Xtank/Copyright

Copyright
Computer software (like books, films or music) is a work of authorship. The laws of many nations implement the concept of copyright which grants, to authors of such works, certain exclusive rights to those works. The authors or copyright holders must grant permission before a work may be copied or modified. They may attach requirements to these permissions.

Software is most useful to the general public when it comes with permission to share and improve the software, and when there are no restrictions to using the software. Thus there is a strong case for "free software" or "open source software". There is also a broader movement for "free cultural works".

However, the authors of Xtank did not grant permission to sell copies of the game. For this reason, Xtank is not free software; thus Xtank is not a free cultural work.

The COPYRIGHT file from Xtank
Xtank is copyright 1988 by Terry Donahue. All rights are reserved, except for the following: Anyone can modify X-Tank and redistribute the modified version provided they do so free of charge and do not omit this file from the distribution. There is no warranty for X-Tank. It is provided as is. I have no responsibility for any problems or damages caused by use of X-Tank.

Analysis of the COPYRIGHT
The COPYRIGHT file of Xtank does not restrict anyone from using Xtank or studying Xtank source code for any reason. You are free to play Xtank frequently, to install it on many computers, to disclose game strategies, to mull the game physics and formulas, to search for bugs and faults, and so on.

In fact, the entire COPYRIGHT file presents only two restrictions:
 * 1) A redistribution of Xtank must be "free of charge".
 * 2) A redistribution must "not omit" the COPYRIGHT file.

The second restriction enforces the preservation of the copyright notice (including the attribution to Terry Donahue), and would be acceptable for free cultural works. However, the first restriction prevents Xtank from qualifying as free software.

(One may construe that there is permission to distribute modified copies of Xtank, but not unmodified copies. This should not be a problem, because someone else has already modified your copy of Xtank from Kernigh, even if you made no modifications.)

Why Xtank is not free
Redistributions of Xtank must be "free of charge". This implies that Xtank is not "free software" and that Xtank is not "open source software".

This is because many persons do charge a fee when distributing free cultural works. For example, they may be trying to make a profit for providing a distribution service, or they may want to finance the continued development of free software. (The seller receives this fee, not the author, unless the seller is the author.)

The Definition of Free Cultural Works states, "Copies may be sold, swapped or given away for free, as part of a larger work, a collection, or independently." The Open Source Definition states, "The license shall not restrict any party from selling or giving away the software as a component of an aggregate software distribution containing programs from several different sources."

Understand, though, that Xtank dates to 1988, when "free software" was known to few and before the "GNU General Public License" came into existence. The probable intention of the "free of charge" restriction was to prevent certain game vendors from creating a modified version of Xtank for gainful purposes, charging a fee for every copy of the software while prevent others from redistributing this software.

This model of proprietary computer games, that everyone must pay a game vendor for the software, is now quite common; today every PlayStation, Wii, or Xbox game is proprietary in this way. In 1976, Bill Gates of Micro-Soft had distributed an Open Letter to Hobbyists in support of proprietary software. However, programmers who make games like Xtank, with no intention of charging a fee for every copy of the software, may wish to avoid exploitation by the vendors of proprietary software. A requirement to distribute the game "free of charge" was one way to accomplish this.

However, because such a way requirement subtracts from anyone's ability to distribute software for a fee without restricting further redistribution, it is not the best way to escape such exploitation. A better way is to employ "copyleft", a requirement that modified versions of a free cultural work remain free under the same turns.

When making a new computer game today, please do not act like it is 1988. Permit users to charge a fee when redistributing the game, but if you wish, apply a copyleft.