Applications and libraries often contain binary or textual data that is really part of the application, rather than user data. For instance Gtk.Builder.Gtk_Builder .ui files, splashscreen images, GMenu 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 Glib.Resource.Gresource API and the [glib-compile-resources][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 its 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 Gdk_Pixdata 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 Glib.Resource.Gresource supports embedding modern image formats just as well. Instead of using it, embed a PNG or SVG file in your Glib.Resource.Gresource.
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 GResource 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][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>
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
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][glib-compile-resources] to compile the XML to a binary bundle that you can load with Glib.Resource.Load. However, its 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][glib-compile-resources]. get_resource returns the generated Glib.Resource.Gresource object. The register and unregister functions register the resource so its files can be accessed using g_resources_lookup_data.
Once a Glib.Resource.Gresource has been created and registered all the data in it can be accessed globally in the process by using API calls like g_resources_open_stream to stream the data or g_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 Glib.GFile.Gfile to access the resource data.
Some higher-level APIs, such as Gtk.Application.Gtk_Application, 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 Glib.Resource.Gresource 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.
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
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 GResource 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.
function Enumerate_Children
(Self : Gresource;
Path : UTF8_String;
Lookup_Flags : Resource_Lookup_Flags;
Error : out Glib.Error.GError) return GNAT.Strings.String_List
Returns all the names of children at the specified Path in the resource. The return result is a null terminated list of strings which should be released with g_strfreev. If Path is invalid or does not exist in the Glib.Resource.Gresource, Glib.Error_Enums.G_Resource_Error_Not_Found will be returned. Lookup_Flags controls the behaviour of the lookup. Since: gtk+ 2.32
A pathname inside the resource
A Glib.Resource.Resource_Lookup_Flags
the return location for a recoverable error
an array of constant strings
function From_Object (Object : System.Address) return Gresource
function From_Object_Free (B : access Gresource'Class) return Gresource
procedure G_New_From_Data
(Self : out Gresource;
Data : Glib.Bytes.Gbytes;
Error : out Glib.Error.GError)
Creates a GResource from a reference to the binary resource bundle. This will keep a reference to Data while the resource lives, so the data should not be modified or freed. If you want to use this resource in the global resource namespace you need to register it with Glib.Resource.Register. Note: Data must be backed by memory that is at least pointer aligned. Otherwise this function will internally create a copy of the memory since GLib 2.56, or in older versions fail and exit the process. If Data is empty or corrupt, Glib.Error_Enums.G_Resource_Error_Internal will be returned. Since: gtk+ 2.32
A Glib.Bytes.Gbytes
the return location for a recoverable error
G_Resource_Lookup_Flags_None : constant Resource_Lookup_Flags := 0;
function Get_Info
(Self : Gresource;
Path : UTF8_String;
Lookup_Flags : Resource_Lookup_Flags;
Size : access Gsize := null;
Flags : access Guint32 := null;
Error : out Glib.Error.GError) return Boolean
Looks for a file at the specified Path in the resource and if found returns information about it. Lookup_Flags controls the behaviour of the lookup. Since: gtk+ 2.32
A pathname inside the resource
A Glib.Resource.Resource_Lookup_Flags
a location to place the length of the contents of the file, or null if the length is not needed
a location to place the flags about the file, or null if the length is not needed
the return location for a recoverable error
True if the file was found. False if there were errors
function Get_Type return Glib.GType
type Gresource is new Glib.C_Boxed with null record;
function Gresource_New_From_Data
(Data : Glib.Bytes.Gbytes;
Error : out Glib.Error.GError) return Gresource
Creates a GResource from a reference to the binary resource bundle. This will keep a reference to Data while the resource lives, so the data should not be modified or freed. If you want to use this resource in the global resource namespace you need to register it with Glib.Resource.Register. Note: Data must be backed by memory that is at least pointer aligned. Otherwise this function will internally create a copy of the memory since GLib 2.56, or in older versions fail and exit the process. If Data is empty or corrupt, Glib.Error_Enums.G_Resource_Error_Internal will be returned. Since: gtk+ 2.32
A Glib.Bytes.Gbytes
the return location for a recoverable error
function Has_Children
(Self : Gresource;
Path : UTF8_String) return Boolean
function Load
(Filename : UTF8_String;
Error : out Glib.Error.GError) return Gresource
Loads a binary resource bundle and creates a Glib.Resource.Gresource representation of it, allowing you to query it for data. If you want to use this resource in the global resource namespace you need to register it with Glib.Resource.Register. If Filename is empty or the data in it is corrupt, Glib.Error_Enums.G_Resource_Error_Internal will be returned. If Filename doesn't exist, or there is an error in reading it, an error from g_mapped_file_new will be returned. Since: gtk+ 2.32
the path of a filename to load, in the GLib filename encoding
the return location for a recoverable error
a new Glib.Resource.Gresource, or null on error. Has transfer-ownership='full'.
function Lookup_Data
(Self : Gresource;
Path : UTF8_String;
Lookup_Flags : Resource_Lookup_Flags;
Error : out Glib.Error.GError) return Glib.Bytes.Gbytes
Looks for a file at the specified Path in the resource and returns a Glib.Bytes.Gbytes that lets you directly access the data in memory. The data is always followed by a zero byte, so you can safely use the data as a C string. However, that byte is not included in the size of the GBytes. For uncompressed resource files this is a pointer directly into the resource bundle, which is typically in some readonly data section in the program binary. For compressed files we allocate memory on the heap and automatically uncompress the data. Lookup_Flags controls the behaviour of the lookup. Since: gtk+ 2.32
A pathname inside the resource
A Glib.Resource.Resource_Lookup_Flags
the return location for a recoverable error
Glib.Bytes.Gbytes or null on error. Free the returned object with Glib.Bytes.Unref. Has transfer-ownership='full'.
Null_Gresource : constant Gresource;
function Open_Stream
(Self : Gresource;
Path : UTF8_String;
Lookup_Flags : Resource_Lookup_Flags;
Error : out Glib.Error.GError)
return Glib.Input_Stream.Ginput_Stream
Looks for a file at the specified Path in the resource and returns a Glib.Input_Stream.Ginput_Stream that lets you read the data. Lookup_Flags controls the behaviour of the lookup. Since: gtk+ 2.32
A pathname inside the resource
A Glib.Resource.Resource_Lookup_Flags
the return location for a recoverable error
Glib.Input_Stream.Ginput_Stream or null on error. Free the returned object with g_object_unref. Has transfer-ownership='full'.
type Property_Resource_Lookup_Flags is new Resource_Lookup_Flags_Properties.Property;
function Ref (Self : Gresource) return Gresource
Atomically increments the reference count of Resource by one. This function is MT-safe and may be called from any thread. Since: gtk+ 2.32
The passed in Glib.Resource.Gresource. Has transfer-ownership='full'.
procedure Register (Self : Gresource)
Registers the resource with the process-global set of resources. Once a resource is registered the files in it can be accessed with the global resource lookup functions like g_resources_lookup_data. Since: gtk+ 2.32
type Resource_Lookup_Flags is mod 2 ** Integer'Size;
GResourceLookupFlags determine how resource path lookups are handled.
package Resource_Lookup_Flags_Properties is
new Generic_Internal_Flags_Property (Resource_Lookup_Flags);
procedure Unref (Self : Gresource)
Atomically decrements the reference count of Resource by one. If the reference count drops to 0, all memory allocated by the resource is released. This function is MT-safe and may be called from any thread. Since: gtk+ 2.32
procedure Unregister (Self : Gresource)
Unregisters the resource from the process-global set of resources. Since: gtk+ 2.32