Can an operating system make terms for apps that run on it, or can third parties do the same?
<p>This is in continuation to the question here :<a href="https://law.stackexchange.com/questions/64361/if-an-app-is-built-on-a-gpl-operating-system-must-it-also-be-gpl">If an app is built on a GPL Operating System must it also be GPL?</a>
Say a user is using a default calendar app and the operating system provides an inbuit method or prebuilt executable to call the default calendar and open it. A person builds an app that, as a side-feature required for its functioning, opens the default app using operating system facilities openly provided to everyone like console commands.</p>
<p>Can the operating system provide terms for such apps which are legally enforceabe like payments to be made to Operating System deveopers? Can the calendar maker do so also?</p>
<p>Lastly will making a console call (opencalendar) [where opencalendar is an operating system executable] or in code (someoperatingsystemlibrary.opencalendar();) create a copyright issue and will it make developers liable for practices of the operating system and calendar? Answers to the last sentence are optional.</p>

<p>First of all, if you have the right to run the OS at all, you have the right to make calls to its published functions, so there is no copyright issue.</p>
<p>The OS or calendar app could cone with a license requiring payment to call the app, I suppose, provided the requirements were communicated to the user. But I have never heard of an OS with that sort of terms in its license. Sometimes particular features are only enabled to those who have bought a more expensive license. That is the difference between Windows Home and Windows PRO, for example. But I don't know of any open-source OS with that sort of license model.</p>
<p>An OS could also have a development version, for which a higher fee is charged, but I don't see hoe it could charge the developer every tiem an app made an OS call. In any case no developer would write for an OS with terms like that, I would think.</p>
<p>MS offers a thing known as MSDN for developers, and charges an extra fee for it. It contains many useful tools for developers, but is not <strong>required</strong>. That is the closest match I know of.</p>

<p>A programmer can write and distribute whatever product he wants. It's possible that an executable module created with a specific compiler cannot, according to the EULA for the compiler, be distributed commercially, so the programmer would need to pick a different compiler. That, plus contractual limits imposed by the vendor, are the basic legal limits on the programmer.</p>
<p>Note that the programmer does not run the program, the user does. It is possible but very unlikely that an OS could impose a restriction on what programs could be installed / run under the OS, coming from the EULA of the operating system (which the user agreed to). The programmer does not agree to anything w.r.t. the user's installation of the OS, so there is no contractual relation between the OS creator and the program creator, and neither has an obligation to the other.</p>
<p>Still, there is an indirect way for the OS creator to limit what the program-writer does. The OS might use technological means to prevent user from running certain programs, by requiring a separate license for a certain function to work. The programmer might devise a work-around that overcomes that limitation, making it physically possible to use the OS in an forbidden way. But this would be illegal: and the programmer could be held liable for contributory copyright infringement. This is an obviously very limited context where the OS creator &quot;gets a say&quot; in what the programmer can sell to the user.</p>

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