I think you're seeing the results of a race where sometimes the parent dies before the child sees it's parent's PID. See http://www.erlenstar.demon.co.uk/unix/faq_2.html#SEC14 for info on why you see this on unix-like systems.

In reply to Re: Broken getppid() on Darwin/OSX by superfrink
in thread Broken getppid() on Darwin/OSX by fuzzyping

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