For child processes, the wait() and waitpid() functions can be used to suspends execution of the current process until a child has exited. But this function can not be used for non-child processes.
Is there another function, which can wait for exit of any process ?
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.
So far I've found three ways to do this on Linux:
kill or by testing for the existence of /proc/$pid, as in most of the other answersptrace system call to attach to the process like a debugger so you get notified when it exits, as in a3nm's answernetlink 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.
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.
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;
}
You could attach to the process with ptrace(2). From the shell, strace -p PID >/dev/null 2>&1 seems to work. This avoid the busy-waiting, though it will slow down the traced process, and will not work on all processes (only yours, which is a bit better than only child processes).
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.
Maybe it could be possible to wait for /proc/[pid] or /proc/[pid]/[something] to disappear?
There are poll() and other file event waiting functions, maybe that could help?
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：
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
inotifywait)This is based on linux's /proc filesystem.
My need was to start a 2nd (overall) backup, once containers backups is done. Containers backups is started by cron.
cron tasksread -r wpid < <(ps -C backup.sh ho pid)
ls -l /proc/$wpid/fd
total 0
lr-x------ 1 user user 64 1 aoû 09:13 0 -> pipe:[455151052]
lrwx------ 1 user user 64 1 aoû 09:13 1 -> /tmp/#41418 (deleted)
lrwx------ 1 user user 64 1 aoû 09:13 2 -> /tmp/#41418 (deleted)
Where deleted entries was created by cron. But even if deleted, you could watch for file descriptor directly:
inotifywait /proc/$wpid/fd/1
/proc/511945/fd/1 CLOSE_WRITE,CLOSE
or
inotifywait /proc/$wpid/fd/0
/proc/511945/fd/0 CLOSE_NOWRITE,CLOSE
Note: My overall backup is run as root user! If no this could require sudo because command is run under cron session!
Just test: In a 1st window, hit:
sleep 0.42m <<<'' >/dev/null 2>&1
Then in another window:
read -r wpid < <(ps -C sleep wwho pid,cmd| sed 's/ sleep 0\.42m$//p;d')
ls -l /proc/$wpid/fd
total 0
lr-x------ 1 user user 64 1 aoû 09:38 0 -> pipe:[455288137]
l-wx------ 1 user user 64 1 aoû 09:38 1 -> /dev/null
l-wx------ 1 user user 64 1 aoû 09:38 2 -> /dev/null
Don't try to watch for 1 or 2! Because they point to /dev/null, any process acessing to them will trig inotifywait.
inotifywait /proc/$wpid/fd/0
/proc/531119/fd/0 CLOSE_NOWRITE,CLOSE
1st window:
sleep 0.42m <<<'' >/dev/null 2>&1
2nd window:
read -r wpid < <(ps -C sleep wwho pid,cmd| sed 's/ sleep 0\.42m$//p;d')
startedAt=$(ps ho lstart $wpid | date -f - +%s)
inotifywait /proc/$wpid/fd/0;echo $((EPOCHSECONDS-startedAt))
/proc/533967/fd/0 CLOSE_NOWRITE,CLOSE
25
Using inotifywait seem to be a good solution, mostly watching for command's standard input (fd/0). But this must be tested case by case.
