. The allowed header files are are

(these relative paths are assuming that you are using the include paths that are specified in the gtk+-2.0.pc file, as returned by pkg-config --cflags gtk+-2.0.pc.)

To check that your application only includes the allowed headers, you can use defines to disable inclusion of individual headers, as follows:

    make CFLAGS+="-DGTK_DISABLE_SINGLE_INCLUDES"
    

Over the years, a number of functions, and in some cases, entire widgets have been deprecated. These deprecations are clearly spelled out in the API reference, with hints about the recommended replacements. The API reference also includes an index of all deprecated symbols.

To verify that your program does not use any deprecated symbols, you can use defines to remove deprecated symbols from the header files, as follows:

    make CFLAGS+="-DGDK_DISABLE_DEPRECATED -DGTK_DISABLE_DEPRECATED"
    

GTK+ 3 removes many implementation details and struct members from its public headers.

To ensure that your application does not have problems with this, you define the preprocessor symbol GSEAL_ENABLE while building your application against GTK+ 2.x. This will make the compiler catch all uses of direct access to struct fields so that you can go through them one by one and replace them with a call to an accessor function instead.

    make CFLAGS+="-DGSEAL_ENABLE"
    

Key constants have gained a _KEY_ infix. For example, GDK_a is now GDK_KEY_a. In GTK+ 2, the old names continue to be available. In GTK+ 3 however, the old names will require an explicit include of the gdkkeysyms-compat.h header.

The gdk_spawn family of functions has been deprecated in GDK 2.24 and removed from GDK 3. Various replacements exist; the best replacement depends on the circumstances:

In GTK+ 3, the GDK drawing API (which closely mimics the X drawing API, which is itself modeled after PostScript) has been removed. All drawing in GTK+ 3 is done via cairo.

The GdkGC and GdkImage objects, as well as all the functions using them, are gone. This includes the gdk_draw family of functions like gdk_draw_rectangle() and gdk_draw_drawable(). As GdkGC is roughly equivalent to cairo_t and GdkImage was used for drawing images to GdkWindows, which cairo supports automatically, a transition is usually straightforward.

The following examples show a few common drawing idioms used by applications that have been ported to use cairo and how the code was replaced.

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gdk_draw_pixbuf (window,
                 gtk_widget_get_style (widget)->black_gc,
                 pixbuf,
                 0, 0
                 x, y,
                 gdk_pixbuf_get_width (pixbuf),
                 gdk_pixbuf_get_height (pixbuf),
                 GDK_RGB_DITHER_NORMAL,
                 0, 0);

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cairo_t *cr = gdk_cairo_create (window);
gdk_cairo_set_source_pixbuf (cr, pixbuf, x, y);
cairo_paint (cr);
cairo_destroy (cr);

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GdkGCValues gc_values;
GdkGC *gc;

/* setup */
gc = gtk_widget_get_style (widget)->black_gc;
gdk_gc_set_tile (gc, pixmap);
gdk_gc_set_fill (gc, GDK_TILED);
gdk_gc_set_ts_origin (gc, x_origin, y_origin);
/* use */
gdk_draw_rectangle (window, gc, TRUE, 0, 0, width, height);
/* restore */
gdk_gc_set_tile (gc, NULL);
gdk_gc_set_fill (gc, GDK_SOLID);
gdk_gc_set_ts_origin (gc, 0, 0);

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cairo_t *cr;
cairo_surface_t *surface;

surface = ...
cr = gdk_cairo_create (window);
cairo_set_source_surface (cr, surface, x_origin, y_origin);
cairo_pattern_set_extend (cairo_get_source (cr), CAIRO_EXTEND_REPEAT);
cairo_rectangle (cr, 0, 0, width, height);
cairo_fill (cr);
cairo_destroy (cr);

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GdkGC *gc;

/* setup */
gc = gtk_widget_get_style (widget)->text_gc[state];
gdk_gc_set_clip_rectangle (gc, &area);
/* use */
gdk_draw_layout (drawable, gc, x, y, layout);
/* restore */
gdk_gc_set_clip_rectangle (gc, NULL);

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GtkStyleContext *context;
GtkStateFlags flags;
GdkRGBA rgba;
cairo_t *cr;

cr = gdk_cairo_create (drawable);
/* clip */
gdk_cairo_rectangle (cr, &area);
cairo_clip (cr);
/* set the correct source color */
context = gtk_widget_get_style_context (widget));
state = gtk_widget_get_state_flags (widget);
gtk_style_context_get_color (context, state, &rgba);
gdk_cairo_set_source_rgba (cr, &rgba);
/* draw the text */
cairo_move_to (cr, x, y);
pango_cairo_show_layout (cr, layout);
cairo_destroy (cr);

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static cairo_pattern_t *
gtk_color_button_get_checkered (void)
{
    /* need to respect pixman's stride being a multiple of 4 */
    static unsigned char data[8] = { 0xFF, 0x00, 0x00, 0x00,
                                     0x00, 0xFF, 0x00, 0x00 };
    cairo_surface_t *surface;
    cairo_pattern_t *pattern;

    surface = cairo_image_surface_create_for_data (data,
                                                   CAIRO_FORMAT_A8,
                                                   2, 2,
                                                   4);
    pattern = cairo_pattern_create_for_surface (surface);
    cairo_surface_destroy (surface);
    cairo_pattern_set_extend (pattern, CAIRO_EXTEND_REPEAT);
    cairo_pattern_set_filter (pattern, CAIRO_FILTER_NEAREST);

    return pattern;
}

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gdk_draw_drawable (pixmap,
                   gc,
                   pixmap,
                   area.x + dx, area.y + dy,
                   area.x, area.y,
                   area.width, area.height);

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cairo_t *cr = cairo_create (surface);
/* clipping restricts the intermediate surface's size, so it's a good idea
 * to use it. */
gdk_cairo_rectangle (cr, &area);
cairo_clip (cr);
/* Now push a group to change the target */
cairo_push_group (cr);
cairo_set_source_surface (cr, surface, dx, dy);
cairo_paint (cr);
/* Now copy the intermediate target back */
cairo_pop_group_to_source (cr);
cairo_paint (cr);
cairo_destroy (cr);