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This chapter covers the different aspects of developing your own libraries for use in Arm Mbed devices, as well as items to keep in mind during development, such as licensing. It covers:

Licensing binaries and libraries

When you write original code, you own the copyright and can choose to make it available to others under a license of your choice. A license gives rights and puts limitations on the reuse of your code by others. Not having a license means others cannot use your code. We encourage you to choose a license that makes possible (and encourages!) reuse by others.

If you create new software, such as drivers, libraries and examples, you can apply whatever license you like as the author and copyright holder of that code. Having said that, we encourage you to use a well-known license such as one of the ones listed OSI-approved, permissive open source software license. Specifically, we recommend the following:

  • For original source code, use the Apache 2.0 license.  

  • For binary releases (for example, private source code you can’t or don’t want to release but want to share as a binary library and headers available for others to use), consider the Permissive Binary License (PBL). This is designed to be compatible with Apache 2.0 and the Mbed OS codebase.

  • If your software incorporates or is derived from other third party open source code, please be sure to retain all notices and identify the license for the third party licensed code in the same manner as described below. You cannot change the license on someone else’s code, because you are not the copyright holder. Instead, choose a license that is compatible with the license used by the third party open source code, or use the same license as that code. For example, if your software is derived from GNU General Public License (GPL) source code, GPL requires you to license the rest of your code in that software under the GPL too. Many commercial users cannot use GPL source code in their products, so we don't recommend this license if you're not obligated to use it.

You must either write all the code you provide yourself, or have the necessary rights to provide code written by someone else.

In all cases, whatever license you use, please use an SPDX license identifier in every source file following the recommendations to make it easier for users to understand and review licenses.

When to use Apache 2.0

Apache 2.0 is a permissive, free and open source software license that allows other parties to use, modify and redistribute the code in source and binary form. Compared to the often used Berkeley Software Distribution (BSD) license, Apache 2.0 provides an express patent grant from contributors to users.

The full text of the license is on the Apache website. For more information about Apache 2.0, see the FAQ.

How to apply Apache 2.0 correctly

To clearly reflect the Apache 2.0 license, please create two text files:

  • A LICENSE.md file with the following text:

    Unless specifically indicated otherwise in a file, files are licensed under the Apache 2.0 license,
    as can be found in: LICENSE-apache-2.0.txt
  • The full original Apache 2.0 license text in LICENSE-apache-2.0.txt

    Each source header should start with your copyright line, the SPDX identifier and the Apache 2.0 header as shown here:

    Copyright (c) [First year]-[Last year], **Your Name or Company Here**
    SPDX-License-Identifier: Apache-2.0
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
    You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0
    Unless required by applicable law or agreed to in writing, software
    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND,
    either express or implied.
    See the License for the specific language governing permissions and limitations under the License.

When to use PBL

PBL is a permissive license based on BSD-3-Clause and designed specifically for binary blobs. It is minimal but covers the basics, including an express patent grant.

It allows you to share a binary blob and the relevant headers, and allows others to use that binary blob as part of their product - as long as they provide it with all the relevant dependencies and don't modify or reverse engineer it.

The full text is at mbed.com.

How to apply PBL correctly

To clearly reflect the PBL license, please create the following three text files:

  • A LICENSE.md file with:

    Unless specifically indicated otherwise in a file, files are licensed under the Permissive Binary License, as can be found in LICENSE-permissive-binary-license-1.0.txt
  • The full original Permissive Binary License 1.0 text in LICENSE-permissive-binary-license-1.0.txt.

  • A DEPENDENCIES file with the dependencies that this binary requires to work properly. This is to make sure that third parties integrating the binary in their own distribution are aware that they need to include the relevant dependencies. If your binary does not have any dependencies, the file should state so (that is, say “No dependencies”); don't omit this file.

    Each source header should start with the copyright line, SPDX identifier and PBL header:

    Copyright (c) [First year]-[Last year], **Your Name Here**, All Rights Reserved SPDX-License-Identifier: LicenseRef-PBL
    This file and the related binary are licensed under the Permissive Binary License, Version 1.0 (the "License"); you may not use these files except in compliance with the License.
    You may obtain a copy of the License here: LICENSE-permissive-binary-license-1.0.txt and at https://www.mbed.com/licenses/PBL-1.0
    See the License for the specific language governing permissions and limitations under the License.

Using a different license

If you decide to use a different license for your work, follow the same pattern:

  • Create a LICENSE.md file with a description of the license situation, following the pattern described in the sections above.

  • Put the full original license texts in separate documents named LICENSE-XYZ.txt, where XYZ is the corresponding SPDX identifier for your license.

  • Begin each source header with your copyright line, the SPDX identifier and the standard header for the license that applies to that single file, if it has one. (See SPDX Specification, Appendix V.)

  • If more than one license applies to the source file, use an SPDX license expression (see SPDX Specification, Appendix IV), to reflect the presence of multiple licenses in your LICENSE.md file and in each source file.

License check in pull requests

We use scancode-toolkit to enforce the license in binaries and libraries. Please see "Continuous integration (CI) testing" section for more information.

To scan the files in the SCANCODE folder, please use:

scancode --license --json-pp mbed-os.json SCANCODE

Run our script to check if files confirm to this license guide:

python tools/test/travis-ci/scancode-evaluate.py -f mbed-os.json

Contributing to the Mbed OS codebase

Mbed OS principles

By creating a pull request on GitHub against Mbed OS, you agree to license your contributions under the same license as the original code. Mbed software is released under a permissive license for all new Mbed contributions. Arm will not accept proprietary code or any code under a "copyleft" license. Accepting such code would prevent us from distributing our code under the permissive license and stall adoption of our code by third parties.

Mbed OS uses these same basic principles for its source code and library distributions. Source code we own is distributed under the Apache 2.0 license, and binary blobs are released under the Permissive Binary License. Software parts from third parties that were already licensed under a different license are available under the original license.

All source code and binary blobs in Mbed OS are maintained in public GitHub repositories.

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