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Process IDs are strictly increasing, but if your system runs long enough and there is a lot of processes terminating and starting, you could at some point reach the limit of the underlying integral type (on my system it seems to be a signed int) where no larger pid would exists. Would this cause old unused ids (of processes that finished) to be recycled (i.e. handed out a second time)? What if somebody is waiting for that pid to terminate but didn't check in a looong time?

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This seems like a near duplicate of… – Bruce Ediger Dec 13 '11 at 4:47

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Process IDs are not strictly increasing on any UNIX-like operating system I know of. Your question is based on a false assumption.

The only requirements on process IDs are:

A process ID shall not be reused by the system until the process lifetime ends. In addition, if there exists a process group whose process group ID is equal to that process ID, the process ID shall not be reused by the system until the process group lifetime ends. A process that is not a system process shall not have a process ID of 1.

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So that means, they are reused. But what if I wait for a child process to finish? By coincidence its process id could be reused before I get a chance to check it, right? – bitmask Dec 13 '11 at 2:32
No, that's impossible. The process is not finished until it reports its exit value to its parent. That is one of its jobs, and until it has done that, it is not finished. – David Schwartz Dec 13 '11 at 2:38
@DavidSchwartz - not sure how readable this will be in a comment but - linux is pretty sequential. [adrian@iceweasel ~]$ a.out 24506 [adrian@iceweasel ~]$ a.out 24507 [adrian@iceweasel ~]$ a.out 24508 [adrian@iceweasel ~]$ a.out 24509 [adrian@iceweasel ~]$ cat #include <stdio.h> #include <sys/types.h> #include <unistd.h> int main(int argc, char *argv[]) { printf("%d\n", getpid()); return 0; } – Adrian Cornish Dec 13 '11 at 4:34
@DavidSchwartz - That was my point - you said "are not strictly increasing on any UNIX-like operating system I know of" - you remember that super security flaw in Debian because they were using the PID as the salt for hashing ;-) - with only 65K of PID's not the best salt – Adrian Cornish Dec 13 '11 at 4:56
Yes, they are not strictly increasing on an UNIX-like operating system I know of. If you know of one where they are strictly increasing, you haven't mentioned it yet. (Strictly increasing means that they never decrease.) I could imagine an OS with 64-bit PIDs that did that, but I don't know of any. – David Schwartz Dec 13 '11 at 5:03

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