Either a) when the license has an explicit exemption (such as at glibc or the kernel's userspace interfaces) or b) when something ceases to be a "derivative work" in copyright terms (which is ultimately a legal question for lawyers).
>>surajr+CQ3
No. The FSF takes the position that a program that is deeply entangled with another is a derivative work and can be subject to the GPL, even if that deep entanglement takes place over IPC.