If I port my program to GNU/Linux, does that mean I have to release it as Free Software under the GPL or some other Software Livre license?
In general, the answer is no–this is not a legal requirement. In specific, the answer depends on which libraries you want to use and what their licenses are. Most system libraries either use the GNU Lesser GPL, or use the GNU GPL plus an exception permitting linking the library with anything. These libraries can be used in non-free programs; but in the case of the Lesser GPL, it does have some requirements you must follow. Some libraries are released under the GNU GPL alone; you must use a GPL-compatible license to use those libraries. But these are normally the more specialized libraries, and you would not have had anything much like them on another platform, so you probably won’t find yourself wanting to use these libraries for simple porting. Of course, your software is not a contribution to our community if it is not free, and people who value their freedom will refuse to use it. Only people willing to give up their freedom will use your software, which means that it will effectiv
