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 for GTK 3 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"

Note that some parts of our API, such as enumeration values, are not well covered by the deprecation warnings. In most cases, using them will require you to also use deprecated functions, which will trigger warnings.

Deprecations of properties and signals cannot be caught at compile time, as both properties and signals are installed and used after types have been instantiated. In order to catch deprecations and changes in the run time components, you should use the G_ENABLE_DIAGNOSTIC environment variable when running your application, e.g.:

G_ENABLE_DIAGNOSTIC=1 ./your-app

GTK4 does not parse command line arguments any more. If you are using command line arguments like --gtk-debug you should use the GTK_DEBUG environment variable instead. If you are using --g-fatal-warnings for debugging purposes, you should use the G_DEBUG environment variable, as specified by the GLib documentation.

Style properties do not exist in GTK 4. You should stop using them in your custom CSS and in your code.

GTK 4 removes the GDK_WA_CURSOR flag. Instead, just use gdk_window_set_cursor() to set a cursor on the window after creating it. GTK 4 also removes the GDK_WA_VISUAL flag, and always uses an RGBA visual for windows. To prepare your code for this, use gdk_window_set_visual (gdk_screen_get_rgba_visual()) after creating your window. GTK 4 also removes the GDK_WA_WMCLASS flag. If you need this X11-specific functionality, use XSetClassHint() directly.

In GTK 4, event structs are opaque and immutable. Many fields already have accessors in GTK 3, and you should use those to reduce the amount of porting work you have to do at the time of the switch.

Warping the pointer is disorienting and unfriendly to users. GTK 4 does not support it. In special circumstances (such as when implementing remote connection UIs) it can be necessary to warp the pointer; in this case, use platform APIs such as XWarpPointer() directly.

GTK 4 always uses RGBA visuals for its windows; you should make sure that your code works with such visuals. At the same time, you should stop using GdkVisual APIs, since this object not longer exists in GTK 4. Most of its APIs are deprecated already and not useful when dealing with RGBA visuals.

GTK 4 removes these GtkBox child properties, so you should stop using them. You can replace GtkBox:padding using the “margin” properties on your GtkBox child widgets.

The fill child property can be replaced by setting appropriate values for the “halign” and “valign” properties of the child widgets. If you previously set the fill child property to TRUE, you can achieve the same effect by setting the halign or valign properties to GTK_ALIGN_FILL, depending on the parent box – halign for a horizontal box, valign for a vertical one.

GtkBox also uses the expand child property. It can be replaced by setting “hexpand” or “vexpand” on the child widgets. To match the old behavior of the GtkBox’s expand child property, you need to set “hexpand” on the child widgets of a horizontal GtkBox and “vexpand” on the child widgets of a vertical GtkBox.

Note that there’s a subtle but important difference between GtkBox’s expand and fill child properties and the ones in GtkWidget: setting “hexpand” or “vexpand” to TRUE will propagate up the widget hierarchy, so a pixel-perfect port might require you to reset the expansion flags to FALSE in a parent widget higher up the hierarchy.

The getters in the GtkStyleContext API, such as gtk_style_context_get_property(), gtk_style_context_get(), or gtk_style_context_get_color() only accept the context’s current state for their state argument. You should update all callers to pass the current state.

These functions are not supported in GTK 4. Instead, either use backend-specific APIs, or render your widgets using GtkWidgetClass.snapshot() (once you are using GTK 4).

Stop using GtkButton’s image-related API

The functions and properties related to automatically add a GtkImage to a GtkButton, and using a GtkSetting to control its visibility, are not supported in GTK 4. Instead, you can just pack a GtkImage inside a GtkButton, and control its visibility like you would for any other widget. If you only want to add a named icon to a GtkButton, you can use gtk_button_new_from_icon_name().

Event controllers and gestures replace event signals in GTK 4.

Most of them have been backported to GTK 3.x so you can prepare for this change.

Event signals which are not directly related to input have to be dealt with on a one-by-one basis. If you were using ::configure-event and ::window-state-event to save window state, you should use property notification for corresponding GtkWindow properties, such as “default-width”, “default-height”, “maximized” or “fullscreened”.

In GTK 4 we want the application’s GApplication “application-id” (and therefore the D-Bus name), the desktop file basename and Wayland’s xdg-shell app_id to match. In order to achieve this with GTK 3.x call g_set_prgname() with the same application ID you passed to GtkApplication. Rename your desktop files to match the application ID if needed.

The call to g_set_prgname() can be removed once you fully migrated to GTK 4.

You should be aware that changing the application ID makes your application appear as a new, different app to application installers. You should consult the appstream documentation for best practices around renaming applications.

GTK 4 removes the gtk_main_ family of APIs. The recommended replacement is GtkApplication, but you can also iterate the GLib mainloop directly, using GMainContext APIs. The replacement for gtk_events_pending() is g_main_context_pending(), the replacement for gtk_main_iteration() is g_main_context_iteration().

GTK 4 introduces a gtk_window_destroy() api. While that is not available in GTK 3, you can prepare for the switch by using gtk_widget_destroy() only on toplevel windows, and replace all other uses with gtk_container_remove() or g_object_unref().

GTK 4 removes gtk_container_add() and gtk_container_remove(). While there is not always a replacement for gtk_container_remove() in GTK 3, you can replace many uses of gtk_container_add() with equivalent container-specific APIs such as gtk_box_pack_start() or gtk_grid_attach(), and thereby reduce the amount of work you have to do at the time of the switch.

When using icons as resources, the behavior of GTK 4 is different in one respect: Icons that are directly in $APP_ID/icons/ are treated as unthemed icons, which also means that symbolic icons are not recolored. If you want your icon resources to have icon theme semantics, they need to be placed into theme subdirectories such as $APP_ID/icons/16x16/actions or $APP_ID/icons/scalable/status.

This location works fine in GTK 3 too, so you can prepare for this change before switching to GTK 4.