Apple Public Source License

The Apple Public Source License, commonly abbreviated as APSL, is an open-source and free-software license created by Apple for the public release of source code associated with Darwin, the Unix-based operating-system foundation used in Apple's software platforms. Apple introduced the license in 2000 as part of an effort to release Darwin source code and involve the broader software community in developing and reviewing technology derived from projects that had originated outside Apple as well as from Apple's own engineering work. APSL is a copyleft-style license, but its obligations are narrower than those of a fully reciprocal license such as the GNU General Public License. In general, when a party distributes externally a modified version of APSL-covered source code, the corresponding source for those modifications must be made available under the license's terms. This approach was intended to preserve public access to changes made to the covered code while allowing Apple and other users to combine the licensed code with proprietary components in ways that would not generally be possible under stronger copyleft licenses. The first version of APSL received approval from the Open Source Initiative. Apple subsequently revised the license, issuing version 2.0 on July 29, 2003. APSL 2.0 was accepted by the Free Software Foundation as a free-software license, although the FSF did not recommend it for new projects. Its reservations included the license's partial-copyleft structure, incompatibility with the GNU General Public License, and permission to link APSL-covered files with wholly proprietary software. The FSF also compared APSL's source-disclosure requirement for externally released derivative works to a similar concept found in the GNU Affero General Public License. APSL is best understood as a licensing framework rather than a conventional consumer brand or commercial product line. Its significance comes from the way it governs reuse, modification, and redistribution of Apple-originated open-source code. Darwin remains the principal historical context for the license, and much of the source code for Apple's operating-system components has continued to be distributed under APSL or related open-source terms. At the same time, Apple has relicensed some projects under more permissive licenses, including the Apache License. Bonjour's Zeroconf networking stack is an example of an Apple software release that moved to a more liberal licensing model. The license occupies a distinctive position in the open-source ecosystem: it provides meaningful access to source code and requires disclosure of externally distributed derivatives, while stopping short of requiring that all software linked with the covered code be released under the same terms. This balance reflects Apple's preference for publishing selected operating-system components without imposing the broad reciprocal obligations associated with the GPL family. Developers evaluating APSL-covered software must therefore consider both the version of the license involved and the circumstances of distribution, modification, and linkage.

Apple created the Apple Public Source License in the context of releasing Darwin, the open-source Unix-based core that underpins important parts of Apple's operating-system technologies. Darwin incorporated code and technical traditions from community and academic software projects, and Apple adopted an open-source licensing model to enable outside participation in the development and examination of the released source. The first APSL version appeared in 2000 and was approved by the Open Source Initiative, giving it formal recognition within the emerging open-source licensing system. The license was designed to combine public access to source code with obligations concerning derivative works. Its central disclosure requirement applies when modified versions of APSL-covered code are released externally: the relevant source for the derivative changes must be made available under the license's conditions. Unlike the GNU General Public License, however, APSL does not impose a broad requirement that every program linked with the covered code become open source. This made it possible to use APSL-licensed components alongside proprietary software, a feature that supported Apple's ability to publish parts of its operating-system stack without committing all adjacent software to the same licensing terms. Apple released version 2.0 on July 29, 2003. The revision was accepted by the Free Software Foundation as a free-software license, although the FSF's assessment was qualified. It considered APSL acceptable for developers working on projects already distributed under the license but recommended that developers not choose it for new projects. The FSF identified the license's partial-copyleft provisions, its incompatibility with the GPL, and its allowance for linking with entirely proprietary files as important limitations from the perspective of GPL-oriented software freedom. The FSF also noted a conceptual similarity between APSL's external-derivative source requirement and the source-disclosure approach associated with the GNU Affero General Public License. APSL's most important practical setting has been Apple's publication of Darwin and related operating-system components. The license has not governed every Apple open-source project permanently. As Apple's open-source portfolio evolved, some projects were relicensed under more permissive terms, including the Apache License. The Bonjour Zeroconf networking stack is identified as one example of software that moved away from APSL to a more liberal license. Nevertheless, a substantial portion of source code for Apple's operating-system components has remained associated with APSL. Historically, APSL represents an intermediate licensing strategy. It is more protective of public access to modifications than a fully permissive license, because externally distributed derivatives trigger source-availability obligations. At the same time, it is less expansive than strong copyleft licenses because it permits integration with proprietary files and does not automatically extend its requirements to all linked software. Its development reflects Apple's attempt to support community involvement and source transparency around selected system components while retaining flexibility over the broader architecture and commercial software ecosystem built around them.

1. 2003Version 2.0 released

   Apple released APSL 2.0 on July 29, 2003. The revised license was later recognized by the Free Software Foundation as a free-software license, with reservations about its suitability for new projects.

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2. 2000Initial Apple Public Source License adopted

   Apple introduced the first version of APSL for source-code releases associated with Darwin. The license was approved by the Open Source Initiative.

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A permissive-to-partial-copyleft open-source license designed to publish Apple-developed and Apple-distributed operating-system source code while requiring source availability for externally released derivatives.

• Apple Public Source License 1.0Open-source software license2000

  The initial APSL release used by Apple for early public source-code distribution associated with Darwin. It established the license's model of allowing access to source while requiring source availability for externally released derivative works.

• Apple Public Source License 2.0Open-source software license2003

  The revised APSL published by Apple on July 29, 2003. It retained the license's partial-copyleft approach and became recognized by the Free Software Foundation as a free-software license, though the FSF advised against selecting it for new projects.

• Darwin source-code releasesOperating-system source code2000

  Darwin is the principal software ecosystem historically associated with APSL. Apple released source code for parts of the operating-system foundation under APSL, enabling community review, modification, and redistribution subject to the license's requirements.

• 2003Apple publishes Apple Public Source License 2.0Generation change

  Apple revised its open-source licensing terms after the initial APSL release and issued a new version on July 29, 2003.

  What changed. Apple released version 2.0, which retained partial copyleft and external-derivative source-disclosure requirements.

  Aftermath. The Free Software Foundation recognized APSL 2.0 as a free-software license but recommended that developers not use it for new projects because of its GPL incompatibility and proprietary-linking provisions.

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• 2000Apple adopts an open-source license for DarwinStrategy

  Apple sought to involve the software community in the development of source code associated with Darwin and to publish selected operating-system components under a defined open-source framework.

  What changed. Apple released Darwin-related source code under the first version of the Apple Public Source License.

  Aftermath. The move established APSL as Apple's principal licensing framework for a significant portion of its publicly released operating-system source code.

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• Selected Apple projects move to more permissive licensesStrategy

  Apple's open-source licensing practices evolved, and some software projects were no longer distributed under APSL.

  What changed. Apple relicensed certain projects, including the Bonjour Zeroconf networking stack, under the more permissive Apache License.

  Aftermath. APSL continued to be associated with much of Apple's operating-system component source code, while selected projects adopted less restrictive licensing terms.

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• 2003Apple Public Source License version 2.0 released

  Apple issued version 2.0 of the Apple Public Source License on July 29, 2003. The revised license was subsequently recognized by the Free Software Foundation as a free-software license, while the FSF continued to advise against using it for new projects because of its partial-copyleft structure and GPL incompatibility.

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• 2000Apple releases Darwin under the Apple Public Source License

  Apple used the newly created license for public source-code releases associated with Darwin, opening selected operating-system components to community inspection, modification, and redistribution under defined conditions.

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1. Apple Public Source License
2. Apple Public Source License text