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Is it possible to start/run a process with a predefined PID? In Linux or in OS X (or BSD)?

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Why would you need that? – slhck Aug 15 '13 at 18:35
to set a monitor for a specific PID before the process started – static Aug 15 '13 at 18:45
Then maybe you're doing the "monitoring" thing wrong... – grawity Aug 15 '13 at 19:23
I was going to say. Maybe you can explain what you really need to do? (And see please.) – slhck Aug 15 '13 at 19:25
@grawity: why wrong? its a bit more then monitoring (instrumentation). I just want to see what is going on in the process from start on. – static Aug 15 '13 at 23:50
up vote 2 down vote accepted

It is impossible to start a process with a predefined PID.

If you wanted to do this you'd have to change the implementation of the fork() (and subsequent) system calls when a new process is spawned, and somehow create a utility that lets you intercept these calls. Also don't forget that the specific implementation of PID allocation depends on the operating system's kernel – it's not the same for OS X and Linux.

See also: How does Linux determine the next PID?

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Odd, I could have sworn the Linux clone() syscall had a custom PID flag from the "checkpoint/resume" patches. But I guess those weren't merged. – grawity Aug 15 '13 at 19:28
is it possible (maybe with any workaround) to use a dtrace/gdb to start a process, pause it immediately after assigning a pid, get it's pid, start a monitor/filter for monitor for that pid and then resume the process? – static Aug 15 '13 at 19:29
@static: You don't even need a workaround for that – it's basic fork() and exec() usage. Instead of forking a process that immediately execs another program, fork a process that stores its PID and then execs another program. – grawity Aug 15 '13 at 19:31
@grawity: hm, is it possible to rewrite/customize a fork for an OS X and for a binary program, which process is interesting for me? – static Aug 15 '13 at 19:32
@grawity I think you meant Checkpoint/Restart? I wasn't aware this existed, so thanks for the info. Looked into it a little and it seems it wasn't ever merged, no. – slhck Aug 15 '13 at 19:47

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