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Technical Team/SPDX Meta Tags
SPDX Meta Tagging
This is currently a DRAFT and work in progress.
Last Updated: 13 August 2015
The need to identify the license for open source software is critical for both reporting purposes and license compliance. However, determining the license can be difficult due to a lack of information or ambiguous information. Even when licensing information is present, a lack of consistent notation for providing license information can make automating the task of license detection very difficult, thus requiring vast amounts of human effort.
Meta Tagging is a proposal by the SPDX Work-group to use short license identifiers from the SPDX License List to indicate license info at the file level. The advantages of doing this are numerous but include:
- It is precise, there is no ambiguity due to variations in license header text.
- It is language neutral.
- It is easy to machine process.
- It is concise
- Leads to code that is easier to read.
- The license travels with the file (as sometimes not entire projects are used or license files are removed).
- It is simple and can be used without much cost in interpreted environments like java Script, etc.,.
- It is a standard and can be universal. There is no need for all the variation.
- An SPDX license identifier is immutable.
- It provides simple guidance for developer's who want to make sure the license for their code is respected.
Although discussed by SPDX for some time, with proposals by Windriver, the U-boot project was the first open source project to adopt (that we know about). This was the commit message by Wolfgang Denk the project maintainer:
Like many other projects, U-Boot has a tradition of including big blocks of License headers in all files. This not only blows up the source code with mostly redundant information, but also makes it very difficult to generate License Clearing Reports. An additional problem is that even the same licenses are referred to by a number of slightly varying text blocks (full, abbreviated, different indentation, line wrapping and/or white space, with obsolete address information, ...) which makes automatic processing a nightmare. To make this easier, such license headers in the source files will be replaced with a single line reference to Unique Lincense Identifiers as defined by the Linux Foundation's SPDX project . For example, in a source file the full "GPL v2.0 or later" header text will be replaced by a single line: SPDX-License-Identifier: GPL-2.0+
Meta-tags can be used in source code for example which can make it easier for tools that run over source code to determine where a particular source code file belongs and what its license is. The following meta-tags are available currently;
This tag declares the license the file is under and should be placed at or near the top of the file. To the extent that the file contains existing copyright and license information it is our general recommendation that the tag be used to supplement not replace that information especially for files with a long history of changes, multiple copyright holders, or complex licensing. Of course, this is the ultimate decision of the copyright holders of the file.
The tag should appear on its own line in the source file, generally as part of a comment.
SPDX-License-Identifier: <SPDX License Expression>
Where an SPDX license expression is defined in SPDX 2.0 Specification see Appendix IV.
The license expression syntax permits references to a single license (represented by a short form license id from the SPDX license list ) or a compound set of licenses (represented by joining together multiple licenses using the license expression syntax).
Representing Individual Licenses
Each license can be represented by the short form license id from the SPDX license list. Optionally, a unary operator "+" (plus sign) may be included to indicate "or later".
- SPDX-License-Identifier: MIT
- SPDX-License-Identifier: GPL-2.0+
Representing Multiple Licenses
A license set should be enclosed in parentheses. When there is a choice between licenses ("disjunctive license"), they should be separated with "OR". Similarly when multiple licenses need to be applied ("conjunctive license"), they should be separated with "AND". In some cases, a set of license terms apply except under special circumstances, in this case, use the "WITH" operator followed by one of the recognized exception identifiers. Please see Appendix IV of SPDX 2.0 Specification for more details of the specific syntax.
- SPDX-License-Identifier: (GPL-2.0 OR MIT)
- SPDX-License-Identifier: (LGPL-2.1 AND BSD-2-CLAUSE)
- SPDX-License-Identifier: (GPL-2.0+ WITH Bison-exception-2.2)
What to do if your license is not on the list?
At this point, its probably best to just put the text of your license header in the file if it doesn't exist in the license list. Please contact the SPDX legal work group to get your license added to the License List.
These are examples of communities which are using a Meta Tag.
U-boot is using SPDX-License-Identifiers in place of a license header in source.
When there is a standard header provided by the license author, it is recommended to use such standard header (alone or in combination with the SPDX short identifier). See, for example: https://github.com/ARMmbed/mbed-mesh-api/blob/master/source/AbstractMesh.cpp
If using SPDX short identifiers in individual files, it is recommended to reproduce the full license in the projects LICENSE file and indicate that SPDX short identifiers are being used to refer to it. See, for example: pocoproject/poco and https://github.com/pocoproject/poco/blob/develop/XML/src/AbstractContainerNode.cpp
Proposing New Meta Tags
To propose a new Meta Tag, send your proposal to the SPDX Technical Workgroup using their mailing list.