Applications and libraries often contain binary or textual data that is really part of the application, rather than user data. For instance GtkBuilder .ui files, splashscreen images, Gio.Menu markup XML, CSS files, icons, etc. These are often shipped as files in $datadir/appname, or manually included as literal strings in the code.

The Gio.Resource API and the glib-compile-resources program provide a convenient and efficient alternative to this which has some nice properties. You maintain the files as normal files, so it’s easy to edit them, but during the build the files are combined into a binary bundle that is linked into the executable. This means that loading the resource files are efficient (as they are already in memory, shared with other instances) and simple (no need to check for things like I/O errors or locate the files in the filesystem). It also makes it easier to create relocatable applications.

Resource files can also be marked as compressed. Such files will be included in the resource bundle in a compressed form, but will be automatically uncompressed when the resource is used. This is very useful e.g. for larger text files that are parsed once (or rarely) and then thrown away.

Resource files can also be marked to be preprocessed, by setting the value of the preprocess attribute to a comma-separated list of preprocessing options. The only options currently supported are:

• xml-stripblanks which will use the xmllint command to strip ignorable whitespace from the XML file. For this to work, the XMLLINT environment variable must be set to the full path to the xmllint executable, or xmllint must be in the PATH; otherwise the preprocessing step is skipped.

• to-pixdata (deprecated since gdk-pixbuf 2.32) which will use the gdk-pixbuf-pixdata command to convert images to the GdkPixdata format, which allows you to create pixbufs directly using the data inside the resource file, rather than an (uncompressed) copy of it. For this, the gdk-pixbuf-pixdata program must be in the PATH, or the GDK_PIXBUF_PIXDATA environment variable must be set to the full path to the gdk-pixbuf-pixdata executable; otherwise the resource compiler will abort. to-pixdata has been deprecated since gdk-pixbuf 2.32, as Gio.Resource supports embedding modern image formats just as well. Instead of using it, embed a PNG or SVG file in your Gio.Resource.

• json-stripblanks which will use the json-glib-format command to strip ignorable whitespace from the JSON file. For this to work, the JSON_GLIB_FORMAT environment variable must be set to the full path to the json-glib-format executable, or it must be in the PATH; otherwise the preprocessing step is skipped. In addition, at least version 1.6 of json-glib-format is required.

Resource files will be exported in the Gio.Resource namespace using the combination of the given prefix and the filename from the file element. The alias attribute can be used to alter the filename to expose them at a different location in the resource namespace. Typically, this is used to include files from a different source directory without exposing the source directory in the resource namespace, as in the example below.

Resource bundles are created by the glib-compile-resources program which takes an XML file that describes the bundle, and a set of files that the XML references. These are combined into a binary resource bundle.

An example resource description:

<?xml version="1.0" encoding="UTF-8"?>
<gresources>
  <gresource prefix="/org/gtk/Example">
    <file>data/splashscreen.png</file>
    <file compressed="true">dialog.ui</file>
    <file preprocess="xml-stripblanks">menumarkup.xml</file>
    <file alias="example.css">data/example.css</file>
  </gresource>
</gresources>
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This will create a resource bundle with the following files:

/org/gtk/Example/data/splashscreen.png
/org/gtk/Example/dialog.ui
/org/gtk/Example/menumarkup.xml
/org/gtk/Example/example.css
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Note that all resources in the process share the same namespace, so use Java-style path prefixes (like in the above example) to avoid conflicts.

You can then use glib-compile-resources to compile the XML to a binary bundle that you can load with Gio.Resource.load. However, it’s more common to use the --generate-source and --generate-header arguments to create a source file and header to link directly into your application. This will generate get_resource(), register_resource() and unregister_resource() functions, prefixed by the --c-name argument passed to glib-compile-resources. get_resource() returns the generated Gio.Resource object. The register and unregister functions register the resource so its files can be accessed using Gio.resources_lookup_data.

Once a Gio.Resource has been created and registered all the data in it can be accessed globally in the process by using API calls like Gio.resources_open_stream to stream the data or Gio.resources_lookup_data to get a direct pointer to the data. You can also use URIs like resource:///org/gtk/Example/data/splashscreen.png with Gio.File to access the resource data.

Some higher-level APIs, such as GtkApplication, will automatically load resources from certain well-known paths in the resource namespace as a convenience. See the documentation for those APIs for details.

There are two forms of the generated source, the default version uses the compiler support for constructor and destructor functions (where available) to automatically create and register the Gio.Resource on startup or library load time. If you pass --manual-register, two functions to register/unregister the resource are created instead. This requires an explicit initialization call in your application/library, but it works on all platforms, even on the minor ones where constructors are not supported. (Constructor support is available for at least Win32, Mac OS and Linux.)

Note that resource data can point directly into the data segment of e.g. a library, so if you are unloading libraries during runtime you need to be very careful with keeping around pointers to data from a resource, as this goes away when the library is unloaded. However, in practice this is not generally a problem, since most resource accesses are for your own resources, and resource data is often used once, during parsing, and then released.

Overlays

When debugging a program or testing a change to an installed version, it is often useful to be able to replace resources in the program or library, without recompiling, for debugging or quick hacking and testing purposes. Since GLib 2.50, it is possible to use the G_RESOURCE_OVERLAYS environment variable to selectively overlay resources with replacements from the filesystem. It is a G_SEARCHPATH_SEPARATOR-separated list of substitutions to perform during resource lookups. It is ignored when running in a setuid process.

A substitution has the form

/org/gtk/libgtk=/home/desrt/gtk-overlay
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The part before the = is the resource subpath for which the overlay applies. The part after is a filesystem path which contains files and subdirectories as you would like to be loaded as resources with the equivalent names.

In the example above, if an application tried to load a resource with the resource path /org/gtk/libgtk/ui/gtkdialog.ui then Gio.Resource would check the filesystem path /home/desrt/gtk-overlay/ui/gtkdialog.ui. If a file was found there, it would be used instead. This is an overlay, not an outright replacement, which means that if a file is not found at that path, the built-in version will be used instead. Whiteouts are not currently supported.

Substitutions must start with a slash, and must not contain a trailing slash before the =. The path after the slash should ideally be absolute, but this is not strictly required. It is possible to overlay the location of a single resource with an individual file.