How to wait for exit of non-children processes

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.

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.

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.