IIRC in SGI, getpid() would return the thread ID. They had a syscall that could create a new subprocess that would share or not share various resources (e.g. memory, signals, file descriptors) so by setting or clearing bits you could implement a continuum of variations between fork and thread creation (for thread creation you'd share everything). Maybe a similar thing exists in Linux? But IRIX is dead so I think it's safe to kill.