computer science

Kmscon

Kmscon is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Kmscon rather than just read about it. In short: Kmscon is a virtual console that runs in userspace which David Hermann created to replace the Linux console, a terminal built into the Linux kernel. Kmscon uses the KMS driver for its output, it is multiseat-capable, and supports internationalized keyboard input and UTF-8 terminal output.

Key takeaways

Kmscon belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Kmscon to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Kmscon from memory before moving on to harder problems.

Reference excerpt

Kmscon is a virtual console that runs in userspace which David Hermann created to replace the Linux console, a terminal built into the Linux kernel. Kmscon uses the KMS driver for its output, it is multiseat-capable, and supports internationalized keyboard input and UTF-8 terminal output. The input support is implemented using X keyboard extension (XKB). Development of Kmscon was halted in March 2015. The project officially resumed in 2025, adopting the codebase from a fork maintained by Aetf since 2011. There was a successor project called systemd-consoled but that project was later dropped in July 2015.

Features Kmscon supports printing the full set of Unicode glyphs and is not limited by console encoding as the Linux console. While the only hard dependency is udev, kmscon can optionally be compiled to use Mesa for hardware acceleration of the console, and the pango library for improved font rendering. The adoption of XKB for input allows kmscon to accept the full range of available keyboard layouts for the X.Org Server and Wayland compositors for input and makes it possible to use the same layout both in graphical environment and in terminal. It has mouse support including select/copy/paste and scrolling.

Multiseat support

The VT system in the Linux kernel dates to 1993, and does not implement out-of-the-box multiseat support. It supports up to 63 VTs, but only one VT can be active at any given time. This necessitates additional steps to configure multiseat support. kmscon will enable multiseat out-of-the-box. If one seat's display server is running on VT 7 and another seat's display server is running on VT 8, then only one of these two seats can be used at a time. To use the other seat, a VT switch must be initiated. To make all seats usable at the same time, there are a few options:

Associate all display servers with the same VT: any user can switch VTs and in that case all users switch to the new VT. This makes VT switching (and thus fast user switching) impractical. X.Org Server command-line option -sharevts Don't associate any display server with a VT: fast user switching is impossible in this case. Text-based console logins are only possible if an input and display device are reserved for this purpose. Associate only one of the display servers with a VT: the other display servers can't do VT switching, but the display server associated with a VT can. VT switching on that one seat won't affect the other seats. This is the approach favored and assumed by systemd. command-line option vt7 for user 1 and -novtswitch for all other users.

Development In 2011, Jesse Barnes wrote in his blog about the possible userspace DRM-based implementation of the virtual terminal, that would dissolve the need for the Linux framebuffer and virtual terminal (VT) subsystems in the Linux kernel. Motivated by this blog post, David Herrmann implemented the basic functionality of virtual terminal. In October 2013, terminal emulator state machine (libtsm) library, a state machine for DEC VT100–VT520 compatible terminal emulators, was split out of kmscon and made available separately. It was amended with wlterm, an example Wayland terminal emulator.

Illustrations

Worked examples

Example 1 — a first encounter with Kmscon

Start with the simplest possible case. Write down what Kmscon claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Kmscon before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Kmscon ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Kmscon

In research: Kmscon appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Kmscon in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Kmscon is common in secondary-school and first-year university syllabi. It links to neighbouring topics Free system software, Freedesktop.org, Linux kernel-related software, so understanding it makes those chapters shorter.
In everyday life: Look for Kmscon outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Kmscon in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Kmscon means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Kmscon out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Kmscon in simple terms?

Why does Kmscon matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Kmscon?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Kmscon.