Gtk – 4.0: Running and debugging GTK Applications
Description: Reference for Gtk-4.0: Running and debugging GTK Applications

## 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.

Note that environment variables are generally used for debugging purposes. They are not guaranteed to be API stable, and should not be used for end-user configuration and customization.

### `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
`builder-objects`
  Unused GtkBuilder objects
`geometry`
  Size allocation
`icontheme`
  Icon themes
`iconfallback`
  Information about icon fallback
`keybindings`
  Keyboard shortcuts
`modules`
  Modules and extensions
`printing`
  Printing
`size-request`
  Size requests
`text`
  Text widget internals
`tree`
  Tree widget internals
`constraints`
  Constraints and the constraint solver
`layout`
  Layout managers
`acccessibility`
  Accessibility state changs

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
`snapshot`
  Include debug render nodes in the generated snapshots
`invert-text-dir`
  Invert the text direction, compared to the locale

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 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
`portals`
  Force the use of portals
`no-portals`
  Disable use of portals
`gl-disable`
  Disable OpenGL support
`gl-fractional`
  Enable fractional scaling for OpenGL. This is experimental
`gl-legacy`
  Use a legacy OpenGL context
`gl-gles`
  Use a GLES OpenGL context
`gl-egl`
  Use an EGL context on X11 or Windows
`gl-glx`
  Use GLX on X11
`gl-wgl`
  Use WGL on Windows
`vulkan-disable`
  Disable Vulkan support
`vulkan-validate`
  Load the Vulkan validation layer, if available
`default-settings`
  Force default values for xsettings
`high-depth`
  Use high bit depth rendering if possible

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:

`macos`
  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
`opengl`
  Selects the default OpenGL renderer
`gl`
  Selects the “gl” OpenGL renderer
`vulkan`
  Selects the Vulkan renderer

Note that on Windows, if one is running Nahimic 3 on a system with nVidia graphics, one needs to stop the “Nahimic service” or insert the GTK application into the Nahimic blacklist, as noted in https://www.nvidia.com/en-us/geforce/forums/game-ready-drivers/13/297952/nahimic-and-nvidia-drivers-conflict/2334568/, or use the cairo renderer (at the cost of being unable to use OpenGL features), or use GDK_DEBUG=gl-gles if you know that GLES support is enabled for the build.

This is a known issue, as the above link indicates, and affects quite a number of applications—sadly, since this issue lies within the nVidia graphics driver and/or the Nahimic 3 code, we are not able to rememdy this on the GTK side; the best bet before trying the above workarounds is to try to update your graphics drivers and Nahimic installation.

### `GSK_MAX_TEXTURE_SIZE`

Limit texture size to the minimum of this value and the OpenGL limit for texture sizes. This can be used to debug issues with texture slicing on systems where the OpenGL texture size limit would otherwise make texture slicing difficult to test.

### `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_DATA_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

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.