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Kind of clickbait/misleading title? The article even states that PID is a userspace concept. There is no PID 0. PIDs is how userspace refers to processes in syscalls. You can't refer to a PID 0 because `kill(0)` has a special meaning.

But "What is TGID 0?" is much less catchy of a title. Hence, clickbait.

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Specifically, kill()ing pid 0 kills every process in the process group of the calling process. Pid 0 is more like a NULL pointer. You can put something in memory there, but a lot of systems will treat it as an invalid identifier for their own purposes.

I had some ruby code that called kill(str.to_i) and killed every process when the string wasn't an integer, which is when I painfully found out that to_i returns 0 on invalid strings and kill(0) kills (potentially) a lot of processes.

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