The GNU's General Public License, or GPL, is one of the foundation open source licenses. Created by the Free Software Foundation (FSF), which has made many contributions to open source coding, it is the preferred license for projects authorized by the FSF, including the GNU Emacs Editor and the GNU C Compiler, among literally scores of others, including the GNU/Linux kernel.

The intentions behind the license and the premise underlying it are explained in the license's preamble, which is included here in its entirety. The preamble follows the copyright notice,^[2] and a notice that prevents modifications, ironically enough, to the license itself: "Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed." While the license permits the creation of derivative works from the licensed code, it does not permit the creation of derivative licenses from the license itself.

This preamble clearly and concisely sets out the three main purposes of the GPL. The first, and by far the most important, is to keep software free, in the sense that it can be distributed and modified without additional permission of the licensor. This imposes a mirror-image restriction on the licensee: while the licensee has free access to the licensed work, the licensee must distribute any derivative works subject to the same limitations and restrictions as the licensed work. The second purpose of the GPL is to ensure that licensees are aware that software under the license is distributed "as is" and without warranty. This purpose is not unique to the GPL, as we have seen. The third purpose (which is really a variant of the first) is that the licensed software be free of restrictive patents: to the extent that a patent applies to the licensed software, it must be licensed in parallel with the code. As we discussed in Chapter 1, a given piece of code may be subject to both a copyright and a patent. In order for the GPL to function properly, both copyright and patent licenses must be subject to the terms of the GPL.

The individual provisions of the license articulate each of these purposes in some detail. The GPL License is written with a great deal more specificity and in substantially more detail than the licenses described in the previous chapter. This meticulousness is obvious in the license's first provision, which defines the scope of the license and its critical terms.

The term "Program" is roughly equivalent to what this book has described previously as "work," the term "work based on the Program" to derivative work, and the term "you" to licensee.^[4] The exclusion of activities other than copying, modifying, or distributing the program or a work based on it is typical of the meticulousness of this license. This exclusion could reasonably be assumed to apply to the licenses discussed in Chapter 2, but only here is it specifically described.

The next provision describes all of the limitations that apply to distribution of the licensed work.

This provision embodies the most important principles with regard to the distribution of the original work. The original licensed work can be distributed or sold by a licensee. This provision by itself creates the opportunity for a profitable business—any person can simply acquire and package GPL-licensed software, perhaps bundle it with an appropriate manual,^[5] market it, and sell it. There is no need to "add value" other than by making the work available in a format convenient to consumers. The limitation, obviously, to this business model is that any other person is equally free to start a business on the same principles and distribute the same work or works. This is not necessarily fatal to such businesses. Businesses do not need to be monopolies in order to prosper. The FSF itself derives a substantial amount of income from distributing its own "free" works.

The other business model identified by this provision is the warrantying of a particular work. Any person can take a GPL-licensed work and sell a guarantee that the work will perform a particular function and make whatever changes or modifications to the work are necessary to achieve that goal. As previously noted, most open source licenses, including the GPL, expressly disclaim warranties of any kind. However, businesses strongly prefer to have reliable software, and, in particular, to have software that is backed up by knowledgeable professionals who are capable of adapting it to particular purposes and situations. This type of "value-adding" is expressly authorized by the GPL.

The second paragraph of the GPL is its most important, as it embodies the FSF idea of " copyleft," a variety of the generational limitation described in Chapter 1, which requires that derivative works be subject to the terms of the GPL and only the terms of the GPL.

This is the first part of copyleft: subject to certain restrictions, modifications to the work or any part of it are permitted.

Like the restriction imposed by the BSD license, this provision serves to ensure that users are aware that the derivative work is not identical to the original work and to identify the person or persons who are responsible for the changes. This is intended to protect users and to protect the reputations of creators of work against injury arising from flawed derivative works.

This key part of the second paragraph of the GPL is the most important provision of the license. Derivative works must be licensed under the GPL and be subject to all of its restrictions. Unlike works licensed under the MIT or the BSD License, works derivative of work licensed under the GPL (or the original work itself) may not be made proprietary or otherwise limited in their distribution. If a programmer is looking to create proprietary works, the entire universe of GPL-licensed software is closed off to her. Indeed, as described in Chapter 6, the inclusion of any GPL-licensed code in purportedly proprietary software could prevent the creator of that software from enforcing any of the rights otherwise available under copyright: any person could distribute, sell, or modify that software, in disregard of any rights that would otherwise be granted the creator under the copyright laws.^[6]

This provision is a necessary complement to provision 2(b). The fact that licensees of the derivative work may freely exercise rights under the GPL is of little importance unless those licensees know that they can exercise those rights. This provision attempts to inform those licensees of those rights.

It is not uncommon for a particular program to be capable of both integration with other software to form a unified whole, such as into a calculator program that performs a variety of functions, and also functioning with minimal or no modifications as a separate entity, such as a program that only calculates square roots. This provision of the GPL allows the author of such software to license the software under another license (typically, a proprietary one) when distributed by itself and under the GPL when the program is distributed as part of a larger work, including GPL-licensed programs. This may provide some benefit to the software developer, but probably not if the developer chooses to distribute the GPL-licensed software publicly. In general, consumers would prefer to acquire the GPL-licensed work, which will likely have greater functionality and be more cheaply available, than to acquire the more limited proprietary work.

This provision may be of some comfort to software developers who are creating software primarily for their own use "in-house." Presumably, such developers could write programs or functions designed to work with GPL-licensed programs and simply limit the distribution of that GPL-licensed code to persons within the organization. If the developers' own code got to the point where it could be commercially distributed on its own, the developers could, with confidence, "disengage" that code from the GPL-licensed code and distribute it as part of standalone programs under a proprietary license or otherwise.

This provision serves as a safeguard against overly broad interpretations of the GPL. This makes explicit what is implicit in the rest of the license: the provisions of the GPL are not contagious, like a cold. Mere proximity does not cause the license to govern a particular piece of code. To fall under the copyleft, the code must be integral to and/or derivative of a program that is GPL-licensed.

The following provisions of the GPL require that the licensees of the GPL-licensed code make available in one of two ways the source code to the program. The right to create derivative works from a program is obviously limited in practice if the source code is not available.

Note that this requirement is equally applicable to derivative works created under Section 2 of the GPL.

This is the most favored way to make source code available. It requires no additional effort from the distributee and is not time-limited. This is the best way to comply with Section 3 for all but the largest programs.

This option furthers the purposes of open source and free software but does so in a way that imposes additional costs on both licensors and licensees. The licensor must maintain a facility for providing copies of the source code; the licensee interested in creating the derivative work must contact and pay for the copying of the source code. Moreover, this provision is limited to three years, which could result in potentially useful software "going closed" as a practical matter (at least for the creation of derivative works) once the licensor ceases making the source code available.

Section 3(c) allows noncommercial distributors of GPL-licensed software to "piggyback" on the original licensor's offer to make the source code available, if the source code of such software was originally made available under Section 3(b).

The following paragraph of the GPL defines "source code" as that term is used in the license.

This limits the size of the source code that needs to be provided by narrowing the definition of program to exclude major components, like the operating system the program is intended to run on. Obviously, if the GPL-licensed programs being distributed (or one or more of them) are themselves major components of an operating system, the source code for those components must be made available, as described in 3(a-c).

This is another provision that explains in greater detail something already implicitly stated elsewhere in the license. Offering access to copy the source in the same manner and with the same degree of ease as the executable code is sufficient to comply with the requirements of Section 3(a).

The first part of Section 4 of the GPL identifies the license as the exclusive license for use of the licensed software.

In the event that a licensee violates any term of the GPL by, for example, distributing a proprietary derivative work based on GPL-licensed code, all rights under the GPL are voided. This brings back into play the ordinary protections of copyright law (and of patent law, if applicable) described in Chapter 1. In the event of such a breach, the ex-licensee would become legally liable to the licensor for violation of the copyright. The licensor could enjoin the ex-licensee from distributing the derivative work and could sue for damages, which could include, among other things, any and all profits the ex-licensee made from distributing the derivative work. This scenario is described in more detail in Chapter 7.

This sentence acts as a savings clause, preventing liability from attaching to those persons who received the licensed work or a GPL-distributed derivative work from the ex-licensee.

Section 5 addresses a problem that applies to almost all software licenses: the uncertainty as to whether a binding contract is in fact created between the licensor and licensee.

While no court has yet ruled on the effect of this provision, it is likely enforceable. As noted in Chapter 1, courts have found that "shrinkwrap" licenses—proprietary licenses that the licensee accepts by breaking the shrinkwrap on commercial software—are enforceable. The GPL can rest firmly on the fundamental (and intrinsic) protection of copyright. The licensor owns every part of the work and any use of it (excepting "fair use") is infringement. The potential licensee is thus faced with a choice: either refuse the GPL, which bars almost every use of the licensed work, or accept it, and use the work as permitted by the GPL. As described in more detail in Chapter 6, knowledge of the applicable license should be implied even as to putative licensees who have no actual knowledge of the license. Some degree of diligence should be required of such users: if they truly believed that there was "no license" applicable to the program, they should have made no use of it at all other than the very limited uses permitted by copyright law.

Section 6 of the GPL creates a relationship between the licensor and each of the licensees, regardless of the number of generations of distribution that may lay between them.

The GPL should be effective, regardless of the number of distributions through which it passes, because of the limitations and requirements of Sections 1 through 4. This provision, therefore, acts in some way as a back-up to those sections. More importantly, however, it also tries to create contractual privity between the licensor of the original work and all the licensees of that work.

Contractual privity is the legal state between two (or more) parties in which they are bound by contractual obligations to each other. In the GPL, it safeguards the standing of the licensor to bring a lawsuit against all the licensees of the work. Standing is a legal term of art, but, in simple terms, it means essentially that the person with standing to sue has been directly injured in some way by another such that that person has the right to bring an action for relief. Without this contractual privity creating the standing on the part of the licensor to sue, a licensee of the work could argue that she did not receive the license to use the work from the original licensor, but rather from some intermediate distributor (who may have no interest at all in defending the terms of the license), and that, accordingly, only that intermediate distributor has standing to sue for putative violations of the license. Section 6 attempts to head off this argument, by creating a relationship between the original licensor and all licensees of the work, regardless of the number of distributors.

The second sentence of Section 6 is the mirror image of Section 2(b). As that section required that derivative works be distributed subject to the restrictions of the license, so this sentence prohibits the addition of any restrictions to those present in the GPL.

As described in Chapter 6, this limitation has significant consequences on the compatibility of the GPL with other licenses.

The third sentence prevents liability from attaching to innocent distributors for license violations committed by distributees or any other person.

Section 7 prevents any outside act, including court judgments premised on patent rulings or otherwise, from limiting or altering the terms of the license.

This section is constructed so that in the event that any court attempts to limit or modify the license by imposing obligations or restrictions inconsistent with the GPL, the license for all practical purposes ceases to exist.^[7] Because of this, patent issues remain something GPL developers need to watch.

In practice, this could have dire consequences on the consumers of GPL-licensed software. Say, for example, that a small software company determines that part of a widely distributed and used GPL-licensed program, such as the GNU/Linux kernel, infringes on a software patent that it holds. The company brings suit and a court determines that the program infringes on the patent. Because the infringing part of the program is relatively trivial, the court determines that the appropriate remedy is for every licensee to pay a one-time fee of one dollar to the company. While both current and future licensees (or at least some of them) would gladly pay the fee and continue to use the software, this payment, because it is a restriction not part of the GPL license, is inconsistent with the license. Accordingly, Section 7, were it to be enforced, would bar any distribution of the program after the court judgment.^[8]

Like the rest of the GPL, this section has not been interpreted by a court. However, it is unlikely that a court would allow this section to limit its own power to grant relief. It is certainly not impossible that a court in adjudicating such a dispute would give notice to the licensor and permit the licensor to make appropriate arguments concerning the license, but would then grant relief that would essentially rewrite the GPL in favor of the injured party and permit licensees to continue to copy, distribute, and modify the affected program. This is, of course, only my speculation.

This sentence is another variety of the savings clause, intended to preserve the remaining parts of this section even if some part of it is invalidated by a court. It is hard to see, however, what effect any part of this section could have if the critical part of it is superseded by a court, as described earlier.

The following part of the section does not really apply a legal limitation on licensees as much as it articulates a defense of Section 7.

The thesis is that the licensor's choice to use the GPL license is, in some sense, a political one, and that choice should be protected and defended against encroachment. Licensees, obviously, may see the situation differently.

Although the potential results from the application of Section 7 may seem draconian, Section 7 is probably necessary to protect the integrity of the GPL and of the GPL distribution model. The license prevents licensees from altering the GPL contractually, through provisions that are very likely to be enforced. However, private parties are not the only entities capable of altering legal obligations. Courts have an even greater power, to alter, to cancel, and to rewrite contracts to effect appropriate relief on any number of grounds. The GPL's use of a strategy of "if we're not playing my game, I'm taking my ball and going home" is probably necessary to prevent the model from being undermined by courts. How the courts will react to the restriction of Section 7, however, is still unknown.

Section 8 addresses a similar problem, where the laws of certain jurisdictions would limit or otherwise modify the GPL.

To the extent that there are jurisdictions in which the licensor is limited from licensing the program due by pre-existing patents or copyrights, the licensor is free to carve them out from the area in which the GPL is effective. This gives the licensor maximum flexibility, by permitting the GPL-licensed software to spread as widely as possible, if it is restricted in certain jurisdictions. This is an example of an area in which the GPL can itself be modified, at least under one set of circumstances.

Section 9 of the GPL gives notice that the FSF may issue updated or revised versions of the license.

Unlike most of the other parts of the GPL, this provision really serves to give notice to potential licensors—i.e., those who choose to use the GPL to license a new program—not to licensees.

This paragraph makes clear that in order to preserve specific guarantees or rights, licensors should identify the GPL version used by version number. If they do not, the licensee can exercise rights under any of the GPL licenses. Moreover, if the licensor adds the language "and any later license" following the identification of the version number, the licensee can exercise the rights under that version and any subsequent version of the GPL.

A different option, albeit not one permitted by the GPL, would be the inclusion of language to the effect that "this software is licensed under GPL Version X.Y. This license is subject to periodic revision and amendment by the Free Software Foundation. Upon publication of such a revised or amended license by the Free Software Foundation, such revised or amended license is deemed to have superseded the license previously applicable to this software, and such revised or amended license shall from that time govern the contractual relationship between licensors and licensees. Accordingly, any further copying, distribution, or modification of this software after that time will be subject to the terms of the revised and amended license. You have the obligation to track such revisions and amendments to the GPL."^[9]

This option was not included in the GPL, most likely because it further complicates the already somewhat thorny issues related to providing notice of the license to licensees and forming a binding contract between licensor and licensee described in Chapter 6. It is one thing to expect a licensee to be bound by the terms of a license, which are made clear to the licensee upon the first use of the program; it may be something entirely different to require that licensee to track the actions of the FSF and conform behavior accordingly. Nonetheless, if those issues could be addressed, this option could offer some benefits, particularly in allowing the FSF to address threats to the GPL, such as those described in Section 8.

Section 10 is less a binding provision than an explanation to licensees as to how to address the GPL's incompatibility with other licenses.

As we have seen, the second sentence of Section 6 of the GPL bars licensees from imposing any additional restrictions on recipients' exercise of rights under the license, and Section 4 terminates all rights under the license in the event that any provision is not complied with. The effect of these two sections is to make the GPL incompatible with most other open source licenses.^[10] Section 10 provides a possible solution, although one that may be impractical in many situations. The original licensor of the program, holding the copyright to the software and having licensed the software under the GPL, cannot withdraw or alter the terms of the license already granted; the licensor, however, in addition to licensing the software under the GPL License, can also license it under another license, such as the Artistic License. If the original licensor is willing to undertake such parallel licensing, the code can be made available under a non-GPL compatible license and thereby avoid the problem.

The rest of the license consists substantially of disclaimers of warranty similar to those in the licenses described in the previous chapters. These disclaimers are also in all-caps.

One interesting, potentially significant distinction between this disclaimer of liability and those discussed in the previous chapter is that the disclaimer does not expressly disclaim liability for "direct damages." As discussed in the previous chapter, direct damages are measured by the price of the software alleged to be defective. This decision to exclude direct damages may be deliberate—it would not be inconsistent with the ideas underlying the GPL to hold distributors liable for the price of the software, for example, if it was poorly copied. However, the better reading of the provision is that it disclaims all damages, and that the list of "general, specific [etc.]" damages that are disclaimed is illustrative, not definitive.

More importantly, Sections 11 and 12 permit one kind of modification to the GPL, in that they permit a separate written agreement between two parties to establish warranties or permit suits for damages. One business model that is available for open source is the provision of warranties and maintenance of open source and free software. The GPL does not prohibit the provision of such services by inserting these exceptions into Sections 11 and 12. Such services are also explicitly authorized in Section 1.

As noted in the notice immediately following Section 12, this is the close of the provisions of the license. The remainder of the GPL text is instructions for implementing the license, which follow here: