... > And yes, I'm tracking a bug. A service monitor, like systemd or > some watchdog, uses kill() to check if a pid is valid or not: > 1. Store service pid into cache. > 2. Check if pid in cache is valid by kill(pid, 0). > 3. Check if pid in cache is the service to watch. > > So if kill(pid, 0) returns success but no process info shows on 'ps' > command, the service monitor could be confused. The monitor could > check if pid is tid, but this means the odd behavior would be used > intentionally. And this workaround may be unsafe on other OS? That looks pretty broken to me. On Linux a pid can be reused immediately a process exits. So there is really no guarantee that the pid is the one you want. IIRC there are some recent changes that mean opening /proc/<pid> will stop the pid being reused - allowing checks before sending a signal. (Netbsd won't reuse a pid for a reasonable number of forks and then uses a semi-random pid allocator. Don't know whether any other 'bsd picked up that change.) Also using signals in multi-threaded programs is pretty much non-portable. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
