How far does GPL go?

In a previous post I wrote about how the GPL software licence is recursive and based on copyleft principles. The basic idea of a copyleft license is that the freedoms it grants, which relieve the automatic copyrights associated with a piece of software, should also pertain to any further modifications or distribution of the said piece of software by any of its future recipients. A brief discussion about this point with a professional programmer friend of mine helped me to get a better idea as to the extent of the obligations imposed on a recipient of GPLd software.

Here is a gist of what I gathered: if you use any piece of a GPLd software source code in your own code, your code is required by the licence to be GPL licenced too. This also happens to be the case even if you only link your own code to a GPLd library (e.g. to import functions). However, if the GPLd software runs as a binary (i.e. independently) in your software system, the other software components in your system are not susceptible to the GPLd software’s GPL.

So e.g. the popular American digital video recorder TIVO has a linux kernel, which is an integral piece of GPLd software. This does not mean however, that the entire software system, which runs TIVO, is also GPLd. Presumably this is because the linux kernel runs independently from the rest of the TIVO software system…