What is Copyleft?

John Cowan jcowan at reutershealth.com
Thu Feb 22 20:17:26 UTC 2001

Ryan S. Dancey wrote:

> If I write a copyleft free program for Windows, I should be able to load and
> link at runtime to any DLL in the system, regardless of whether or not that
> DLL is free code or not, shouldn't I?  How else could a Windows program ever
> be written using the GPL?

By a special exception in the GNU GPL, GPLed programs may link with
unfree code if it is routinely distributed with the operating system,
such as VBRUNxxx.DLL or MSCVRTxxx.DLL or KERNEL32.DLL for that matter.
Either static or dynamic linking is acceptable.

Dynamic linking with unfree libraries *not* distributed with the OS is a
gray area in the GPL.  When it was written, dynamic linking was a
marginal concept.  The FSF believes that linking with unfree dynamic
libraries, except as mentioned above, is not allowed.

> (I don't know enough about Linux to have an
> opinion about Linux code).

There are no differences, except that unfree libraries are less frequent
on Linux.

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