Daemonize Python

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Python Daemonize source

This short module shows the correct way to create a UNIX daemon process in Python. Creating a proper UNIX daemon is not as trivial as it sounds; although, some UNIX variants have system calls specifically for creating a daemon process.

#!/usr/bin/env python
import sys, os, signal

'''This module is used to fork the current process into a daemon.

Almost none of this is necessary (or advisable) if your daemon 
is being started by inetd. In that case, stdin, stdout and stderr are 
all set up for you to refer to the network connection, and the fork()s 
and session manipulation should not be done (to avoid confusing inetd). 
Only the chdir() and umask() steps remain as useful. 

References:
    UNIX Programming FAQ
        1.7 How do I get my program to act like a daemon?
        http://www.erlenstar.demon.co.uk/unix/faq_2.html#SEC16
   
    Advanced Programming in the Unix Environment
        W. Richard Stevens, 1992, Addison-Wesley, ISBN 0-201-56317-7.

Noah Spurrier
'''

def daemonize (stdin='/dev/null', stdout='/dev/null', stderr='/dev/null'):
    '''This forks the current process into a daemon.
    The stdin, stdout, and stderr arguments are file names that
    will be opened and be used to replace the standard file descriptors
    in sys.stdin, sys.stdout, and sys.stderr.
    These arguments are optional and default to /dev/null.
    Note that stderr is opened unbuffered, so
    if it shares a file with stdout then interleaved output
    may not appear in the order that you expect.
    '''

    # Do first fork.
    try: 
        pid = os.fork() 
        if pid > 0:
            return pid
    except OSError, e: 
	return (e.errno, e.strerror) # Fork failed.

    # Decouple from parent environment.
    os.setsid() 

    # When the first child terminates, all processes in the second child
    # are sent a SIGHUP. This causes it to be ignored.
    signal.signal(signal.SIGHUP, signal.SIG_IGN)

    # Do second fork.
    try: 
        pid = os.fork() 
        if pid > 0:
            os._exit(0)   # Exit second parent.
    except OSError, e: 
        sys.stderr.write ("fork #2 failed: (%d) %s\n" % (e.errno, e.strerror) )
        os._exit(1)

    # Now I am a daemon!
    
    os.chdir("/") # don't hold open any directories
    os.umask(0) 

    # Close open files. Try the system configuration variable, SC_OPEN_MAX,
    # for the maximum number of open files to close. If it doesn't exist, use 1024.
    try:
        maxfd = os.sysconf("SC_OPEN_MAX")
    except (AttributeError, ValueError):
        maxfd = 1024
    for fd in range(3, maxfd):
        try:
            os.close(fd)
        except OSError:   # ERROR (ignore)
            pass

    # Redirect standard file descriptors.
    si = open(stdin, 'r')
    so = open(stdout, 'a+')
    se = open(stderr, 'a+', 0)
    os.dup2(si.fileno(), sys.stdin.fileno())
    os.dup2(so.fileno(), sys.stdout.fileno())
    os.dup2(se.fileno(), sys.stderr.fileno())
    return 0

def main ():
    '''This is an example main function run by the daemon.
    This prints a count and timestamp once per second.
    '''
    import time
    sys.stdout.write ('Daemon started with pid %d\n' % os.getpid() )
    sys.stdout.write ('Daemon stdout output\n')
    sys.stderr.write ('Daemon stderr output\n')
    c = 0
    while 1:
        sys.stdout.write ('%d: %s\n' % (c, time.ctime(time.time())) )
        sys.stdout.flush()
        c = c + 1
        time.sleep(1)

if __name__ == "__main__":
    print str(daemonize('/dev/null','/tmp/daemon.log','/tmp/daemon.log'))
    main()