Licence for Windows API
<p>Is a licence required for calling operating system functions provided by Windows or calling its kernel? Is any licence required to use dll files of win API (linking but not distributing)? If yes, what are the obligations of a developer who makes software and not of user who uses it?</p>

<p>As mentioned in <a href="https://stackoverflow.com/a/25915904/3943090">this SO answer</a></p>
<blockquote>
<p>The license that applies to Windows system DLLs is the end-user license for the desktop or server OS that they came with.</p>
</blockquote>
<p>Such files are not normally distributed except when installing or upgrading the OS, and the only license needed to use them is the license that is part of the individual windows installation.</p>
<p>The Windows 10 software developer kit (SDK) which most developers use in creating software for windows is at  <a href="https://docs.microsoft.com/en-us/legal/windows-sdk/license-terms-ewdk" rel="nofollow noreferrer">https://docs.microsoft.com/en-us/legal/windows-sdk/license-terms-ewdk</a> and contains various restrictions such as:</p>
<blockquote>
<p>One user may install and use any number of copies of the software on your devices to design, develop and test your device drivers and supporting components, as defined by DCHU. Further, you may install, use and/or deploy via a network management system or as part of a desktop image, any number of copies of the software on computer devices within your internal corporate network to design, develop and test your device drivers and supporting components, as defined by DCHU, that run on a Microsoft operating system. Each copy must be complete, including all copyright and trademark notices. You must require end users to agree to terms that protect the software as much as these license terms.</p>
<p>...</p>
<p>API Use. We may monitor and collect data related to a program’s use of APIs in order to provide, improve and personalize Microsoft products and services. End user information collected by Microsoft’s monitoring and data collection related to your program’s use of APIs is subject to the Microsoft Consumer Privacy Statement.</p>
<p>...</p>
<p>The software may collect information about you and your use of the software and send that to Microsoft. Microsoft may use this information to provide services and improve our products and services. Your opt-out rights, if any, are described in the product documentation. Some features in the software may enable collection of data from users of your applications that access or use the software. If you use these features to enable data collection in your applications, you must comply with applicable law, including getting any required user consent, and maintain a prominent privacy policy that accurately informs users about how you use, collect, and share their data.</p>
</blockquote>
<p>This license imposes some obligations on developers, but none that interfere with normal development and distribution of applications.</p>
<p>This means that a developer can use the documented OS API calls without needing any special license as long as they are used as described in the Microsoft documentation. A user is covered by the general license on an installed copy of Windows for any calls made by programs running on that machine, as long as the local copy of the OS is properly licensed.</p>
<p>The various products, such as Office 365 each has a license for use of its API. In particular these prohibit creating an application that just exposes the API to others, and requires appropriate acknowledgement of the API. But that is all separate from ordinary windows API calls, weather Win-16, Win-32, or Win-64.</p>

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