daemonize.py

import sys
import os

# maximum file descriptors to close if sysconf(SC_OPEN_MAX)
# is indeterminate
BD_MAX_CLOSE = 8192

def become_daemon(no_umask0=False,
                  no_chdir=False,
                  no_close_open_files=False,
                  no_reopen_standard_fds=False):
    try:
        pid = os.fork()
        if pid == 0:
            pass    # child falls through
        else:
            exit(0) # parent terminates
    except OSError:
        return -1

    if os.setsid() == -1: # become the leader of a new session
        return -1

    try: # ensure we are not session leader
        pid = os.fork()
        if pid == 0:
            pass
        else:
            exit(0)
    except OSError:
        return -1

    if not no_umask0:
        os.umask(0) # clear file mode creation mask

    if not no_chdir:
        os.chdir('/')  # change to root directory

    if not no_close_open_files: # close all open files
        max_fd = os.sysconf('SC_OPEN_MAX')
        if max_fd == -1:
            max_fd = BD_MAX_CLOSE
        os.closerange(0, max_fd)

    if not no_reopen_standard_fds:
        try:
            os.close(sys.stdin.fileno())
        except OSError:
            pass

        fd = os.open('/dev/null', os.O_RDWR)
        os.dup2(sys.stdin.fileno(), sys.stdout.fileno())
        os.dup2(sys.stdin.fileno(), sys.stderr.fileno())

    return 0