GPL is the "bad" one, right?

Excerpt:

The licensee is allowed to charge a fee for this service, or do this free of charge. This latter point distinguishes the GPL from software licenses that prohibit commercial redistribution. The FSF argues that free software should not place restrictions on commercial use,[21] and the GPL explicitly states that GPL works may be sold at any price.

So to answer the question, “no,” its use does not “infect” other software with the deadly “this has to be free” virus. That being said, there are licenses that do, including the Creative Commons noncommercial licenses.

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Wait!

I thought you were asking about commercial use. The GPL does have a copylefting component, as do CC sharealike licenses. That requires sharing source code.

Forget that previous answer, which is a hot mess. Here you go, straight from the GPL FAQ:

The GPL does not require you to release your modified version, or any part of it. You are free to make modifications and use them privately, without ever releasing them. This applies to organizations (including companies), too; an organization can make a modified version and use it internally without ever releasing it outside the organization.

But if you release the modified version to the public in some way, the GPL requires you to make the modified source code available to the program’s users, under the GPL.

Thus, the GPL gives permission to release the modified program in certain ways, and not in other ways; but the decision of whether to release it is up to you.

The thing that gives you the right to use the GPL’d code in the first place requires you to make the source public if you release a modified version. The BSD and MIT licenses do not have that requirement in them, only a requirement that you acknowledge the use of the source.

So if you created an embedded router operating system based on Linux, and sold routers, you’d need to make the source code to your operating system public, because Linux is released under the GPL and you’re releasing a derivative work.

But if you created an embedded router operating system based on OpenBSD, and sold routers, you would not need to make the source code to your operating system public, because OpenBSD is released under the BSD license and you can use the code freely for whatever purpose you like.

This is why Mac OS X is based on an amalgam of the BSDs, and not on Linux.

The real question is whether linking to a GPL library binds your other code to GPL.

From the wiki:

A GPL linking exception modifies the GNU General Public License (GPL) to create a new, modified license. Such modified licenses enable software projects which provide ‘library’ code, that is software code which is designed to be used by (in technical terms, ‘linked to’) other software, to distribute the software code of the library itself under terms essentially identical to the GPL without forcing others distributing code which merely uses, but therefore integrates, the code of the software library to apply the terms of the GPL to their own code.

Is that what you’re asking?

@andrew: I don’t think so, but if I were in doubt, I’d only use libraries covered by the LGPL.