It is possible to write and distribute software, with no license whatsoever, but in the past decade it has become popular to attach to any released software some sort of license, defining the obligations of the person who downloads or modifies the software.
Software (or any other creative work) released with no strings whatsoever (and, in particular, with an irrevocable termination of any creator rights under copyright law) is said to be in the public domain. One large category of public-domain works are those for which copyright has lapsed; this category does not include any working software as copyright has a substantial lifetime (70 years from the author's death, in the United States). Perhaps some of Ada Lovelace's work for the Difference Engine (in the 1840's) can be considered software (though it never ran on anything); it is certainly now in the public domain. Alan Turing died in 1954; his work on programming does not enter the public domain until 2024. There is some legal question whether an author even has the legal ability to place his or her work irrevocably in the public domain, on the theory that potential rights under copyright can never be terminated.
The GNU public license is the earliest, perhaps mostly because the GPL tries to accomplish something legally tricky: it requires that any modifications must remain as open source. When the GPL was first written, people not concerned about this sort of thing would most likely use no license at all.
We will, however, start with the MIT license, which is perhaps the simplest. Here it is, from opensource.org/licenses/MIT:
Copyright <YEAR> <COPYRIGHT HOLDER>
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
The only restrictions are the inclusion of the copyright notice and the waiver of liability. There is no rule that the source must be distributed; the copyright notice can be included in the executable. This is not spelled out explicitly however.
The waiver of liability might not be legally binding (though I am aware of no cases where this has been contested, if the software was distributed for free). You might think this odd, but the legal theory is that no creator of a product can escape negligence liability simply with a waiver. Were this not the case, nothing would stop vehicle manufacturers from claiming they were not liable for poor design. It is easy to claim in court that software errors are due to "negligence".
In 2009 the European Union proposed new laws on software that were intended to make it harder for companies to escape liability for software problems. The way the draft was worded, it appeared to make it impossible for Open Source to escape such liability. However, the laws were ultimately not adopted. Ironically, one version of the laws would have made it possible for software vendors to require that customers waive liability at the time the software was sold. Free software, not being sold, could not benefit from such waivers.
The X Consortium added to the MIT license a paragraph restricting use of the X Consortium name.
Software released under the MIT license can be:
People who see their open-source work as a contribution to society sometimes have an issue with one or more of these. On the other hand, if you want your software to be used, you may find that GPL-style licenses are too restrictive.
Daniel Haxx wrote the cURL package and released it under the MIT license. As a result, Haxx's email address appears in the license terms in odd places. In-car sound systems, in particular, often incorporate cURL, and so desperate users occasionally contact Haxx for help (daniel.haxx.se/blog/2016/11/14/i-have-toyota-corola):
I have Avalon 2016
Regarding the audio player, why there delay between audio and video when connect throw Bluetooth and how to fix it.
Haxx appears to find this mildly entertaining. Sometimes it can be a nuisance.
The original Berkeley Software Distribution of Unix came with the original "four-clause" BSD license. The most common BSD license version today is the following "three-clause" version (opensource.org/licenses/BSD-3-Clause):
Copyright <YEAR> <COPYRIGHT HOLDER>
Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
3. Neither the name of the copyright holder nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
The waiver of liability is more elaborate. Binary distribution is explicitly permitted, as is redistribution. The new clause is that the names of the copyright holders (any of them, as each new contributor may add his or her name) may not be used to promote the program.
It is not clear if the BSD license was originally understood to apply cleanly to improvements added by others; the BSD group likely thought that clause 3 above would apply only to them.
The original BSD license included a fourth clause:
All advertising materials mentioning features
or use of this software must display the following acknowledgement:
This product includes software developed by the <organization>.
That actually is a fairly intrusive requirement, which is why it went away.
The license from the Apache Software Foundation is a bit long to paste in; here's the link: www.apache.org/licenses/LICENSE-2.0. The part you paste in to the source files is shorter:
Copyright [yyyy] [name of copyright owner]
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
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.
The first section of the actual license includes a definition of multiple terms.
The second section includes an authorization under copyright. This authorization allows the creation of derivative works; that is, you can modify the software.
Section three includes a new feature: each contributor must waive any patent rights, or, more specifically, grant a free license to any users of the software. The idea here is that if you have a patent, and release or modify software under the Apache license, you cannot sue other users for patent infringement. Even if those other users make further modifications to the software.
Of course, if a third party holds the patent, they are not bound by the license.
Section four spells out the requirements for redistributing the software. This clause lets you redistribute for sale, but you must attach the original license terms.
Section five says that any contributions by default have the same license, but you are allowed to negotiate different terms.
This is the one that says that if you make changes, and you distribute them, then they too must be open source.
Richard Stallman wrote the first version of the GPL in 1989. This was followed in 1991 by GP v2. To allow the use of GPL libraries in non-free projects, it was accompanied by the "Library GPL", or LGPLv2. GPLv3 was released in 2007.
The actual texts of the licenses are here:
Here is the important clause, that defines the copyleft feature [here and elsewhere, all bold emphasis is added]
By way of explanation, the following clause also appears:
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.
And this, which defines the legal nature of "copyleft":
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.
As far as patents are concerned, GPLv2 states:
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.
Then there is the Library GPLv2. In the preamble it states:
The reason we have a separate public license for some libraries is that they blur the distinction we usually make between modifying or adding to a program and simply using it. Linking a program with a library, without changing the library, is in some sense simply using the library, and is analogous to running a utility program or application program. However, in a textual and legal sense, the linked executable is a combined work, a derivative of the original library, and the ordinary General Public License treats it as such. Because of this blurred distinction, using the ordinary General Public License for libraries did not effectively promote software sharing, because most developers did not use the libraries. We concluded that weaker conditions might promote sharing better.
Here is the LGPL's Section 2 (left), side-by-side with the GPLv2 Section 2 (right).
2. You may modify your copy or copies of the Library or any portion of it, thus forming a work based on the Library, and copy and distribute such modifications or work under the terms of Section 1 above, provided that you also meet all of these conditions: a) The modified work must itself be a software library. b) You must cause the files modified to carry prominent notices stating that you changed the files and the date of any change. c) You must cause the whole of the work to be licensed at no charge to all third parties under the terms of this License. d) If a facility in the modified Library refers to a function or a table of data to be supplied by an application program that uses the facility, other than as an argument passed when the facility is invoked, then you must make a good faith effort to ensure that, in the event an application does not supply such function or table, the facility still operates, and performs whatever part of its purpose remains meaningful.
| 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:
a) You must cause the modified files to carry prominent notices
stating that you changed the files and the date of any change.
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.
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.)
Clause (d) appears to be an effort to ensure that the LGPL can only in fact be used for libraries.
More specific library-related clauses are the following:
5. A program that contains no derivative of any portion of the Library, but is designed to work with the Library by being compiled or linked with it, is called a "work that uses the Library". Such a work, in isolation, is not a derivative work of the Library, and therefore falls outside the scope of this License. However, linking a "work that uses the Library" with the Library creates an executable that is a derivative of the Library (because it contains portions of the Library), rather than a "work that uses the library". The executable is therefore covered by this License. Section 6 states terms for distribution of such executables.
6. As an exception to the Sections above, you may also compile or link a "work that uses the Library" with the Library to produce a work containing portions of the Library, and distribute that work under terms of your choice, provided that the terms permit modification of the work for the customer's own use and reverse engineering for debugging such modifications.
In version 3 of the GPL, the language is, overall, more readable. The following clause is new:
No covered work shall be deemed part of an effective technological measure under any applicable law fulfilling obligations under article 11 of the WIPO copyright treaty adopted on 20 December 1996, or similar laws prohibiting or restricting circumvention of such measures.
You cannot use GPL-covered software as a basis for DRM.
Section 5 contains the copyleft feature:
Section 6 addresses distribution of binary code:
You may convey a covered work in object code form under the terms of sections 4 and 5, provided that you also convey the machine-readable Corresponding Source under the terms of this License, in one of these ways:
Section 6 also addresses another issue, which Stallman calls "Tivoization". TiVo built their DVR device with GNU/linux, and you can get their source code modifications, but their hardware does not allow you to install software with any further modifications made by you or others. The license includes a definition of "User Product" that excludes, say, medical systems.
“Installation Information” for a User Product means any methods, procedures, authorization keys, or other information required to install and execute modified versions of a covered work in that User Product from a modified version of its Corresponding Source. The information must suffice to ensure that the continued functioning of the modified object code is in no case prevented or interfered with solely because modification has been made.
If you convey an object code work under this section in, or with, or specifically for use in, a User Product, and the conveying occurs as part of a transaction in which the right of possession and use of the User Product is transferred to the recipient in perpetuity or for a fixed term (regardless of how the transaction is characterized), the Corresponding Source conveyed under this section must be accompanied by the Installation Information....
The requirement to provide Installation Information does not include a requirement to continue to provide support service, warranty, or updates for a work that has been modified or installed by the recipient, or for the User Product in which it has been modified or installed.
The apparent reason TiVo included this hardware lockdown was to prevent users from grabbing and saving the recorded content in raw digital form.
Section 11 addresses patent claims by contributors:
Each contributor grants you a non-exclusive, worldwide, royalty-free patent license under the contributor's essential patent claims, to make, use, sell, offer for sale, import and otherwise run, modify and propagate the contents of its contributor version.
Also this, in order to address the apparent licensing by Microsoft of some of its patents to Novell only:
You [Novell -- pld] may not convey a covered work if you are a party to an arrangement with a third party [MS -- pld] that is in the business of distributing software, under which you make payment to the third party based on the extent of your activity of conveying the work, and under which the third party grants, to any of the parties who would receive the covered work from you, a discriminatory patent license....
In 2005, Fortinet apparently used GPL-licensed code in ways that violated the license. Ultimately they had to release source code for their FortiOS system.
In 2006 a German court upheld the validity of the GPL in a lawsuit against D-Link.
In 2007, developers of the GPL BusyBox package sued Monsoon, which incorporated BusyBox but refused to release their modified source code. Monsoon eventually settled, opening their source and paying unspecified damages.
In 2013 a Hamburg court found that Fantec GmbH had violated the GPL in the distribution of a game module that made use of netfilter.
In 2016 a German court dismissed kernel dev Christoph Hellwig's lawsuit against VMware for GPL violation. Ultimately the court dismissed the case, on the grounds that Hellwig could not identify the specific code sections written by Hellwig that VMware had used.
In April 2017, California federal judge Jacqueline Corley ruled, in the case Artifex v Hancom, that the GPL was binding even though Hancom never signed anything. Artifex offered its Ghostscript pdf-rendering software on both GPL and commercial licensing terms. Hancom used the GPL version, modified it, violated the GPL, and tried to claim the GPL was non-binding.
See also copyleft.org/guide/comprehensive-gpl-guidepa3.html.
As one final observation, see Open Source Software: An Open Door to Intellectual Property Liability. It concludes with
In the end, it may be less expensive to pay more for commercial software, if only to purchase the benefit of the indemnification that typically runs with the license.
But open-source licensing is not that hard to deal with. The intent is usually very clear.