Environment variables

GTK inspects a number of environment variables in addition to standard variables like LANG, PATH, HOME or DISPLAY; mostly to determine paths to look for certain files. The X11, Wayland, Windows and Broadway GDK backends use some additional environment variables.

GTK_DEBUG

This variable can be set to a list of debug options, which cause GTK to print out different types of debugging information. Some of these options are only available when GTK has been configured with -Ddebug=true.

actions
Actions and menu models
builder
GtkBuilder support
geometry
Size allocation
icontheme
Icon themes
keybindings
Keybindings
modules
Loading of modules
printing
Printing support
size-request
Size requests
text
Text widget internals
tree
Tree widget internals

A number of keys are influencing behavior instead of just logging:

interactive
Open the interactive debugger
no-css-cache
Bypass caching for CSS style properties
touchscreen
Pretend the pointer is a touchscreen device
updates
Visual feedback about window updates
resize
Highlight resizing widgets
layout
Show layout borders
snapshot
Include debug render nodes in the generated snapshots

The special value all can be used to turn on all debug options. The special value help can be used to obtain a list of all supported debug options.

GTK_PATH

Specifies a list of directories to search when GTK is looking for dynamically loaded objects such as input method modules and print backends. If the path to the dynamically loaded object is given as an absolute path name, then GTK loads it directly. Otherwise, GTK goes in turn through the directories in GTK_PATH, followed by the directory .gtk-4.0 in the user’s home directory, followed by the system default directory, which is libdir/gtk-4.0/modules. (If GTK_EXE_PREFIX is defined, libdir is $GTK_EXE_PREFIX/lib. Otherwise it is the libdir specified when GTK was configured, usually /usr/lib, or /usr/local/lib.)

For each directory in this list, GTK actually looks in a subdirectory directory/version/host/type. Where version is derived from the version of GTK (use pkg-config --variable=gtk_binary_version gtk4 to determine this from a script), host is the architecture on which GTK was built. (use pkg-config --variable=gtk_host gtk4 to determine this from a script), and type is a directory specific to the type of modules; currently it can be modules, immodules or printbackends, corresponding to the types of modules mentioned above. Either version, host, or both may be omitted. GTK looks first in the most specific directory, then in directories with fewer components. The components of GTK_PATH are separated by the ‘:’ character on Linux and Unix, and the ‘;’ character on Windows.

Note that this environment variable is read by GTK 2.x and GTK 3.x too, which makes it unsuitable for setting it system-wide (or session-wide), since doing so will cause applications using different GTK versions to see incompatible modules.

GTK_IM_MODULE

Specifies an IM module to use in preference to the one determined from the locale. If this isn’t set and you are running on the system that enables XSETTINGS and has a value in Gtk/IMModule, that will be used for the default IM module. This also can be a colon-separated list of input-methods, which GTK will try in turn until it finds one available on the system.

GTK_MEDIA

Specifies what backend to load for GtkMediaFile. The possible values depend on what options GTK was built with, and can include ‘gstreamer’, ‘ffmpeg’ and ‘none’. If set to ‘none’, media playback will be unavailable. The special value ‘help’ can be used to obtain a list of all supported media backends.

GTK_EXE_PREFIX

If set, GTK uses $GTK_EXE_PREFIX/lib instead of the libdir configured when GTK was compiled.

GTK_DATA_PREFIX

If set, GTK uses $GTK_DATA_PREFIX instead of the prefix configured when GTK was compiled.

GTK_THEME

If set, makes GTK use the named theme instead of the theme that is specified by the gtk-theme-name setting. This is intended mainly for easy debugging of theme issues.

It is also possible to specify a theme variant to load, by appending the variant name with a colon, like this: GTK_THEME=Adwaita:dark.

The following environment variables are used by GdkPixbuf, GDK or Pango, not by GTK itself, but we list them here for completeness nevertheless.

GDK_PIXBUF_MODULE_FILE

Specifies the file listing the GdkPixbuf loader modules to load. This environment variable overrides the default value libdir/gtk-4.0/4.0.0/loaders.cache (libdir is the sysconfdir specified when GTK was configured, usually /usr/lib.)

The loaders.cache file is generated by the gdk-pixbuf-query-loaders utility.

GDK_DEBUG

This variable can be set to a list of debug options, which cause GDK to print out different types of debugging information. Some of these options are only available when GTK has been configured with -Ddebug=true.

cursor
Information about cursor objects (only win32)
eventloop
Information about event loop operation (mostly macOS)
misc
Miscellaneous information
frames
Information about the frame clock
settings
Information about xsettings
selection
Information about selections
clipboard
Information about clipboards
dnd
Information about drag-and-drop
opengl
Information about OpenGL
vulkan
Information about Vulkan

A number of options affect behavior instead of logging:

nograbs
Turn off all pointer and keyboard grabs
gl-disable
Disable OpenGL support
gl-software
Force OpenGL software rendering
gl-texture-rect
Use the OpenGL texture rectangle extension, if available
gl-legacy
Use a legacy OpenGL context
gl-gles
Use a GLES OpenGL context
vulkan-disable
Disable Vulkan support
vulkan-validate
Load the Vulkan validation layer, if available

The special value all can be used to turn on all debug options. The special value help can be used to obtain a list of all supported debug options.

GSK_DEBUG

This variable can be set to a list of debug options, which cause GSK to print out different types of debugging information. Some of these options are only available when GTK has been configured with -Ddebug=true.

renderer
General renderer information
cairo
cairo renderer information
opengl
OpenGL renderer information
shaders
Shaders
surface
Surfaces
vulkan
Vulkan renderer information
fallback
Information about fallbacks
glyphcache
Information about glyph caching

A number of options affect behavior instead of logging:

diff
Show differences
geometry
Show borders
full-redraw
Force full redraws for every frame
sync
Sync after each frame
vulkan-staging-image
Use a staging image for Vulkan texture upload
vulkan-staging-buffer
Use a staging buffer for Vulkan texture upload

The special value all can be used to turn on all debug options. The special value help can be used to obtain a list of all supported debug options.

GDK_BACKEND

If set, selects the GDK backend to use. Selecting a backend requires that GTK is compiled with support for that backend. The following backends can be selected, provided they are included in the GDK libraries you are using:

quartz
Selects the native Quartz backend
win32
Selects the native backend for Microsoft Windows
x11
Selects the native backend for connecting to X11 servers
broadway
Selects the Broadway backend for display in web browsers
wayland
Selects the Wayland backend for connecting to Wayland compositors

This environment variable can contain a comma-separated list of backend names, which are tried in order. The list may also contain a *, which means: try all remaining backends. The special value help can be used to make GDK print out a list of all available backends. For more information about selecting backends, see the gdk_display_manager_get() function.

GDK_VULKAN_DEVICE

This variable can be set to the index of a Vulkan device to override the default selection of the device that is used for Vulkan rendering. The special value list can be used to obtain a list of all Vulkan devices.

GSK_RENDERER

If set, selects the GSK renderer to use. The following renderers can be selected, provided they are included in the GTK library you are using and the GDK backend supports them:

help
Prints information about available options
broadway
Selects the Broadway-backend specific renderer
cairo
Selects the fallback Cairo renderer
gl
Selects the default OpenGL renderer
vulkan
Selects the Vulkan renderer

GTK_CSD

The default value of this environment variable is 1. If changed to 0, this disables the default use of client-side decorations on GTK windows, thus making the window manager responsible for drawing the decorations of windows that do not have a custom titlebar widget.

CSD is always used for windows with a custom titlebar widget set, as the WM should not draw another titlebar or other decorations around the custom one.

GTK_A11Y

If set, selects the accessibility backend to use. The following backends can be selected, provided they are included in the GTK library you are using:

help
Prints information about available options
atspi
Selects the AT-SPI accessibility backend
test
Selects the test backend
none
Disables the accessibility backend

The test accessibility backend is recommended for test suites and remote continuous integration pipelines.

XDG_DTA_HOME, XDG_DATA_DIRS

GTK uses these environment variables to locate icon themes and MIME information. For more information, see the Icon Theme Specification the Shared MIME-Info Database and the Base Directory Specification.

DESKTOP_STARTUP_ID

GTK uses this environment variable to provide startup notification according to the Startup Notification Spec. Following the specification, GTK unsets this variable after reading it (to keep it from leaking to child processes). So, if you need its value for your own purposes, you have to read it before calling gtk_init().

Interactive debugging

The inspector

GTK includes an interactive debugger, called the GTK Inspector, which lets you explore the widget tree of any GTK application at runtime, as well as tweak the theme and trigger visual debugging aids. You can easily try out changes at runtime before putting them into the code.

Note that the GTK inspector can only show GTK internals. It can not understand the application-specific logic of a GTK application. Also, the fact that the GTK inspector is running in the application process limits what it can do. It is meant as a complement to full-blown debuggers and system tracing facilities such as DTrace, not as a replacement.

To enable the GTK inspector, you can use the Control+Shift+I or Control+Shift+D keyboard shortcuts, or set the GTK_DEBUG=interactive environment variable.

There are a few more environment variables that can be set to influence how the inspector renders its UI. GTK_INSPECTOR_DISPLAY and GTK_INSPECTOR_RENDERER determine the GDK display and the GSK renderer that the inspector is using.

In some situations, it may be inappropriate to give users access to the GTK inspector. The keyboard shortcuts can be disabled with the enable-inspector-keybinding key in the org.gtk.Settings.Debug GSettings schema.

Profiling

GTK supports profiling with sysprof. It exports timing information about frameclock phases and various characteristics of GskRenderers in a format that can be displayed by sysprof or GNOME Builder.

A simple way to capture data is to run your application under the sysprof-cli wrapper, which will write profiling data to a file called capture.syscap.

When launching the application from sysprof, it will set the SYSPROF_TRACE_FD environment variable to point GTK at a file descriptor to write profiling data to.