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.

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.

Here is a way to wait for any process (not necessarily a child) in linux to exit (or get killed) without polling:

Using inotify to wait for the /proc'pid' to be deleted would be the perfect solution, but unfortunately inotify does not work with pseudo file systems like /proc. However we can use it with the executable file of the process. While the process still exists, this file is being held open. So we can use inotify with IN_CLOSE_NOWRITE to block until the file is closed. Of course it can be closed for other reasons (e.g. if another process with the same executable exits) so we have to filter those events by other means.

We can use kill(pid, 0), but that can't guarantee if it is still the same process. If we are really paranoid about this, we can do something else.

Here is a way that should be 100% safe against pid-reuse trouble: we open the pseudo directory /proc/'pid', and keep it open until we are done. If a new process is created in the meantime with the same pid, the directory file descriptor that we hold will still refer to the original one (or become invalid, if the old process cease to exist), but will NEVER refer the new process with the reused pid. Then we can check if the original process still exists by checking, for example, if the file "cmdline" exists in the directory with openat(). When a process exits or is killed, those pseudo files cease to exist too, so openat() will fail.

here is an example code:

// return -1 on error, or 0 if everything went well
int wait_for_pid(int pid)
{
    char path[32];
    int in_fd = inotify_init();
    sprintf(path, "/proc/%i/exe", pid);
    if (inotify_add_watch(in_fd, path, IN_CLOSE_NOWRITE) < 0) {
        close(in_fd);
        return -1;
    }
    sprintf(path, "/proc/%i", pid);
    int dir_fd = open(path, 0);
    if (dir_fd < 0) {
        close(in_fd);
        return -1;
    }

    int res = 0;
    while (1) {
        struct inotify_event event;
        if (read(in_fd, &event, sizeof(event)) < 0) {
            res = -1;
            break;
        }
        int f = openat(dir_fd, "fd", 0);
        if (f < 0) break;
        close(f);
    }

    close(dir_fd);
    close(in_fd);
    return res;
}

None I am aware of. Apart from the solution from chaos, you can use semaphores if you can change the program you want to wait for.

The library functions are sem_open(3), sem_init(3), sem_wait(3), ...

sem_wait(3) performs a wait, so you don´t have to do busy waiting as in chaos´ solution. Of course, using semaphores makes your programs more complex and it may not be worth the trouble.

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.

Simply poll values number 22 and 2 of the /proc/[PID]/stat. The value 2 contains name of the executable and 22 contains start time. If they change, some other process has taken the same (freed) PID. Thus the method is very reliable.

You can use eBPF to achieve this.

The bcc toolkit implements many excellent monitoring capabilities based on eBPF. Among them, exitsnoop traces process termination, showing the command name and reason for termination, either an exit or a fatal signal.

   It catches processes of all users, processes in containers,  as  well  as  processes  that
   become zombie.

   This  works by tracing the kernel sched_process_exit() function using dynamic tracing, and
   will need updating to match any changes to this function.

   Since this uses BPF, only the root user can use this tool.

You can refer to this tool for related implementation.

You can get more information about this tool from the link below：

• Github repo: tools/exitsnoop: Trace process termination (exit and fatal signals). Examples.
• Linux Extended BPF (eBPF) Tracing Tools
• ubuntu manpages: exitsnoop-bpfcc

You can first install this tool and use it to see if it meets your needs, and then refer to its implementation for coding, or use some of the libraries it provides to implement your own functions.

exitsnoop examples:

   Trace all process termination
          # exitsnoop

   Trace all process termination, and include timestamps:
          # exitsnoop -t

   Exclude successful exits, only include non-zero exit codes and fatal signals:
          # exitsnoop -x

   Trace PID 181 only:
          # exitsnoop -p 181

   Label each output line with 'EXIT':
          # exitsnoop --label EXIT

Another option

Wait for a (non-child) process' exit using Linux's PROC_EVENTS

Reference project: https://github.com/stormc/waitforpid

mentioned in the project：

Wait for a (non-child) process' exit using Linux's PROC_EVENTS. Thanks to the CAP_NET_ADMIN POSIX capability permitted to the waitforpid binary, it does not need to be set suid root. You need a Linux kernel having CONFIG_PROC_EVENTS enabled.

Appricate @Hongli's answer for macOS with kqueue. I implement it with swift

/// Wait any pids, including non-child pid. Block until all pids exit.
/// - Parameters:
///   - timeout: wait until interval, nil means no timeout
/// - Throws: WaitOtherPidError
/// - Returns: isTimeout
func waitOtherPids(_ pids: [Int32], timeout: TimeInterval? = nil) throws -> Bool {
    
    // create a kqueue
    let kq = kqueue()
    if kq == -1 {
        throw WaitOtherPidError.createKqueueFailed(String(cString: strerror(errno)!))
    }
    
    // input
    // multiple changes is OR relation, kevent will return if any is match
    var changes: [Darwin.kevent] = pids.map({ pid in
        Darwin.kevent.init(ident: UInt(pid), filter: Int16(EVFILT_PROC), flags: UInt16(EV_ADD | EV_ENABLE), fflags: NOTE_EXIT, data: 0, udata: nil)
    })
    
    let timeoutDeadline = timeout.map({ Date(timeIntervalSinceNow: $0)})
    let remainTimeout: () ->timespec? = {
        if let deadline = timeoutDeadline {
            let d = max(deadline.timeIntervalSinceNow, 0)
            let fractionalPart = d - TimeInterval(Int(d))
            return timespec(tv_sec: Int(d), tv_nsec: Int(fractionalPart * 1000 * 1000 * 1000))
        } else {
            return nil
        }
    }
    
    // output
    var events = changes.map{ _ in Darwin.kevent.init() }
    
    while !changes.isEmpty {
        
        // watch changes
        // sync method
        let numOfEvent: Int32
        if var timeout = remainTimeout() {
            numOfEvent = kevent(kq, changes, Int32(changes.count), &events, Int32(events.count), &timeout);
        } else {
            numOfEvent = kevent(kq, changes, Int32(changes.count), &events, Int32(events.count), nil);
        }
        
        if numOfEvent < 0 {
            throw WaitOtherPidError.keventFailed(String(cString: strerror(errno)!))
        }
        if numOfEvent == 0 {
            // timeout. Return directly.
            return true
        }
        
        // handle the result
        let realEvents = events[0..<Int(numOfEvent)]
        let handledPids = Set(realEvents.map({ $0.ident }))
        changes = changes.filter({ c in
            !handledPids.contains(c.ident)
        })

        for event in realEvents {
            if Int32(event.flags) & EV_ERROR > 0 { // @see 'man kevent'
                let errorCode = event.data
                if errorCode == ESRCH {
                    // "The specified process to attach to does not exist"
                    // ingored
                } else {
                    print("[Error] kevent result failed with code \(errorCode), pid \(event.ident)")
                }
            } else {
                // succeeded event, pid exit
            }
        }
    }
    return false
}
enum WaitOtherPidError: Error {
    case createKqueueFailed(String)
    case keventFailed(String)
}

PR_SET_PDEATHSIG can be used to wait for parent process termination