Daemons, Orphan Processes, Zombie Processes, and waitpid

A daemon is a long-running background process detached from a controlling terminal. Traditional daemonization forks, creates a new session with setsid, changes working directory and file mode mask, redirects standard streams, and often forks again so the process cannot reacquire a terminal.

An orphan is a running child whose parent has exited. The operating system reparents it to a system process that can eventually collect its exit status. A zombie is different: it has already exited, but its parent has not yet read that status. The kernel retains a small process-table entry until the parent waits for it.

Parents should reap children using wait or waitpid. waitpid(pid, options) can wait for one child, any child, or a process group; WNOHANG makes the call non-blocking. A SIGCHLD handler commonly loops because one signal may correspond to several exited children:

while True:
    try:
        pid, status = os.waitpid(-1, os.WNOHANG)
        if pid == 0:
            break
    except OSError:
        break

Modern service managers usually handle daemon lifecycle, logs, restarts, and file descriptors, so self-daemonization is often unnecessary for new services.

Originally published on SegmentFault.