How does a linux GUI work at the lowest level?

Question

I'm basically trying to figure out how one would go about making a GUI from absolute scratch with nothing but the linux kernel and programming in C.

I am not looking to create a GUI desktop environment from scratch, but I would like to create some desktop applications and in my search for knowledge, all the information I have been able to find is on GUI APIs and toolkits. I would like to know, at the very least for my understanding of the fundamentals of how linux GUI is made, how one would go about making a GUI environment or a GUI appllication without using any APIs or toolkits.

I am wondering if for example:

1. existing APIs and toolkits work via system calls to the kernel (and the kernel is responsible at the lowest level for constructing a GUI image in pixels or something)

2. these toolkits perform syscalls which simply pass information to screen drivers (is there a standard format for sending this information that all screen drivers abide by or do GUI APIs need to be able to output this information in multiple formats depending on the specific screen/driver?) and also if this is roughly true, does the the raw linux kernel usually just send information to the screen in the form of 8-bit characters?

I just really want to understand what happens between the linux kernel, and what I see on my screen (control/information flow through both software and hardware if you know, what format the information takes, etc). I would so greatly appreciate a detailed explanation, I understand this might be a dousie to explain in sufficient detail, but I think such an explanation would be a great resource for others who are curious and learning. For context I'm a 3rd year comp sci student who recently started programming in C for my Systems Programming course and I have an intermediate(or so I would describe it) understanding of linux and programming. Again Thank you to anyone who helps me out!!!

Answer 1

How it works (Gnu/Linux + X11)

Overview

It looks something like this (not draws to scale)

┌───────────────────────────────────────────────┐
│                       User                    │
│     ┌─────────────────────────────────────────┤
│     │             Application                 │
│     │            ┌──────────┬─────┬─────┬─────┤
│     │            │      ... │ SDL │ GTK │ QT  │
│     │            ├──────────┴─────┴─────┴─────┤
│     │            │            xLib            │
│     │            ├────────────────────────────┤
├─────┴───┬────────┴──┐         X11             │
│   Gnu   │ Libraries │        Server           │
│   Tools │           │                         │
├─────────┘           │                         │ 
├─────────────────────┤                         │
│   Linux (kernel)    │                         │
├─────────────────────┴─────────────────────────┤
│                    Hardware                   │
└───────────────────────────────────────────────┘

We see from the diagram that X11 talks mostly with the hardware. However it needs to talk via the kernel, to initially get access to this hardware.

I am a bit hazy on the detail (and I think it changed since I last looked into it). There is a device /dev/mem that gives access to the whole of memory (I think physical memory), as most of the graphics hardware is memory mapped, this file (see everything is a file) can be used to access it. X11 would open the file (kernel uses file permissions to see if it can do this), then X11 uses mmap to map the file into virtual memory (make it look like memory), now the memory looks like memory. After mmap, the kernel is not involved.

X11 needs to know about the various graphics hardware, as it accesses it directly, via memory.

(this may have changes, specifically the security model, may no longer give access to ALL of the memory.)

Linux

At the bottom is Linux (the kernel): a small part of the system. It provides access to hardware, and implements security.

Gnu

Then Gnu (Libraries; bash; tools:ls, etc; C compiler, etc). Most of the operating system.

X11 server (e.g. x.org)

Then X11 (Or Wayland, or ...), the base GUI subsystem. This runs in user-land (outside of the kernel): it is just another process, with some privileges. The kernel does not get involved, except to give access to the hardware. And providing inter-process communication, so that other processes can talk with the X11 server.

X11 library

A simple abstraction to allow you to write code for X11.

GUI libraries

Libraries such as qt, gtk, sdl, are next — they make it easier to use X11, and work on other systems such as wayland, Microsoft's Windows, or MacOS.

Applications

Applications sit on top of the libraries.

Some low-level entry points, for programming

xlib

Using xlib, is a good way to learn about X11. However do some reading about X11 first.

SDL

SDL will give you low level access, direct to bit-planes for you to directly draw to.

Going lower

If you want to go lower, then I am not sure what good current options are, but here are some ideas.

• Get an old Amiga, or simulator. And some good documentation. e.g. https://archive.org/details/Amiga_System_Programmers_Guide_1988_Abacus/mode/2up (I had 2 books, this one and similar).
• Look at what can be done on a raspberry pi. I have not looked into this.

Links

X11

https://en.wikipedia.org/wiki/X_Window_System

Modern ways

Writing this got my interest, so I had a look at what the modern fast way to do it is. Here are some links:

https://blogs.igalia.com/itoral/2014/07/29/a-brief-introduction-to-the-linux-graphics-stack/

Answer 2

ctrl-alt-delor's answer gives you a good overview of the general architecture. For a more hands-on approach, I give you an answer regarding "nothing but the linux kernel and programming in C".

I like writing to the frame-buffer directly every now and then. The frame-buffer device driver will do all the tedious close-to-the-hardware "how will this eventually end up on a screen" stuff for you. You can do so right away with a root shell:

echo -n -e '\x00\x00\xFF' > /dev/fb0

It sets the very first (top left) pixel to red on my 32 bit framebuffer:

You can totally do so from within C by opening /dev/fb0 and write bytes. Memory mapping can become your friend. This does only work without an X server or in a virtual console. Press Ctrl+Alt+F1 to access it.

PS: Visualising random data like your mouse movement can also be fun:

cat /dev/input/mouse0 > /dev/fb0

PPS: Please also note that virtually any real-world desktop application wants more direct access to the hardware for some fancy stuff like hardware acceleration for drawing, 3D and video rendering. The simple frame-buffer device won't do any of this well.

Answer 3

I would strongly recommend starting with ncurses.

Unlike more complex graphical systems, it is based purely on text, so there is no need to get bogged down in the details of screen drivers and graphics libraries. However the basic principles of putting windows on a screen, moving focus between windows, and so on, still hold true. And you can still do some drawing, at the level of single character blocks and ASCII art.

Of course you're still building this on top of a library, but it's a library which you can easily understand. And more than that, it's a library where the source code is freely available, fairly well documented, and not too impenetrable if you want to read it. You can even modify it yourself if you want to. Or you could look at all the library functions in there to find what the API needs to be, and write it yourself from scratch based on that design.

Answer 4

SunOS 5 had the DGA library, which provided device independent access to the different cg[3,6,14], TCX or LEO graphics adapters, which also was the thing which supported DOOM on the SPARC machines.

cg6 was a 8 bit, usually used in X11 as a pseudocolor visual but it could also provide an 8 bit truecolor while the tcx and leo is a 24 bit accelerated 3d display frame buffer (pseudocolor = a byte in videoram is an index into a large table which gives an 3x8 RGB value, the table's content can be changed easily.) The cg3 had about the same ability but it wasn't accelerated (the cg6 designers started afterwards another firm ... nVidia.)

The later devices like the PGX which was based on ATI Rage Pro chipset couldn't support truecolor and pseudocolor at the same time, which the earlier ones did. This forced a user to choose between old applications written for the pseudocolor model (or upgrade the sw if possible) and running only truecolor oriented apps.

Pseudocolor existed basically because what was videoram was awfully expensive in the mid 80s until 1992 or so. A color display which supported an usable workstation type resolution was also rather expensive (the 1984 Sun 2 black and white had a resolution of 1152x864 while a 1989 or so MG1 had 1600x1280 but b&w.)

I write this because i want to show the different requirements which X11 had to support.