How to wait for exit of non-children processes

On BSDs and OS X, you can use kqueue with EVFILT_PROC+NOTE_EXIT to do exactly that. No polling required. Unfortunately there's no Linux equivalent.

You could also create a socket or a FIFO and read on them. The FIFO is especially simple: Connect the standard output of your child with the FIFO and read. The read will block until the child exits (for any reason) or until it emits some data. So you'll need a little loop to discard the unwanted text data.

If you have access to the source of the child, open the FIFO for writing when it starts and then simply forget about it. The OS will clean the open file descriptor when the child terminates and your waiting "parent" process will wake up.

Now this might be a process which you didn't start or own. In that case, you can replace the binary executable with a script that starts the real binary but also adds monitoring as explained above.

So far I've found three ways to do this on Linux:

• Polling: you check for the existence of the process every so often, either by using kill or by testing for the existence of /proc/$pid, as in most of the other answers
• Use the ptrace system call to attach to the process like a debugger so you get notified when it exits, as in a3nm's answer
• Use the netlink interface to listen for PROC_EVENT_EXIT messages - this way the kernel tells your program every time a process exits and you just wait for the right process ID. I've only seen this described in one place on the internet.

Shameless plug: I'm working on a program (open source of course; GPLv2) that does any of the three.

Nothing equivalent to wait(). The usual practice is to poll using kill(pid, 0) and looking for return value -1 and errno of ESRCH to indicate that the process is gone.

Update: Since linux kernel 5.3 there is a pidfd_open syscall, which creates an fd for a given pid, which can be polled to get notification when pid has exited.