united-states copyright licensing
Is a copyright license by default revocable or irrevocable?
<p>If I grant someone a license to use a copyrighted work of mine, do I need to detail my ability to revoke the license within the terms of the license grant, or is that ability inherent to all licenses?</p>

<p>I understand that a copyright license could have terms like, "This license is irrevocable" (as the <a href="http://www.gnu.org/licenses/gpl.html">GPLv3 does</a>) or, "This license may be revoked under any of the following circumstances..." Absent any explicit language about revocability or irrevocability, what is the default rule for revoking a license?</p>

<p>I'm interested in copyright licenses that are granted publicly to anyone who wants them (e.g., how open source software licenses are typically granted), but also interested in the general rule, including licenses granted to specific parties.</p>

<p>Basically, if you own it you get a chance in 35 years to make changes. You must provide notice, date, 2-10 years before the date, and record the notice with the copyright office. Any derivative works during the time of the license continue to be licensed even after revocation.</p>

<p>Companies that have a GPL and want to change it typically just increase the version number and change the license at that time. There, the new license applies to that version on, and the older versions stay with the GPL.</p>

<hr>

<p><a href="https://www.law.cornell.edu/uscode/text/17/203">17 U.S.C. S203</a>:</p>

<blockquote>
  <p>In the case of a grant executed by one author, termination of the grant may be effected by that author or, if the author is dead, by the person or persons who, under clause (2) of this subsection, own and are entitled to exercise a total of more than one-half of that author’s termination interest. [There are many conditions on if one person or another has rights.] . . . </p>
  
  <p>Termination of the grant may be effected at any time during a period of five years beginning at the end of thirty-five years from the date of execution of the grant; or, if the grant covers the right of publication of the work, the period begins at the end of thirty-five years from the date of publication of the work under the grant or at the end of forty years from the date of execution of the grant, whichever term ends earlier. </p>
  
  <p>The termination shall be effected by serving an advance notice in writing, signed by the number and proportion of owners of termination interests required under clauses (1) and (2) of this subsection, or by their duly authorized agents, upon the grantee or the grantee’s successor in title.  </p>
  
  <blockquote>
    <p>(A) The notice shall state the effective date of the termination, which shall fall within the five-year period specified by clause (3) of this subsection, and the notice shall be served not less than two or more than ten years before that date. A copy of the notice shall be recorded in the Copyright Office before the effective date of termination, as a condition to its taking effect.  </p>
    
    <p>(B) The notice shall comply, in form, content, and manner of service, with requirements that the Register of Copyrights shall prescribe by regulation.  </p>
  </blockquote>
  
  <p>(5) Termination of the grant may be effected notwithstanding any agreement to the contrary, including an agreement to make a will or to make any future grant. </p>
</blockquote>

<p>While the question assumes that irrevocability clauses are legally effective, it is sometimes suggested that even the GPL is a gratuitous licence which is revocable at will. For example, see:</p>
<ul>
<li><p>this <a href="https://softwareengineering.stackexchange.com/questions/209036/does-an-open-source-license-exist-that-allows-me-to-retain-rights-to-revoke-usag#209064">Software Engineering Stack Exchange answer</a> (2013) citing an <a href="https://web.archive.org/web/20160312022558/http://www.ilaw.com.au/public/licencearticle.html" rel="nofollow noreferrer">article by an Australian lawyer</a> (2003)</p>
</li>
<li><p>this <a href="https://news.ycombinator.com/item?id=14556931" rel="nofollow noreferrer">Hacker News thread</a> (2017)</p>
</li>
<li><p>this <a href="https://hackaday.com/2018/09/27/can-you-take-back-open-source-code/" rel="nofollow noreferrer">Hackaday article</a> (2018) citing a <a href="https://lkml.org/lkml/2018/9/23/212" rel="nofollow noreferrer">post to the Linux Kernel Mailing list</a> (2018)</p>
</li>
</ul>
<p>Andrew's answer confirms that in the U.S., even a licence which was paid for and agreed to be irrevocable can be revoked after 35 years. However, as explained in the comments, I am not sure if 17 U.S.C. § 203 prevents revocation of a gratuitous licence within 35 years. In any case, I am more interested in the law in Commonwealth countries.</p>
<p>The most authoritative analysis I have been able to find is contained in Fitzgerald and Suzor, <a href="http://www.mulr.com.au/issues/29_2/29_2_3.pdf#page=26" rel="nofollow noreferrer">Legal issues for the use of free and open source software in government</a> (2005) 29(2) <em>Melbourne University Law Review</em> 412 at 437 (footnotes omitted):</p>
<blockquote>
<p>… a gratuitous licence can normally be revoked at will. This means that, in the case where one single entity controls a significant portion of the copyright in the source code for a free software package, that entity may be able to terminate the licence and users will no longer be entitled to copy or redistribute the software … <strong>In the event that a licence is revoked, it is likely that the doctrine of estoppel would prevent the copyright owner from asserting his or her rights.</strong> Equitable estoppel has been developed to prevent a person from unconscionably denying an expectation where they induce in another party (here the licensee) an assumption that a particular legal relationship exists between them, and that party subsequently acts, reasonably, in reliance upon that expectation. If a licensor releases software under a free software licence, they are essentially inviting others to perpetually use, reproduce, modify and distribute that software. If another person does in fact make use of the software, and the original licensor purports to revoke the licence (a departure clearly to that person’s detriment), the doctrine of equitable estoppel would arguably prevent the licensor from denying that the licence could not be revoked.</p>
</blockquote>
<p>The authors conclude that the issue is ultimately of more theoretical than practical concern:</p>
<blockquote>
<p>In practical terms, however, it would be hard for any single licensor to revoke a licence partially supporting a program — especially one which forms part of a large, distributed project … While revocation may be technically possible, it is unlikely to occur in the face of public opposition and a vigilant open source community. Regardless, as has been demonstrated over the last 12 months by the <em><a href="https://en.wikipedia.org/wiki/SCO_Group,_Inc._v._International_Business_Machines_Corp." rel="nofollow noreferrer">SCO Group Inc v International Business Machines Corp</a></em> litigation, the developer community is more than willing to replace any code for which the licence has been revoked or that otherwise infringes copyright. For these reasons, the issue of revocability is much more a theoretical than a practical concern.</p>
</blockquote>
