is it possible that a folder like /proc/4587 exists even though there isn't a process that has PID 4587?

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    Are you wondering if checking for /proc/N is a safe/reliable way to see if PID N is running? – Andy Lester Sep 8 '17 at 15:21
  • I was wondering if theoretically there could be residue in /proc after process is gone. – SparedWhisle Sep 12 '17 at 9:11

If /proc is only the proc mount (and no one is playing tricks with overlays), no, a pid-based folder only exists as long as the corresponding process exists in some state (including as a zombie). In fact, just before returning a directory entry for a process id, the kernel re-validates the process’ existence — so at the instant a directory entry is returned, the corresponding process is still there. Accessing a directory also starts by looking up the corresponding process. (If the line numbers change, look for proc_pident_instantiate and proc_pident_lookup.)

You can run into issues caused by listing /proc and using the results later (even a few microseconds later): a process can be running when you list /proc, and stop before you act on the results.

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    What about processes that finished execution but wait() hadn't been called on them? – el.pescado Sep 8 '17 at 9:30
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    Good point @el.pescado, a quick check indicates that zombie processes still have their directory. I’ll update my answer, thanks! – Stephen Kitt Sep 8 '17 at 9:36
  • Basically, if ps shows the process, its /proc directory should exist. – Barmar Sep 8 '17 at 22:18
  • @Barmar “must” in fact: ps accesses /proc to find the information it displays. – Stephen Kitt Sep 9 '17 at 8:27

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