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In the past, and the practice still continues, programs covered by one of the GNU licenses, such as utilities from the GNU Project, have been ported to Microsoft Windows by linking to the MSVCRT.DLL library. That's something frowned upon by Microsoft because it's a system library not intended for application use.

The reason for this is that the GPL has a system library exception: GPLed programs can be linked to a proprietary library, if it's part of the target platform (and not a third-party piece that is separately installed). So linking to a redistributable Visual C/C++ run-time would be a GPL violation, but linking to the MSVCRT.DLL that the operating system installation places into the system folder is okay.

More recently, Microsoft developed a library that is fine for linking by applications, called the Universal C Run-Time. This is bundled in Windows 10 and 11. So it seems that a GPLed program running on Windows 10 or 11 can link to this library.

However, the Universal C Run-Time can also be installed on older Windows, as far back as Vista and Server 2008. Is there a GPL violation if a GPLed program developed for Windows 10, linking with the Universal CRT is run on older Windows where the UCRT has been installed as an add-on?

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  • Declaring that linking to a redistributable Visual C/C++ run-time would be a "GPL violation" does not seem like a settled issue. One interpretation is that it is indeed covered by the System Libraries exception (the wording in the GPL v3 is broad enough that you could read it that way), and another interpretation is that it is not covered (i.e. you could say that the Redistributable package is not 'essential' or is not a Major Component). See also: opensource.stackexchange.com/questions/7182/…
    – Brandin
    Aug 9, 2023 at 5:39
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    I think the 'Answer' is in this FAQ here: As long as you don't distribute that closed-source runtime along with the program, then those libraries can still be considered System Libraries, even if they were installed as a so-called 'add-on': gnu.org/licenses/gpl-faq.html#WindowsRuntimeAndGPL
    – Brandin
    Aug 9, 2023 at 6:13

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