Basics of Linux GUI stack

DISCLAIMER: This post is written by a complete novice. Information stated may be inaccurate and if so, feel free to correct any mistakes.
RESEARCH TOPIC: Linux GUI stack
WHAT PROMPTED THIS RESEARCH?: I stumbled across a process called 'xorg' and wondered what that is. A quick Google search shows it has something to do with displaying graphics and so begins the journey of trying to understand the Linux GUI stack on a surface level.

Windowing System

• In a windowing system, there are 2 main components: the display server and the window manager.

  • Display server
    • The display server protocol specifies communication between a client (GUI application) and the display server
    • Relays inputs (keyboards, mouse, touchscreen etc.) to clients
    • Displays the output onto a screen
  • Window manager
    • Works hand-in-hand with a display server
    • Manages the placement and appearance of windows

• There are 2 main types of windowing system used in GUI Linux operating systems today: X11 and Wayland.

  • X11

    • X.Org server (display server) + Mutter/KWin (window manager)
    • X.Org server is the dominant implementation of the display server for X Window System (X11 Protocol).

    "In the case of X11, A window manager is a regular X client. It doesn’t have any superuser privileges or keys to kernel backdoors; it is a normal user process that is allowed by the X server to call a set of special APIs. X ensures that no more than one window manager is running at any given point by denying a client access to these APIs if another client currently has access. The first client to attempt to access these APIs always succeeds." - How X Window Managers Work, And How To Write One (Part I)

  • Wayland

    • Mutter/KWin Wayland compositor (Display server + window manager)
    • Wayland, unlike its X11 counterpart, does not have network transparency in order to simplify its architecture.

    I was curious as to how a wayland client would communicate to the display server (wayland compositor). In wayland’s case, it would look for a unix socket (ipc socket) typically named wayland-0. https://wayland.freedesktop.org/docs/html/ch04.html#sect-Protocol-Wire-Format

• Linux GUI toolkits (QT, GTK) now supports both X11 and wayland protocol.

Window Manager

• "A window manager is a system software that controls the placement and appearance of windows within a windowing system in a graphical user interface." - Window Manager on Wikipedia
• There are 2 notable types of Window Managers: Stacking and Tiling window managers.
  • Stacking
    • Openbox
    • Mutter/Kwin - more precisely a compositing window manager
  • Tiling
    • i3 (X11 Tiling window manager)
    • Sway (Tiling wayland compositor)

Display Manager

• A display manager presents the user with a login screen. A session starts when a user successfully enters a valid combination of username and password.
• Examples are GDM, KDM and LightDM.

• Ubuntu uses GDM by default.
• GDM runs on wayland. Display managers that run on X, on the other hand, starts up the X server before displaying the login UI.
• GDM starts in TTY1 and on user login, it starts up the user session on TTY2. Why is my GDM at a different TTY than my desktop environment? Just a question: Is it possible to start greeter and user screen in the same TTY? Another question: *Session manager https://www.systutorials.com/docs/linux/man/1-gnome-session/ So the method of starting a window manager directly without a display manager does not use a session manager? What does that mean for a user?

Desktop Environments

• "A desktop environment (DE) usually rides on top of a Window Manager and adds many features, including panels, status bars, drag-and-drop capabilities, and a suite of integrated applications and tools. " - zeal
• You can run standalone window managers without the need of a desktop environment.
• See desktop environments vs window managers
• Examples include GNOME and KDE.

GUI Toolkits

• Developers use these toolkits to ease development. They do not need to know the intricacies of X11/Wayland to start developing apps for the platform.
• Examples of GUI toolkits are QT and GTK.

Digression

• POP!_OS and Regolith OS looks pretty sick.
• What's Chrome OS GUI stack like?
  • Aura
  • Ozone
    • Overview
    • Chrome Is Reaching The Point Of Good X11 + Wayland Support In Same Build
• Android
  • Has its own windowing system known as SurfaceFlinger
    • What is Android's SurfaceFlinger
  • https://source.android.com/devices/graphics
• MacOS - Quartz Compositor ?