The modern Gtk drawing model

November 4th, 2013 by alexl

Over a few releases now, and culminating in Gtk+ 3.10 we have restructured the way drawing works internally. This hasn’t really been written about a lot, and I still see a lot of people unaware of these changes, so I thought I’d do a writeup of the brave new world.

All Gtk+ applications are mainloop driven, which means that most of the time the app is idle inside a loop that just waits for something to happen and then calls out to the right place when it does. On top of this Gtk+ has a frame clock that gives a “pulse” to the application. This clock beats at a steady rate, which is tied to the framerate of the output (this is synced to the monitor via the window manager/compositor). The clock has several phases:

  • Events
  • Update
  • Layout
  • Paint

The phases happens in this order and we will always run each phase through before going back to the start. The Input events phase is a long stretch of time between each redraw where we get input events from the user and other events (like e.g. network i/o). Some events, like mouse motion are compressed so that we only get a single mouse motion event per clock cycle[1].

Once the event phase is over we pause all external events and run the redraw loop. First is the update phase, where all animations are run to calculate the new state based on the estimated time the next frame will be visible (available via the frame clock). This often involves geometry changes which drives the next phase, Layout. If there are any changes in widget size requirements we calculate a new layout for the widget hierarchy (i.e. we assign sizes and positions). Then we go to the Paint phase where we redraw the regions of the window that needs redrawing.

If nothing requires the update/layout/paint phases we will stay in the event phase forever, as we don’t want to redraw if nothing changes. Each phase can request further processing in the following phases (e.g. the update phase will cause there to be layout work, and layout changes cause repaints).

There are multiple ways to drive the clock, at the lowest level you can request a particular phase by gdk_frame_clock_request_phase() which will schedule a clock beat as needed so that it eventually reaches the requested phase. However in practice most things happen at higher levels:

  • If you are doing an animation, you can use gtk_widget_add_tick_callback() which will cause a regular beating of the clock with a callback in the update phase until you stop the tick.
  • If some state changes that causes the size of your widget to change you call gtk_widget_queue_resize() which will request a layout phase and mark your widget as needing relayout.
  • If some state changes so you need to redraw some area of your widget you use the normal gtk_widget_queue_draw() set of functions. These will request a paint phase and mark the region as needing redraw.

There are also a lot of implicit triggers of these from the CSS layer (which does animations, resizes and repaints as needed).

During the Paint phase we will send a single expose event to the toplevel window[2]. The event handler will create a cairo context for the window and emit a GtkWidget::draw() signal on it, which will propagate down the entire widget hierarchy in back-to-front order, using the clipping and transform of the cairo context. This lets each widget draw its content at the right place and time, correctly handling things like partial transparencies and overlapping widgets.

There are some notable differences here compared to how it used to work:

  • Normally there is only a single cairo context which is used in the entire repaint, rather than one per GdkWindow. This means you have to respect (and not reset) existing clip and transformations set on it.
  • Most widgets, including those that create their own GdkWindows have a transparent background, so they draw on top of whatever widgets are below them. This was not the case before where the theme set the background of most widgets to the default background color. (In fact, transparent GdkWindows used to be impossible.)
  • The whole rendering hierarchy is captured in the call stack, rather than having multiple separate draw emissions as before, so you can use effects like e.g.  cairo_push/pop_group() which will affect all the widgets below you in the hierarchy. This lets you have e.g. partially transparent containers.
  • Due to backgrounds being transparent we no longer use a self-copy operation when GdkWindows move or scroll. We just mark the entire affected area for repainting when these operations are used. This allows (partially) transparent backgrounds, and it also more closely models modern hardware where self-copy operations are problematic (they break the rendering pipeline).
  • Due to the above, old scrolling code is slower, so we do scrolling differently. Containers that scroll a lot (GtkViewport, GtkTextView, GtkTreeView, etc) allocate an offscreen image during scrolling and render their children to it (which is now possible since draw is fully hierarchical). The offscreen image is a bit larger than the visible area, so most of the time when scrolling it just needs to draw the offscreen in a different position.  This matches contemporary graphics hardware much better, as well as allowing efficient transparent backgrounds.
    In order for this to work such containers need to detect when child widgets are redrawn so that it can update the offscreen. This can be done with the new gdk_window_set_invalidate_handler() function.

This skips some of the details, but with this overview you should have a better idea that happens when your code is called, and be in a better position to do further research if necessary.

[1] If you need more mouse event precision you need to look at the mouse device history.

[2] Actually each native subwindow will get one too, but that is not very common these days.

Adventures in Docker land

October 15th, 2013 by alexl

Connoisseurs of this blog know that I have an interest in application deployment systems, having created three different application bunding system (1,2, 3). These were all experiments in the area of desktop applications, but recently there has been some interesting motions in related areas, namely Docker.

Docker is a server application/container deployment system, which nicely sidesteps a lot of the complexity with desktop apps (not having to integrate deeply with the desktop) which makes it a lot easier to deploy. Additionally, docker is more than a deployment system, it also has some interesting ideas about how to create and distribute applications.

Every docker container is a copy-on-write clone of a specific parent Image, which means instantiation of docker containers is very fast and cheap. It also gives some very interesting properties because you can track the changes in a container (compared to t its parent image) and “commit” this to a new image. This creates a git-like hierarchy of images where every commit is a filesystem layer that applies to a previous layer, up to some base image. The git-like workflow is really nice to work with when creating images, and the final result is very easy to share and deploy, and at the same time automatically shares as much as possible with common base images.

Unfortunately Docker relies on AUFS, a union filesystem that is not in the upstream kernel, nor is it likely to ever be there. Also, while AUFS is in the current Ubuntu kernel it is deprecated there and will eventually be removed. This means Docker doesn’t run on Fedora which has a primarily-upstream approach to packaging.

So, the last month or so I’ve been working on making Docker work in Fedora (and thus eventually in RHEL, which is the nr 1 requested Docker feature). Of course, this work will benefit other distributions that don’t have AUFS too.

I started looking at possible replacements for the copy-on-write support, and there are a few possibilities availible:

  • overlayfs
  • btrfs
  • lvm snapshots
  • lvm thin provisioning

Overlayfs is a different union filesystem implementation than AUFS, and the one that seems most likely to land upstream. But that is happening slowly, if at all. Long-term I think this is the best option, but right now it is out of the question.

Btrfs has copy-on-write both using filesystem snapshots and one a file basis using reflink. However, btrfs is not currently used much in production as its not considered stable enough. It would also be a very heavy dependency for Docker, as may users would have to reformat their disks to use it.

Lvm snapshots are useful for doing e.g. backup of a snapshot, but regress badly in performance when you start having many snapshots of the same device.

This leaves us with only lvm thin provisioning. This is a fairly recent, but relatively stable technology that allows you to create copy-on-write block devices that are “thinly” provisioned, meaning they don’t use real space until the device is in use. This is not ideal for Docker as it really wants copy-on-write at the file level, but with some work it is possible to work around this.

Rather than interacting with lvm which is a very generic volume manager I chose to use the lower level device-mapper kernel APIs directly (via libdevmapper). This allows us greater ease of access to the devices programmatically, as well as avoiding confusion with possible system use of LVM. Also it avoids some LVM performance issues with very many devices.

So, we set up a single large block device on which we create a device-mapper “thinp” pool. On this we then creates a single “base” block device formated with ext4. Every image and container are then created as snapshot (in multiple steps) from this base device. So, say you’re starting a container based on an image “apache” which itself is  based on a “fedora” image, we would:

  1. Create a snapshot of the base device.
  2. Mount it and apply the changes in the fedora image.
  3. Create a snapshot based on the fedora device.
  4. Mount it and apply the changes in the apache image.
  5. Create a snapshot based on the apache device.
  6. Mount it and use as the root in the new container.

And of course, these devices will be reused (with corresponding steps skipped) as needed by other images/containers.

The devicemapper pool need to be set up on a large block device that fits all the images and containers that you will be used which would be painful for most people. Docker handles this by automatically creating the a large sparse file, using it as a loopback device for the devicemapper work. Additionally we ensure that DISCARD support is enabled in the filesystem so that any files removed in the conttainer filters down to the loopback file making it sparse again.

This means that there is no need for setup, and space for images and containers will only be used as needed. Of course, there are still issues, like the max size of the loopback mount (100G by default, but this should be easy to grow) and the max size of the base extt4 image (10G by default, resizing is harder after initial construction, but should be possible).

We’re currently in the process of landing this in Docker, and hope to have a 0.7 release out based on my device-mapper work pretty soon. Then I will continue working on making docker a first-class citizen on Fedora.

HiDPI support in Gnome

June 28th, 2013 by alexl

For some time I’ve been working on HiDPI support for Gnome, in order to support all the new laptops with very high resolution displays. This work is now at a stage where I can start showing it off and adventurous users might even want to test it out.

The support happens on many layers, like:

  • Wayland: I’ve added support in the protocol for scaled windows and outputs, and implemented this in Weston (the Wayland compositor reference implementation) and the wayland sample clients. This is currently in the unstable branches and will be in Wayland 1.2.
  • Cairo: In order to seamlessly support hidpi rendering I’ve added support for setting device-scale on cairo surfaces. This code is a prerequisite for the Gtk+ support, and is currently in a cairo branch, but will land in Cairo 1.13 eventually.
  • Gnome-icon-theme: Added high resolution version of most images used in the theme.

But most of the work has been in Gtk+. There is a branch called wip/window-scales that contains this code. It has:

  • Support for scaled windows in the wayland backend, including support for different scale factors on different outputs.
  • Support for scaled windows on X, limited to one scaling factor for all monitors.
  • Support for retina displays on OSX
  • Support for alternative CSS assets for scaled windows, so that you can specify higher resolution background and border images in your themes that will be used on hidpi screens.
  • Support for alternative high-resolution icons for scaled windows, including additions to the icon theme spec so that themes can specify higher resolution images with less details (i.e. 24×24@2x is the same size as 48×48@1x, but has less detail).

Here is how it looks right now: (click for full version)

This is work in progress, but if you want to try it out, here is the code you need to use:

You can then enable the scaling either by setting GDK_SCALE=2 in the environment, or by using gsettings (only works under gnome):

gsettings set org.gnome.settings-daemon.plugins.xsettings overrides 
   "{ 'Gdk/WindowScalingFactor':<2>, 'Gdk/UnscaledDPI':<92160> }"

There is still a lot of work to do, but I hope to have this mostly done for my talk at Guadec. Hopefully I’ll see you there!

And last, but not least, many thanks to Brion Vibber who generously donated a Chromebook Pixel to me so that I could work on this stuff.

More Gtk+ in the cloud

April 3rd, 2013 by alexl

Since my last blog entry I’ve worked a bit more on broadway, and the openshift-broadway cartridge for openshift.

New in this version:

  • Bumped all modules to the final gnome 3.8 releases
  • Chrome support (this used to work, but had regressed)
  • Password protection for the broadway session (optional)
  • Added a simple app launcher to the default session
  • Added gedit, gnome-terminal and glade to build
  • Made it work with https (using a terminating ssl proxy in openshift)
  • General bugfixes and robustness

Here is a video of this:

YouTube Preview Image

Broadway on openshift

March 19th, 2013 by alexl

I spent some time this week making a clean build of Gtk 3 with the broadway backend for deploying on openshift. It works suprisingly well, given that I’m in sweden and the openshift servers are somewhere in the US:

YouTube Preview Image

I made the video instead of linking to the app itself because only one person at a time can use the actual app. However, you can easily create your own openshift module based on this as I made the openshift code availible at gitthub. Just clone that and push to your diy based openshift app. Play with it! Build your favourite gtk3 app! Share it!

It lacks authentication/encryption and there are some minor issues, but it proves that broadway works reasonably well even over the internet.

Developer Hackfest status

February 1st, 2013 by alexl

Today is the third day of the Gnome Developer Experience hackfest. The first day I was in the platform group where we looked at the core gtk+ platform and whats missing from it. We ended up with a pretty large list of items, but we picked a few of them and started on a few of them. More details in cosimocs blog post.

Yesterday we finally had all the people interested in the application deployment and sandboxing story here, so we started looking at that. I’ve historically had a lot of interest in this area with previous experiments like glick, glick2, and bundler, so this is my main interest this hackfest. We have some initial plans for how to approach this.

There are two fundamental, but interrelated problems in this area. One is the deployment of the application (how to create an application, bundle it, “install” it, set up its execution environment and start it) and the second is sandboxing (protecting the user session against the app).

Deployment

For deplyoment we’re considering a bundling model where the app ships the binary and some subset of the libraries it needs, plus a manifest that describes what the kind of system ABIs that the app requires. These are very course grained, unlike the traditional per-library dependencies, and would be on the level of i.e.

  • bare: Just the kernel ABI
  • system: libc, libm, and a few core libs
  • gnome-platform-1.0: The full gnome upstream defined set of “stable supported ABIs”

There is a scale here where apps (like games) can chose to bundle more dependencies, and expect more stability over time, but it will not be very integrated, while on the other hand more integrated desktop apps which will be tied to specific versions of some desktop platform.

What is important here is that once an app specifies a particular profile we guarantee that this is all that the app ever sees, i.e. we support isolation of the app runtime environment vs the distribution environment that the user is running. This is done by containers and namespaces where we mount only the system files and application files into the app namespace.

With this kind of isolation we guarantee two things, first of all there will never be any accidental leaks of dependencies. If your app depends on anything not in the supported platform it will just not run and the developer will immediately notice. Secondly, any incompatible changes in system libraries are handled and the app will still get a copy of the older versions when it runs.

One nice thing about this setup is that the application runtime environment will look like a minimal but standard linux install with the app and its dependencies in the standard prefixes like /usr/lib, etc. This means that when creating an application one can very easily reuse existing .deb or .rpm files by just extracting these and putting in the application bundle. Of course, it also will require a higher level of binary compatibility guarantees than what we have previously handled for modules the platform profiles provide. For instance, internal IPC protocols the platform libraries use absolutely have to be backwards compatible.

Sandboxing

The sandbox model goes hand-in-hand with the isolation model of app deployment, in the sense that whatever the app should not be able to do it will not even see. So, for a fully sandboxed app we will not even mount in the users home directory in the app namespace, rather than not having access rights to it. (We will of course also offer applications with unlimited sandboxes so that existing apps can run.)

In order to talk with the sandboxes app we need a IPC model that handles the domain transition between the namespaces. This implies the kernel being involved, so we have been looking (again) at getting some form of dbus routing support into the kernel. Hopefully this will work out this time.

Unfortunately we also need IPC for the Xserver, which is very hard to secure. We’ve decided to just just ignore this for now however, as it turns out Wayland is a very good fit for this, since it naturally isolates the clients.

We also talked about implementing something similar to the Intents system in android as a way to allow sandboxed applications to communicate without necessarily knowing about each other. This essentially becomes a DBus service which keeps a registry of which apps implements the various interfaces we want to support (e.g. file picking, get-a-photo, share photo) and actually proxies the messages for these to the right destination. We had a long discussion about the name for these and came up with the name “Portals”, reflecting the domain-transition that these calls represent.

We’re continuing to discuss the details of the different parts and hopefully we can start implement parts of this soon. After the hackfest we will continue discussions on the gnome-os mailing list.

Broadway Multi-process support

December 27th, 2012 by alexl

I just landed support for multiple processes in broadway. The way it works is that we run a process (broadwayd) similar to the X server, which all broadway apps talk to. Additionally it has a webserver built in that the browser connects to.

This allows (for instance) broadway processes to start other broadway processes. Basically the webpage turns into something like an X session.

This is best shown with a demo:

YouTube Preview Image

 

Rethinking the shell pipeline

August 10th, 2012 by alexl

I’ve been playing with an idea of how to extend the traditional unix shell pipeline. Historically shell pipelines worked by passing free-form text around. This is very flexible and easy to work with. For instance, its easy to debug or incrementally construct such a pipeline as each individual step is easily readable.

However, the pure textual format is problematic in many cases as you need to interpret the data to work on it. Even something as basic as numerical sorting on a column gets quite complicated.

There has been a few projects trying to generalize the shell pipeline to solve these issues by streaming Objects in the pipeline. For instance the HotWire shell and Microsoft PowerShell. Although these are cool projects I think they step too far away from the traditional interactive shell pipelines, getting closer to “real” programming, with more strict interfaces (rather than freeform) and not being very compatible with existing unix shell tools.

My approach is a kind of middle ground between free-form text and objects. Instead of passing free-form text in the pipeline it uses typed data, in the form of glib GVariants. GVariant is a size-efficient binary data format with a powerful recursive type system and a textual form that is pretty nice. Additionally the type system it is a superset of DBus which is pretty nice as it makes it easier to integrate DBus calls with the shell.

Additionally I created a format negotiation system for pipes such that for “normal” pipes or other types of output we output textual data, one variant per line. But, if the destination process specifies that it supports it we pass the data in raw binary form.

Then I wrote some standard tools to work on this format, so you can sort, filter, limit, and display variant streams. For example, to get some sample data I wrote a “dps” tool similar to ps that gives typed output.

Running it prints something like:

$ dps
 <{'pid': <uint32 1>, 'ppid': <uint32 0>, 'euid': <uint32 0>, 'egid': <uint32 0>, 'user': <'root'>, 'cmd': <'systemd'>, 'cmdline': <'/usr/lib/systemd/systemd'>, 'cmdvec': <['/usr/lib/systemd/systemd']>, 'state': <'S'>, 'utime': <uint64 38>, 'stime': <uint64 138>, 'cutime': <uint64 3867>, 'cstime': <uint64 1273>, 'time': <uint64 1344635046>, 'start': <uint64 1>, 'vsize': <uint64 61488>, 'rss': <uint64 24408>}>
 <{'pid': <uint32 2>, 'ppid': <uint32 0>, 'euid': <uint32 0>, 'egid': <uint32 0>, 'user': <'root'>, 'cmd': <'kthreadd'>, 'cmdline': <'[kthreadd]'>, 'state': <'S'>, 'utime': <uint64 0>, 'stime': <uint64 1>, 'cutime': <uint64 0>, 'cstime': <uint64 0>, 'time': <uint64 1344635046>, 'start': <uint64 1>, 'vsize': <uint64 0>, 'rss': <uint64 0>}>
 ...

Not super-readable, but its a textual format that you could combine with traditional tools like grep and awk.

But, with the type information we can do more interesting things. For instance, we could filter using a numeric comparison, say finding
all system uids:

$ dps | dfilter euid \< 1000
 <{'pid': <uint32 1>, 'ppid': <uint32 0>, 'euid': <uint32 0>, 'egid': <uint32 0>, 'user': <'root'>, 'cmd': <'systemd'>, 'cmdline': <'/usr/lib/systemd/systemd'>, 'cmdvec': <['/usr/lib/systemd/systemd']>, 'state': <'S'>, 'utime': <uint64 38>, 'stime': <uint64 139>, 'cutime': <uint64 4290>, 'cstime': <uint64 1318>, 'time': <uint64 1344635266>, 'start': <uint64 1>, 'vsize': <uint64 61488>, 'rss': <uint64 24408>}>
 <{'pid': <uint32 2>, 'ppid': <uint32 0>, 'euid': <uint32 0>, 'egid': <uint32 0>, 'user': <'root'>, 'cmd': <'kthreadd'>, 'cmdline': <'[kthreadd]'>, 'state': <'S'>, 'utime': <uint64 0>, 'stime': <uint64 1>, 'cutime': <uint64 0>, 'cstime': <uint64 0>, 'time': <uint64 1344635266>, 'start': <uint64 1>, 'vsize': <uint64 0>, 'rss': <uint64 0>}>
 ...

Then we can add numerical sorting:

$ dps | dfilter euid \< 1000 | dsort rss
 <{'pid': <uint32 1>, 'ppid': <uint32 0>, 'euid': <uint32 0>, 'egid': <uint32 0>, 'user': <'root'>, 'cmd': <'systemd'>, 'cmdline': <'/usr/lib/systemd/systemd'>, 'cmdvec': <['/usr/lib/systemd/systemd']>, 'state': <'S'>, 'utime': <uint64 38>, 'stime': <uint64 139>, 'cutime': <uint64 4290>, 'cstime': <uint64 1318>, 'time': <uint64 1344635365>, 'start': <uint64 1>, 'vsize': <uint64 61488>, 'rss': <uint64 24408>}>
 <{'pid': <uint32 769>, 'ppid': <uint32 745>, 'euid': <uint32 0>, 'egid': <uint32 0>, 'user': <'root'>, 'cmd': <'Xorg'>, 'cmdline': <'/usr/bin/Xorg :0 -background none -logverbose 7 -seat seat0 -nolisten tcp vt1'>, 'cmdvec': <['/usr/bin/Xorg', ':0', '-background', 'none', '-logverbose', '7', '-seat', 'seat0', '-nolisten', 'tcp', 'vt1']>, 'state': <'S'>, 'utime': <uint64 1602>, 'stime': <uint64 3145>, 'cutime': <uint64 22>, 'cstime': <uint64 9>, 'time': <uint64 1344634285>, 'start': <uint64 1081>, 'vsize': <uint64 108000>, 'rss': <uint64 16028>}>
 ...

And, nice display:

$ dps | dfilter euid \< 1000 | dsort rss | dhead 4 | dtable pid user rss vsize cmdline
 pid     user      rss    vsize  cmdline
   1   'root'    24408    61488 '/usr/lib/systemd/systemd'
 769   'root'    16028   108000 '/usr/bin/Xorg :0 -background none -logverbose 7 -seat seat0 -nolisten tcp vt1'
 608   'root'    15076   255312 '/usr/bin/python /usr/sbin/firewalld --nofork'
 747   'root'     8276   452604 '/usr/sbin/libvirtd'

Note how we do two type-sensitive operations (filter by numerical comparison and numerical sort) without problems, and that we can do the “head” operation to limit output length without affecting the table header. We also filter on a column (euid) which is not displayed. And, all the data that flows in the pipeline is in binary form (since all targets support that), so we don’t waste a time re-parsing it.

Additionally I think this is a pretty nice example of the Unix idea of “do one thing well”. Rather than having the “ps” app have lots of ways to specify how the output should be sorted/limited/displayed we have separate reusable apps for those parts. Of course, its a lot more typing than “ps aux”, which makes it less practical in real life.

I’ve got some code, which while quite rudimentary, does show that this could work. However, it needs a lot of fleshing out to be actually useful. I’m interested in what people think about this. Does it seem useful?

Moar windows themes!

March 27th, 2012 by alexl

Last time I blogged about the Gtk 3 windows theme we had just landed the initial support. It kinda worked but didn’t look quite right. Since then the css machinery has gotten a bit more capable, and I recently found some time to work on this again.

Testing had revealed that the theme was totally broken on Windows XP. This had two main causes; first of all there was some bugs in the Win32 theme APIs on XP when rendering to surfaces with alpha, and secondly the css file used some windows theme parts that only existed in Vista and later. I added workarounds for the alpha bug and introduced a new css file that is used on XP (although most of the css is shared). So, now XP support is working.

I also went through all the widgets, fixing a lot of details in how they render and adding theming for some less common widgets.

Here is how gtk3-widget-factory looks now:

Windows 7

Windows XP

Not bad for 998 lines of css.

This is all in the new Gtk+ 3.4.0 release. We hope to have window binaries out for it soon.

Resources in glib

January 26th, 2012 by alexl

Last week I landed a new feature in Glib that I’ve been wanting to do for a long time: A resource framework. Resources are things that are naturally part of an application or library, but not really normal code. For instance, our code increasingly uses xml  to describe user interfaces and menus.

Traditionally these either had to be manually inserted into the code, like so:

static const gchar *ui_info =
"<ui>"
"  <menubar name='MenuBar'>"
"    <menu action='FileMenu'>"
"      <menuitem action='Quit'/>"
"    </menu>"
"  </menubar>"
"</ui>";

Or a file was stored in /usr/share/$application/ and you had to write code to manually find and load the file, and cache it if used often. This is not a lot of code, but it can be tricky as all I/O code needs to handle errors and the external file makes it harder to make the library/app relocatable.

Instead, with resources you store your data file as plain files in your source tree, edit them with your favourite editor, with full syntax highlighting, automatic indentation, etc. Then you reference these files from a resource description file (another xml file) and use glib-compile-resources to compile these into a binary chunk which is linked into the application.

The resource framework then automatically registers all such resource bundles in a global namespace, where you can quickly look up resource data by a resource path. There are API calls to get the data as a direct pointer, as well as streaming data, but in most cases there are helper functions that let you just specify the pathname. So, for instance, you can just do:

 gtk_builder_add_from_resource (builder, "/org/gnome/appname/menu.ui", &error);

Which would handle all the work for you. And while this looks like an I/O operation its really just a hashtable lookup on linked-in data, reading from the (shared, readonly) data section in your executable, so its very fast and safe.

Additionally there are some tricks the resource compiler can do for you. For instance, you can specify that a resource should be compressed, which means that the data is stored compressed, and the APIs uncompress for you automatically. You can also specify that xml files should be pre-processed to strip away whitespace, which avoids wasting time and memory on something that is not useful at runtime.

There is also support for resource:// URIs, which means you can easily reference resource data like icons from e.g. CSS and UI files.

On Linux we use some gcc specific extensions to store the resources in separate ELF sections, which means its very easy to extract resource data from the binaries. Glib even ships with a tool that lets you do this:

$ gresource list libgtk-3.so
/org/gtk/libgtk/cursor/dnd-ask.png
/org/gtk/libgtk/cursor/dnd-copy.png
/org/gtk/libgtk/cursor/dnd-link.png
/org/gtk/libgtk/cursor/dnd-move.png
/org/gtk/libgtk/cursor/dnd-none.png
/org/gtk/libgtk/gtk-default.css
/org/gtk/libgtk/gtk-win32.css
$ gresource extract libgtk-3.so /org/gtk/libgtk/gtk-default.css
@define-color fg_color #000;
@define-color bg_color #dcdad5;
@define-color text_color #000;
...

If you’re interested in using resources in your application, check out the documentation, or look at this example commit that converts Nautilus to use resources.