4.7. Tracking licensing information in packages

PTXdist aims to track licensing information for every package. This includes the license(s) under which a package can be distributed, as well as the respective files in the package’s source tree that state those terms. Sadly there is no widely adopted standard for machine-readable licensing information in source code (yet), so here are a few hints where to look.

In that process, we aim to collect the baseline set of licenses which at least apply to a package. There may be other licenses which apply too, but the complete set often cannot be found without a time-consuming review. Still, the extracted license information in PTXdist can serve as a hint for the full license compliance process, and can help to exclude certain software under certain licenses from the build.

There are many older package rules in PTXdist which don’t specify licensing information. If you want to help complete the database, you can use grep -L _LICENSE_FILES rules/*.make (in the PTXdist tree) to find those rules. Note however that this cannot find wrong or incomplete licensing information.

Finding licensing information

You should first select and extract the package in question, and then have a look at in the extracted package sources (usually something like platform-nnn/build-target/mypackage-1.0 in your BSP, if in doubt see ptxdist package-info mypackage).

  • Check for files named COPYING, COPYRIGHT, or LICENSE. These often only contain the license text and, in case of GPL, no information if the code is available under the -only or -or-later variant. Sometimes these files are in a folder /doc or /legal.

  • Check the README, if there is any. Often there is important information there, e.g. in case of GPL if the software is GPL-x.x-or-later or GPL-x.x-only.

  • Check source files, like *.c for license headers. Often additional information can be found here.

  • If you want to be extra sure, use a license compliance toolchain (e.g. FOSSology) on the project.

Ideally you’ll find two pieces of information:

  • A license text (e.g. a GNU General Public License v2.0 text)

  • A license statement that states that a certain license applies to (parts of) the project (often also including copyright statements and a warranty disclaimer)

Some licenses (e.g. BSD-style licenses) are also short enough so that both pieces are combined in a short comment header in a source file or a README. Strictly speaking, both the license text and the license statement must be present for a complete, unambiguous license, but see the next section about edge cases.

On the other hand, there are some parts that can be ignored for our purposes:

  • Everything that is auto-generated, either by a script in the project source, or by the build system previous to packaging. The generator itself cannot hold copyright, although the authors of the templates used for the generation or the authors of the generator can.

  • Most files belonging to the build system don’t make it into the compiled code and can therefore be ignored (e.g. configure scripts, Makefiles). These cases sometimes can be hard to detect – if unsure, include the file in your research.

Some projects also include a COPYING.LIB containing an LGPL text, which is referenced nowhere in the project. In that case, ignore the COPYING.LIB – it probably comes from a boilerplate project skeleton and the maintainer forgot to delete it.

Distillation into license identifiers

In PTXdist, we use SPDX license expressions.

Either the license identifier is clear, e.g. because the README says “GPL 2.0 or later” (check the license text to be sure), or you can use tools like FOSSology, licensecheck, or spdx-license-match to match texts to SPDX license identifiers.

License texts don’t have to match exactly, you should apply the SPDX Matching Guidelines accordingly. The important part here is that the project’s license and the SPDX identifier describe the same licensing terms. “Rather close” or “mostly similar” statements are not enough for a match, but simple unimportant changes like replacing “The Author” with the project’s maintainer’s name, or a change in e-mail adresses, are usually okay.

For software that is not open-source according to the OSI definition, use the identifier proprietary.


If no license identifier matches, or if anything is unclear about the licensing situation, use the identifier custom (for licenses) or custom-exception (for license exceptions, e.g.: GPL-2.0-only WITH custom-exception).

If SPDX doesn’t know about a license yet, and the project is considered open source or free software, you can report its license to be added to the SPDX license list.

Multiple licenses

Open-source software is re-used all the time, so it can happen that some files make their way into a different project. This is usually no problem. If you encounter multiple parts of the project under different licenses, combine their license expressions with AND. For example, in a project that contains both a library and command line tools, the license expression could be GPL-2.0-or-later AND LGPL-2.1-or-later.

Sometimes files are licensed under multiple licenses, and only one license is to be selected. In that case, combine the license expressions with OR. This is often the case with Device Trees in the Linux kernel, e.g.: GPL-2.0-only OR BSD-2-Clause.

No operator precedence is defined, use brackets (…) to group sub-statements.

Conflicting and ambiguous statements

Human interpretation is needed when statements inside the project conflict with each other. Some clues that can help you decide:


If the header in the COPYING file says “GNU General Public License”, but the license text below that is in fact a BSD license, the correct license for the license identifier is the BSD license.

Author Intent:

If the README says “this code is LGPL 2.1”, but COPYING contains a GPL boilerplate license text, the correct license identifier is probably “LGPL 2.1” – the README written by the author prevails over the boilerplate text.


If README and COPYING are both clearly written by the author themselves, and the README says “don’t do $thing” and COPYING says “do $thing”, the more recent file prevails.


If no license statement can be found, but there is a COPYING file containing a license text, infer that the whole project is licensed under that license.

Err on the side of caution:

If all you can find is a GPL license text, this doesn’t yet tell you whether the project is licensed under the -only or the -or-later variant. In that case, interpret the license restrictively and choose the -only variant for the license identifier.

Don’t assume:

If anything is ambiguous or unclear, choose custom as a license identifier.


Any of these cases is considered a bug and should be reported to the upstream maintainers!

“Public Domain” software

For good reasons, SPDX doesn’t supply a license identifier for “Public Domain”. Nevertheless, some PTXdist package rules specify public_domain as their respective license identifier. This is purely for historical reasons, and public_domain should normally not be used for new packages. Some of those “Public Domain” dedications in packages have since been accepted in SPDX, e.g. libselinux or SQLite.

No license information at all

No license - no usage rights!

Definitely report this bug to the upstream maintainer. Maybe even point them in the direction of machine-readablity :)

Adding license files to PTXdist packages

The SPDX license identifier of the package goes into the <PKG>_LICENSE variable in the respective package rule file. All relevant files identified in the steps above are then added to the variable <PKG>_LICENSE_FILES, including a checksum so that PTXdist complains when they change.


DDRESCUE_LICENSE     := GPL-2.0-or-later AND BSD-2-Clause
        file://COPYING;md5=76d6e300ffd8fb9d18bd9b136a9bba13 \
        file://main.cc;startline=1;endline=16;md5=a01d61d3293ce28b883d8ba0c497e968 \

See the section Package Specific Variables for more information about the syntax of those two variables.

The MD5 sum for a block of lines can be generated with sed’s p (print) command applied to a range of lines. For the example above, lines 1 to 16 of main.cc would be:

$ sed -n 1,16p main.cc | md5sum -

Always include the copyright statement (“Copyright YYYY (C) Some Person”) for the calculation of the checksum, even if it means that the checksum changes on package updates when new years are added to the string. While it is not is needed for most licenses to be valid, some licenses require that it must not be removed (e.g. see GPLv2, section 1), and it is proper etiquette to give attribution to the maintainers in the license report document.

If additional information is in the README or license headers in source files are used, also include these files (for source code: one of each is enough), but use md5sum only on the relevant lines, so changes in the rest of the file do not appear as license changes.

For rather chaotic directories with lots of license files, definitely include at least one relevant source file with license headers (if there are any), as some developers tend to accumulate license files without adjusting it to license changes in their source.


For each single license identifier in the license expression, include at least one file with checksum in the <PKG>_LICENSE_FILES variable.

PTXdist will include all files (or their respective lines) that were referenced in <PKG>_LICENSE_FILES as verbatim sources in the license report.