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, 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.
 The license described is Version 2.0 of the GPL and is Copyright © 1989, 1991 Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA.
Copyright (C) 1989, 1991 Free Software Foundation, Inc. 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change free software to make sure the software is free for all its users. This General Public License applies to most of the Free Software Foundation's software and to any other program whose authors commit to using it. (Some other Free Software Foundation software is covered by the GNU Library General Public License instead.) You can apply it to your programs, too.
When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things.
To protect your rights, we need to make restrictions that forbid anyone to deny you these rights or to ask you to surrender the rights. These restrictions translate to certain responsibilities for you if you distribute copies of the software, or if you modify it.
For example, if you distribute copies of such a program, whether gratis or for a fee, you must give the recipients all the rights that you have. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights.
We protect your rights with two steps: (1) copyright the software, and (2) offer you this license which gives you legal permission to copy, distribute and/or modify the software.
Also, for each author's protection and ours, we want to make certain that everyone understands that there is no warranty for this free software. If the software is modified by someone else and passed on, we want its recipients to know that what they have is not the original, so that any problems introduced by others will not reflect on the original authors' reputations.
Finally, any free program is threatened constantly by software patents. We wish to avoid the danger that redistributors of a free program will individually obtain patent licenses, in effect making the program proprietary. To prevent this, we have made it clear that any patent must be licensed for everyone's free use or not licensed at all.
The precise terms and conditions for copying, distribution and modification follow.
 The most current GNU Library General Public License is now known as the GNU Lesser General Public License (LGPL) and is described in more detail later in this chapter.
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.
GNU GENERAL PUBLIC LICENSE
TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
0. This License applies to any program or other work which contains a notice placed by the copyright holder saying it may be distributed under the terms of this General Public License. The "Program", below, refers to any such program or work, and a "work based on the Program" means either the Program or any derivative work under copyright law: that is to say, a work containing the Program or a portion of it, either verbatim or with modifications and/or translated into another language. (Hereinafter, translation is included without limitation in the term "modification".) Each licensee is addressed as "you".
Activities other than copying, distribution and modification are not covered by this License; they are outside its scope. The act of running the Program is not restricted, and the output from the Program is covered only if its contents constitute a work based on the Program (independent of having been made by running the Program). Whether that is true depends on what the Program does.
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. 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 terms "work" and "derivative work" are terms of art defined by copyright law.
The next provision describes all of the limitations that apply to distribution of the licensed work.
1. You may copy and distribute verbatim copies of the Program's source code as you receive it, in any medium, provided that you conspicuously and appropriately publish on each copy an appropriate copyright notice and disclaimer of warranty; keep intact all the notices that refer to this License and to the absence of any warranty; and give any other recipients of the Program a copy of this License along with the Program.
You may charge a fee for the physical act of transferring a copy, and you may at your option offer warranty protection in exchange for a fee.
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, 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 application of open source licensing principles to works other than software, including manuals, is described in Chapter 5.
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.
2. You may modify your copy or copies of the Program or any portion of it, thus forming a work based on the Program, and copy and distribute such modifications or work under the terms of Section 1 above, provided that you also meet all of these conditions:
This is the first part of copyleft: subject to certain restrictions, modifications to the work or any part of it are permitted.
a) You must cause the modified files to carry prominent notices stating that you changed the files and the date of any change.
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.
b) You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License.
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.
 Whether such a person would in turn be bound by the GPL is discussed in more detail in Chapter 6.
c) If the modified program normally reads commands interactively when run, you must cause it, when started running for such interactive use in the most ordinary way, to print or display an announcement including an appropriate copyright notice and a notice that there is no warranty (or else, saying that you provide a warranty) and that users may redistribute the program under these conditions, and telling the user how to view a copy of this License. (Exception: if the Program itself is interactive but does not normally print such an announcement, your work based on the Program is not required to print an announcement.)
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.
These requirements apply to the modified work as a whole. If identifiable sections of that work are not derived from the Program, and can reasonably be considered independent and separate works in themselves, then this License, and its terms, do not apply to those sections when you distribute them as separate works. But when you distribute the same sections as part of a whole which is a work based on the Program, the distribution of the whole must be on the terms of this License, whose permissions for other licensees extend to the entire whole, and thus to each and every part regardless of who wrote it.
Thus, it is not the intent of this section to claim rights or contest your rights to work written entirely by you; rather, the intent is to exercise the right to control the distribution of derivative or collective works based on the Program.
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.
In addition, mere aggregation of another work not based on the Program with the Program (or with a work based on the Program) on a volume of a storage or distribution medium does not bring the other work under the scope of this License.
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.
3. You may copy and distribute the Program (or a work based on it, under Section 2) in object code or executable form under the terms of Sections 1 and 2 above provided that you also do one of the following:
Note that this requirement is equally applicable to derivative works created under Section 2 of the GPL.
a) Accompany it with the complete corresponding machine-readable source code, which must be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or,
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.
b) Accompany it with a written offer, valid for at least three years, to give any third party, for a charge no more than your cost of physically performing source distribution, a complete machine-readable copy of the corresponding source code, to be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or,
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.
c) Accompany it with the information you received as to the offer to distribute corresponding source code. (This alternative is allowed only for noncommercial distribution and only if you received the program in object code or executable form with such an offer, in accord with Subsection b above.)
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.
The source code for a work means the preferred form of the work for making modifications to it. For an executable work, complete source code means all the source code for all modules it contains, plus any associated interface definition files, plus the scripts used to control compilation and installation of the executable. However, as a special exception, the source code distributed need not include anything that is normally distributed (in either source or binary form) with the major components (compiler, kernel, and so on) of the operating system on which the executable runs, unless that component itself accompanies the executable.
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).
If distribution of executable or object code is made by offering access to copy from a designated place, then offering equivalent access to copy the source code from the same place counts as distribution of the source code, even though third parties are not compelled to copy the source along with the object code.
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.
4. You may not copy, modify, sublicense, or distribute the Program except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense or distribute the Program is void, and will automatically terminate your rights under this License.
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.
However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.
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.
5. You are not required to accept this License, since you have not signed it. However, nothing else grants you permission to modify or distribute the Program or its derivative works. These actions are prohibited by law if you do not accept this License. Therefore, by modifying or distributing the Program (or any work based on the Program), you indicate your acceptance of this License to do so, and all its terms and conditions for copying, distributing or modifying the Program or works based on it.
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.
6. Each time you redistribute the Program (or any work based on the Program), the recipient automatically receives a license from the original licensor to copy, distribute or modify the Program subject to these terms and conditions.
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.
You may not impose any further restrictions on the recipients' exercise of the rights granted herein.
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.
You are not responsible for enforcing compliance by third parties to this License.
Section 7 prevents any outside act, including court judgments premised on patent rulings or otherwise, from limiting or altering the terms of the license.
7. If, as a consequence of a court judgment or allegation of patent infringement or for any other reason (not limited to patent issues), conditions are imposed on you (whether by court order, agreement or otherwise) that contradict the conditions of this License, they do not excuse you from the conditions of this License. If you cannot distribute so as to satisfy simultaneously your obligations under this License and any other pertinent obligations, then as a consequence you may not distribute the Program at all. For example, if a patent license would not permit royalty-free redistribution of the Program by all those who receive copies directly or indirectly through you, then the only way you could satisfy both it and this License would be to refrain entirely from distribution of the Program.
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. Because of this, patent issues remain something GPL developers need to watch.
 For an interesting discussion of the effects of the bringing of a patent infringement action on a licensor's ability to continue to distribute under the GPL a work that the licensor itself claims violates its own patent rights and the subsequent effect of this on the GPL's compatibility with the Apache License, v2.0, see http://www.apache.org/licenses/GPL-compatibility.html. As of this writing, the FSF has taken the position that the Apache License, v2.0, is incompatible with the GPL because the Apache License, v2.0, has a slightly different treatment of this scenario.
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.
 Because Section 7 refers only to distribution, and because Section 0 limits the application of the license to "copying, distribution and modification," the licensees could continue to run the affected program. However, the licensees could not copy, distribute, or modify the program, drastically limiting its usefulness in the open source/free software model.
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.
If any portion of this section is held invalid or unenforceable under any particular circumstance, the balance of the section is intended to apply and the section as a whole is intended to apply in other circumstances.
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.
It is not the purpose of this section to induce you to infringe any patents or other property right claims or to contest validity of any such claims; this section has the sole purpose of protecting the integrity of the free software distribution system, which is implemented by public license practices. Many people have made generous contributions to the wide range of software distributed through that system in reliance on consistent application of that system; it is up to the author/donor to decide if he or she is willing to distribute software through any other system and a licensee cannot impose that choice.
This section is intended to make thoroughly clear what is believed to be a consequence of the rest of this License.
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.
8. If the distribution and/or use of the Program is restricted in certain countries either by patents or by copyrighted interfaces, the original copyright holder who places the Program under this License may add an explicit geographical distribution limitation excluding those countries, so that distribution is permitted only in or among countries not thus excluded. In such case, this License incorporates the limitation as if written in the body of this License.
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.
9. The Free Software Foundation may publish revised and/or new versions of the General Public License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns.
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.
Each version is given a distinguishing version number. If the Program specifies a version number of this License which applies to it and "any later version", you have the option of following the terms and conditions either of that version or of any later version published by the Free Software Foundation. If the Program does not specify a version number of this License, you may choose any version ever published by the Free Software Foundation.
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."
 A similar provision is contained in the Mozilla Public License described later in this chapter.
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.
10. If you wish to incorporate parts of the Program into other free programs whose distribution conditions are different, write to the author to ask for permission. For software which is copyrighted by the Free Software Foundation, write to the Free Software Foundation; we sometimes make exceptions for this. Our decision will be guided by the two goals of preserving the free status of all derivatives of our free software and of promoting the sharing and reuse of software generally.
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. 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.
 Not all licenses are incompatible, however. For a list of licenses the Free Software Foundation considers compatible with the GPL, which include the MIT (or X license) and the post-1999 BSD license, see http://www.gnu.org/licenses/license-list.html.
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.
11. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
12. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.
END OF TERMS AND CONDITIONS
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:
How to Apply These Terms to Your New Programs
If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms.
To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the "copyright" line and a pointer to where the full notice is found.
one line to give the program's name and a brief idea of what it does.Copyright (C)
This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
Also add information on how to contact you by electronic and paper mail.
If the program is interactive, make it output a short notice like this when it starts in an interactive mode:
Gnomovision version 69, Copyright (C) year name of author Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'. This is free software, and you are welcome to redistribute it under certain conditions; type `show c' for details.
The hypothetical commands `show w' and `show c' should show the appropriate parts of the General Public License. Of course, the commands you use may be called something other than `show w' and `show c'; they could even be mouse-clicks or menu items whatever suits your program.
You should also get your employer (if you work as a programmer) or your school, if any, to sign a "copyright disclaimer" for the program, if necessary. Here is a sample; alter the names:
Yoyodyne, Inc., hereby disclaims all copyright interest in the program `Gnomovision' (which makes passes at compilers) written by James Hacker.
signature of Ty Coon, 1 April 1989Ty Coon, President of Vice
This General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Library General Public License instead of this License.