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The source code to the Linux version of "Doom" was released by id Software on December 23, 1997 under a non-profit End user license agreement; it was re-released on October 3, 1999, under the terms of the GPL-2.0-or-later license. However, the source codes to the DOS and Windows versions of the game were not released, due to copyright issues concerning the sound library used by original DOS version and id Software having no access to the source code of the Windows port.