Python Cookbook

Forking a Daemon Process on Unix

Credit: Jürgen Hermann

Problem

You need to fork a daemon process on a Unix or Unix-like system, and this, in turn, requires a certain precise sequence of system calls.

Solution

Daemon processes must detach from their controlling terminal and process group. This is not hard, but it does take some care:

import sys, os

def main(  ):
    """ An example daemon main routine; writes a datestamp to file
        /tmp/daemon-log every 10 seconds.
    """
    import time

    f = open("/tmp/daemon-log", "w")
    while 1:
        f.write('%s\n' % time.ctime(time.time(  )))
        f.flush(  )
        time.sleep(10)

if _ _name_ _ == "_ _main_ _":
    # Do the Unix double-fork magic; see Stevens's book "Advanced
    # Programming in the UNIX Environment" (Addison-Wesley) for details
    try:
        pid = os.fork(  )
        if pid > 0:
            # Exit first parent
            sys.exit(0)
    except OSError, e:
        print >>sys.stderr, "fork #1 failed: %d (%s)" % (
            e.errno, e.strerror)
        sys.exit(1)

    # Decouple from parent environment
    os.chdir("/")
    os.setsid(  )
    os.umask(0)

    # Do second fork
    try:
        pid = os.fork(  )
        if pid > 0:
            # Exit from second parent; print eventual PID before exiting
            print "Daemon PID %d" % pid
            sys.exit(0)
    except OSError, e:
        print >>sys.stderr, "fork #2 failed: %d (%s)" % (
            e.errno, e.strerror)
        sys.exit(1)

    # Start the daemon main loop
    main(  )

Discussion

Forking a daemon on Unix requires a certain specific sequence of system calls, which is explained in W. Richard Steven’s seminal book, Advanced Programming in the Unix Environment (Addison-Wesley). We need to fork