What license applies to Cygwin source code?
Cygwin source code is licensed under a modified version of the GNU GPL. Cygwin’s license modification specifically allows third-party software under an open source license to be linked with Cygwin without requiring that the source code for the third-party software be distributed under the terms of the GNU GPL. Cygwin’s developers went to great trouble to obtain this modification and should be thanked for doing so, as without it the modification, Cygwin/X linking to Cygwin would be the subject of endless discussion.