Guidelines and HOWTOs/Licensing

This document explains how to state license information in KDE projects according to the KDE Licensing Policy. Stating licenses correctly is a very important task, because only correctly licensed software can be distributed and delivered to our users.

Introduction

Every source code file shall contain information about its license to inform the user of the file how it may be used. Such information are usually added in the form of source code comments at the top of the file. Traditionally, those comments were stated as so-called "license headers", which are either the complete license texts or short texts that explain where to find the license text for the file along with some legal constraints. Today, this system is getting replaced with "SPDX Expressions", which nowadays should be the default way to state a license (see REUSE.software).

SPDX Identifiers and Expressions

Software Package Data Exchange® (SPDX) is an open standard for communicating software bill of material information. The SPDX specification is developed by the SPDX workgroup, which is hosted by The Linux Foundation. The idea is to have a public registry of all open source licenses and important license exceptions, such that license statements can be reduced to simply stating short license identifiers.

Complex Expressions

Not every source code is licensed under just one license. For example, you might want to state that a file can be used under the terms for the BSD-2-Clause license or under the terms of the GNU Public License version 2 or later. For these cases, the SPDX workgroup also provides a specification how state complex license statements (which we call "SPDX expressions"). The SPDX expression language also allows tooling based syntax checks, which enables us to use tools to check the correctness of license statements.

https://spdx.org/specifications

For SPDX expressions that do not only consist of one SPDX identifier, the following keywords can be used (for details, see SPDX specification):

OR
AND
WITH

The preference order of these operators is as stated above (cf. SPDX Specification, Appendix IV), where a lower order operator is applied before a higher order operator. For example, for GPL-3.0-only OR LGPL-2.1-only_WITH_Qt-LGPL-exception-1.1 the Qt-LGPL-exception-1.1 applies only when using the code under the LGPL-2.1 license.

REUSE.software

REUSE.software is an initiative by the Free Software Foundation Europe (FSFE), which provides recommendations to make licensing easier. Their guidelines state how to best state SPDX based license headers in source files and how to put license texts into the repository. When following their recommendations, the correct statement of license information can be tested by the "reuse" Python tool, which checkscheck the syntactical correctness of the license statements and the overall conformance with the REUSE specification:

https://reuse.software/spec/

SPDX Expression Examples

A SPDX conform license statement is quite simple. It always has the form to first state the copyright holders and then it specifies the license or licenses under which the source code can be used:

/*
 * SPDX-FileCopyrightText: 2019 Jane Doe <[email protected]>
 * SPDX-FileCopyrightText: © 2019 John Doe <[email protected]>
 *
 * SPDX-License-Identifier: LGPL-2.1-or-later
 */

License Statements

We follow the recommendations from reuse.software how to state license and copyright information. You can use the following template and fill it with the missing information:

/*
 * SPDX-FileCopyrightText: <year> <name> <contact-address>
 * SPDX-FileCopyrightText: <year> <name> <contact-address>
 * ...
 *
 * SPDX-License-Identifier: <SPDX-expression>
 */

SPDX-FileCopyrightText or Copyright Statement

A copyright statement shall always contain the name of the copyright holder, the year of publication and a contact address. Any of the following examples is correct:

Copyright 2016, 2018-2019 Joe Anybody <[email protected]>
Copyright (c) Alice Coder
SPDX-FileCopyrightText: 2019 Jane Doe <[email protected]>
SPDX-FileCopyrightText: © 2019 John Doe <[email protected]>
© Example Corporation <https://corp.example.com>

For copyright statements, please use the following checklist:

Prefer "Copyright" or "SPDX-FileCopyrightText:" to state the copyright holders.
State the copyright information in the order: year, name, contact address.
Any contact address should be stated in angle brackets.
The year of publication can be a single year, multiple years, or a span of years.

SPDX-License-Identifier Statement

The SPDX-License-Identifier tag must be followed by a valid SPDX License Expression.

Here are several example statements from the KDE project. For the list of allowed licenses in the KDE project, please see the KDE Licensing Policy:

SPDX-License-Identifier: LGPL-2.1-or-later
SPDX-License-Identifier: LGPL-2.1-only OR LGPL-3.0-only OR LicenseRef-KDE-Accepted-LGPL
SPDX-License-Identifier: LGPL-2.1-only_WITH_Qt-LGPL-exception-1.1
SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only OR LicenseRef-KDE-Accepted-GPL
SPDX-License-Identifier: GPL-2.0-or-later
SPDX-License-Identifier: GPL-3.0-or-later
SPDX-License-Identifier: MIT
SPDX-License-Identifier: BSD-2-Clause

License Texts

Each repository shall contain a folder LICENSES/ in the root of the repository. In this folder, there shall be a license file for all (and only for those!) SPDX Identifiers that are used inside the project. The license file must be in plain text and state the license.

For all SPDX identifiers and exception identifiers that are listed in the SPDX registry, use the https://github.com/fsfe/reuse-tool to download the correct license test with:

reuse download <IDENTIFIER>

For example, in order to download the "LGPL-2.1-or-later" license file use the command reuse download LGPL-2.1-or-later, which places the file into the LICENSES/ folder of your project.

All SPDX identifiers that start with "LicenseRef-" are custom identifiers (cf. SPDX Specification, Section 6), which are not listed in the SPDX registry. The correct license file contents for

LicenseRef-KDE-Accepted-LGPL
LicenseRef-KDE-Accepted-GPL

are listed in the KDE Licensing Policy.

For details, check REUSE.software specification, Section "License Files".

License Tooling

As we follow the reuse.software specification, we can use their compliance tool to check the correctness of license statements. The tool is available via https://github.com/fsfe/reuse-tool or can simply be installed via pip:

pip3 install reuse

For details, see the README.md file. The most important options are:

download : Download the specified license into the LICENSES/ directory.
lint : Verify the project for REUSE compliance.