Software licences¶

Exercise software-licences-annex.pdf · PDF

This year you will write a small game. A classmate copies your folder and sells it online for 2 €. You never signed anything. You never wrote "do not copy". Is he allowed?

No. And the reason is the fourth question from Comparing operating systems: what am I allowed to do with this software? As a future developer you will answer it twice — for the software you use, and for the software you write. On the OS architecture page, modinfo printed license: GPL. Today you find out what those three letters mean.

Copyright and licences¶

Copyright gives the author of a program all the rights over it, automatically, from the moment it is written. Nobody else may copy it, change it or share it.

A licence is the author's written permission: what others may do with the program, and on what conditions.

No licence means no permission. That is why your classmate cannot sell your game — and why you cannot paste code you found on a random web page into your app, even if it is "public". No licence = all rights reserved.

It also means that when you "buy Windows", you do not buy Windows. You buy a licence: permission to use it, with limits.

The recipe and the dish¶

Programmers write source code: text that a person can read. The computer runs a translation of it, the binary, which no person can read.

flowchart LR
    SRC["<b>Source code</b><br>the recipe:<br>text a person reads"] -->|"compiled<br>(translated)"| BIN["<b>Binary</b><br>the dish:<br>what the CPU runs"]
    BIN -->|"you can run it,<br>but not read it"| YOU["<b>You</b>"]
    SRC -.->|"only with the recipe<br>can you study or change it"| YOU

The printer nobody was allowed to fix¶

1980, the Artificial Intelligence Lab at MIT. The lab gets a new laser printer, a Xerox 9700. It jams all the time. A programmer of the lab, Richard Stallman, had solved that on the old printer: he changed its program so it warned everybody when the paper got stuck. For the new one he asks for the source code. The answer is no: it is secret, and the colleague who has it signed a promise not to share it. A machine in his own lab, and he is not allowed to repair it.

• 27 September 1983 — Stallman announces the GNU project: a complete operating system that everybody may study, change and share.
• October 1985 — he founds the Free Software Foundation (FSF).
• 1989 — the first GNU General Public License (GPL).
• 1991 — GNU has almost everything except a kernel. A student in Helsinki publishes one. You know his name.

Free software and the four freedoms¶

A program is free software when its licence gives every user these four freedoms.

The FSF numbers them from zero — programmers count from zero:

Freedoms 1 and 3 are impossible without the source code. That is why free software always comes with its recipe.

Open source¶

In business, "free software" sounded like "software with no price". On 3 February 1998, in a meeting in Palo Alto — shortly after Netscape announced it would publish the source of its browser — Christine Peterson proposed a new name: open source. That same month Eric Raymond and Bruce Perens founded the Open Source Initiative (OSI).

You will see both names used for the same programs. In practice: same recipes, different speeches.

Copyleft and permissive¶

All free licences give the four freedoms. They differ on one point: when you pass on a changed version, what must you do?

With copyleft, the recipe must travel with every dish you hand out. With a permissive licence, you may serve the dish and keep your improved recipe to yourself.

The blue router¶

2003: one of the most popular home routers is the blue Linksys WRT54G. Some users look inside its software and find Linux, under the GPL. But Linksys has published no source code: the dish without the recipe. After public pressure, the company publishes it. In 2004 volunteers use that code to start OpenWrt, a free system for routers that is still developed today.

Copyleft is not a wish; it is a condition. Linksys could use Linux for nothing. What it could not do was keep the recipe for itself.

Android shows the other side. Its kernel is Linux (GPL), but most of the rest is under Apache 2.0 — permissive — so each phone maker may add its own closed parts on top.

How proprietary software is sold¶

Proprietary software keeps the recipe secret and gives you a permission with limits.

The text you accept when you click "I agree" is the EULA (End-User Licence Agreement). Microsoft's licence terms for Windows say it plainly: "The software is licensed, not sold." Among the things they forbid: to reverse engineer, decompile or disassemble the software — that is, trying to get the recipe back from the dish.

The same Windows 11 is sold in several ways:

OEM stands for Original Equipment Manufacturer: the company that made the computer.

And nobody makes money with free software? Wrong too: companies like Red Hat and Canonical sell support and services for it, and selling copies is allowed as well.

The licence hunt: find it on your PC¶

Every program on Ubuntu comes with its licence in a text file. Open a terminal (Ctrl+Alt+T). The Tab key completes file names — use it to avoid typos.

The full text of the most used licences lives in one folder:

ls /usr/share/common-licenses/

You will see names like GPL-2, GPL-3, LGPL-3, Apache-2.0, BSD, MPL-2.0. Look at the start of one:

head -3 /usr/share/common-licenses/GPL-3

                    GNU GENERAL PUBLIC LICENSE
                       Version 3, 29 June 2007

Every installed package keeps its own licence in /usr/share/doc/<package>/copyright. To read just the first licence line:

grep -m1 "^License:" /usr/share/doc/bash/copyright

License: GPL-3+

The + means "this version or any later one". Repeat it changing bash for apt, libc6, dash and zlib1g — the Exercise sheet has a box for each. You will find at least four different licences on one PC. And the kernel's drivers carry one too:

modinfo usbhid | grep license

The Linux kernel is under GPL version 2.

Then compare with the other side: open microsoft.com/useterms, choose Windows 11, and search (Ctrl+F) for not sold.

Choosing a licence for your own code¶

When you publish your first app, choosealicense.com (a site kept by GitHub) helps you pick. Its first page asks what you care about: sharing improvements points you to a copyleft licence; keeping it simple and permissive points you to a permissive one.

Key takeaways¶

• Software is protected by copyright by default; a licence is the author's permission. No licence = no permission.
• Source code is the recipe; the binary is the dish. Studying or changing a program needs the recipe.
• Free software gives the four freedoms (run, study and change, copy, share changes). Free means freedom, not price. Open source is almost the same set of licences, with a different emphasis.
• Copyleft (GPL) obliges changed versions you distribute to stay free; permissive (MIT, BSD, Apache) allows closed products.
• Proprietary software is sold as OEM (tied to the device), retail (movable), volume, subscription or freeware, under a EULA: "licensed, not sold".
• On Ubuntu, /usr/share/doc/<package>/copyright tells you the licence of any package.

Quick check¶

Next: you will write a lab report for every practical this year — learn the format once — Markdown lab reports.