QUIP 18: License specification in Qt's modules

by Lucie Gerard, Jörg Bornemann

Details

Number: 18
Title: License specification in Qt's modules
Author: Lucie Gerard, Jörg Bornemann
Status: Active
Type: Informational
Post History: 043648.html, 045028.html, 046121.html
Created: 26 September 2022

Overview

This QUIP specifies the licenses that are used in different parts of a typical Qt module repository and also provides helpful guidance about how to express which license source code is under.

License header format

The license header follows the SPDX standard¹.

There is at least one copyright line and exactly one license expression line:

Copyright (C) 2023 The Qt Company Ltd.
SPDX-License-Identifier: <spdx-license-expression>

A copyright line for code written by a contributor outside the Qt Company can include that contributor’s name (or a legal entity they represent) in the Copyright header.

The copyright line should include the year the file was created, or - in the case of additional copyright lines - the year the additional copyright notice was added. Updating the copyright year when changes are made to the file is possible.

The copyright line may be prefixed with SPDX-FileCopyrightText:.

The <spdx-license-expression> must be a valid expression, according to the SPDX standard as defined in².

The license identifiers used in a license expression are either from the list of known licenses³ or prefixed with LicenseRef-.

The license files are placed into a LICENSES directory below the Qt module repository’s root. The license files are named after the SPDX license identifier followed by .txt⁴.

Classification of files

There are various kinds of files in a Qt module repository that should include license headers. Here, we classify these files in order to identify which files should be under which licenses.

Note that files that cannot use inline headers need to advertise their copyright and license using an adjacent REUSE.toml file⁵.

The documentation landing page of a Qt module contains a “Licenses and Attributions” section that defines the licenses under which the module and plugin implementation is available. See the list of Qt modules ⁶.

Module and plugin implementation

Files that belong to the implementation of Qt modules and plugins. The usual locations for module source code are the subdirectories src/ and plugins/.

There are two licensing models for module and plugin implementations: GPL-only and one that is additionally licensed under LPGL.

The license expression permitting the LPGL is LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only.

The GPL-only license expression is LicenseRef-Qt-Commercial OR GPL-3.0-only.

The license expression for files that are statically linked with user code - even for dynamic builds of Qt - is LicenseRef-Qt-Commercial OR BSD-3-Clause.

Examples and Snippets

Files that belong to the implementation of Qt examples. The usual locations for examples is the examples/ subdirectory.

Also, any source code that appears in the documentation. The usual locations are snippets/ directories throughout the Qt module repository’s directory tree.

The license expression for examples and snippets is LicenseRef-Qt-Commercial OR BSD-3-Clause.

Tests

Files that belong to the implementation of Qt tests. The usual location is the tests/ subdirectory.

The license expression for tests is LicenseRef-Qt-Commercial OR GPL-3.0-only.

Qt Translations

The usual location and file pattern for Qt translation files is translations/*.ts.

The license expression for Qt translation files is LicenseRef-Qt-Commercial OR BSD-3-Clause.

If the translation file is within a tests/ subdirectory it is licensed as a test file. We also have TS files in some examples. Those would be licensed as examples.

Tools and Utils

Files that belong to the implementation of Qt tools that are for use by Qt users. The usual location is the src/tools/ subdirectory.

Also considered tools are script and shell files, as long as they are not part of an example or a documentation snippet. Such files are usually using the following file name patterns:

*.sh
*.py
*.pl
*.bat
*.ps1

Finally, files that are located in a /util/ directory.

The license expression for tools and utils is LicenseRef-Qt-Commercial OR GPL-3.0-only WITH Qt-GPL-exception-1.0.

Documentation

All files that are needed to generate the documentation:

*.qdoc
*.qdoc.sample
*.qdocinc
*.qdocconf
documentation assets, like images, typically located in doc/images folders, including screencasts or animations.

Following informative files are also considered documentation:

README
qt_attribution.json (also used to generate documentation)

The license expression for documentation is LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only.

Build System Files

All files that contribute to Qt’s build system. That includes the following file name patterns:

CMakeLists.txt
*.cmake
*.cmake.conf
*.pro
*.pri
*.prf
configure
ci_config_linux.json
*.cfg
*.qrc
*.plist
*.tag
*.yaml

and files within the coin folder (unless their suffix have them fall into an other category, like tools for example).

If such a file is present in a snippet directory, it is licensed as a documentation snippets file. If such a file is present in an example directory, it is licensed as an example file.

The license expression for build system files is BSD-3-Clause.

Infrastructure

Files that define development environment configurations and coding standards. These files typically include version control settings, code formatting rules, editor configurations, and license setting and definition. Common file patterns include:

.gitattributes
.gitignore
.gitmodules
.gireview
_clang-format
licenseRule.json
REUSE.toml

The license expression for infrastructure files is LicenseRef-Qt-Commercial OR BSD-3-Clause.

Testing License Usage

The Qt CI can enforce correct usages of license files. This requires a licenseRule.json file to exist in the top-level directory of the Qt module’s repository. This file contain declarative rules, based on file suffix and directory, and should follow the rules set out in this QUIP. They do allow for exceptions and special cases though, if necessary.

The “Software License Agreement between The KDE Free Qt Foundation and The Qt Company Oy”⁷ poses restrictions for the licenses of the Qt Free Edition modules.

To a certain extent, Qt follows REUSE⁸. REUSE is a guideline on how to specify the licenses in a project. We aim at full compliance.

The reuse tool is also used to generate a “software bill of materials” (SBOM) document for the Qt source code. Using SPDX format, such a source SBOM lists copyright and licensing information for all files within a module. In the CI, source SBOM is generated from the SDPX-formatted copyright and license information in the Qt sources and in the REUSE.toml files. The source SBOM is then checked against licenseRule.json.