소프트웨어에 관련된 다섯 가지 의무

GNU 일반 공중 사용 허가서는 누구에게나 다음의 다섯 가지의 의무를 저작권의 한 부분으로서 강제한다.

  1. 컴퓨터 프로그램을 어떠한 목적으로든지 사용할 수 있다. 다만 법으로 제한하는 행위는 할 수 없다.
  2. 컴퓨터 프로그램의 실행 복사본은 언제나 프로그램의 소스 코드와 함께 판매하거나 소스코드를 무료로 배포해야 한다.
  3. 컴퓨터 프로그램의 소스 코드를 용도에 따라 변경할 수 있다.
  4. 변경된 컴퓨터 프로그램 역시 프로그램의 소스 코드를 반드시 공개 배포해야 한다.
  5. 변경된 컴퓨터 프로그램 역시 반드시 똑같은 라이선스를 취해야 한다. 즉 GPL 라이선스를 적용해야 한다.

Copyleft

The distribution rights granted by the GPL for modified versions of the work are not unconditional. When someone distributes a GPL'd work plus his/her own modifications, the requirements for distributing the whole work cannot be any greater than the requirements that are in the GPL.

This requirement is known as copyleft. It earns its legal power from the use of copyright on software programs. Because a GPL work is copyrighted, a licensee has no right to redistribute it, not even in modified form (barring fair use), except under the terms of the license. One is only required to adhere to the terms of the GPL if one wishes to exercise rights normally restricted by copyright law, such as redistribution. Conversely, if one distributes copies of the work without abiding by the terms of the GPL (for instance, by keeping the source code secret), he or she can be sued by the original author under copyright law.

Copyleft thus uses copyright law to accomplish the opposite of its usual purpose: instead of imposing restrictions, it grants rights to other people, in a way that ensures the rights cannot subsequently be taken away. It also ensures that unlimited redistribution rights are not granted, should any legal flaw be found in the copyleft statement.[citation needed]

Many distributors of GPL'ed programs bundle the source code with the executables. An alternative method of satisfying the copyleft is to provide a written offer to provide the source code on a physical medium (such as a CD) upon request. In practice, many GPL'ed programs are distributed over the Internet, and the source code is made available over FTP or HTTP. For Internet distribution, this complies with the license.

Copyleft applies only when a person seeks to redistribute the program. Developers may make private modified versions with no obligation to divulge the modifications, as long as they do not distribute the modified software to anyone else. Note that copyleft applies only to the software, and not to its output (unless that output is itself a derivative work of the program). For example, a public web portal running a modified derivative of a GPL'ed content management system is not required to distribute its changes to the underlying software, because its output is not a derivative.

There has been debate on whether it is a violation of the GPL to release the source code in obfuscated form, such as in cases in which the author is less willing to make the source code available. The consensus was that while unethical, it was not considered a violation. The issue was clarified when the license was altered with v2 to require that the "preferred" version of the source code be made available.