"""Disk And Execution MONitor (Daemon)
Configurable daemon behaviors:
1.) The current working directory set to the "/" directory.
2.) The current file creation mode mask set to 0.
3.) Close all open files (1024).
4.) Redirect standard I/O streams to "/dev/null".
A failed call to fork() now raises an exception.
References:
1) Advanced Programming in the Unix Environment: W. Richard Stevens
2) Unix Programming Frequently Asked Questions:
http://www.erlenstar.demon.co.uk/unix/faq_toc.html
"""
__author__ = "Chad J. Schroeder"
__copyright__ = "Copyright (C) 2005 Chad J. Schroeder"
__revision__ = "$Id$"
__version__ = "0.2"
# Standard Python modules.
import os # Miscellaneous OS interfaces.
import sys # System-specific parameters and functions.
# Default daemon parameters.
# File mode creation mask of the daemon.
UMASK = 0
# Default working directory for the daemon.
WORKDIR = "/"
# Default maximum for the number of available file descriptors.
MAXFD = 1024
# The standard I/O file descriptors are redirected to /dev/null by default.
if (hasattr(os, "devnull")):
REDIRECT_TO = os.devnull
else:
REDIRECT_TO = "/dev/null"
def createDaemon():
"""Detach a process from the controlling terminal and run it in the
background as a daemon.
"""
try:
# Fork a child process so the parent can exit. This returns control to
# the command-line or shell. It also guarantees that the child will not
# be a process group leader, since the child receives a new process ID
# and inherits the parent's process group ID. This step is required
# to insure that the next call to os.setsid is successful.
pid = os.fork()
except OSError, e:
raise Exception, "%s [%d]" % (e.strerror, e.errno)
if (pid == 0): # The first child.
# To become the session leader of this new session and the process group
# leader of the new process group, we call os.setsid(). The process is
# also guaranteed not to have a controlling terminal.
os.setsid()
# Is ignoring SIGHUP necessary?
#
# It's often suggested that the SIGHUP signal should be ignored before
# the second fork to avoid premature termination of the process. The
# reason is that when the first child terminates, all processes, e.g.
# the second child, in the orphaned group will be sent a SIGHUP.
#
# "However, as part of the session management system, there are exactly
# two cases where SIGHUP is sent on the death of a process:
#
# 1) When the process that dies is the session leader of a session that
# is attached to a terminal device, SIGHUP is sent to all processes
# in the foreground process group of that terminal device.
# 2) When the death of a process causes a process group to become
# orphaned, and one or more processes in the orphaned group are
# stopped, then SIGHUP and SIGCONT are sent to all members of the
# orphaned group." [2]
#
# The first case can be ignored since the child is guaranteed not to have
# a controlling terminal. The second case isn't so easy to dismiss.
# The process group is orphaned when the first child terminates and
# POSIX.1 requires that every STOPPED process in an orphaned process
# group be sent a SIGHUP signal followed by a SIGCONT signal. Since the
# second child is not STOPPED though, we can safely forego ignoring the
# SIGHUP signal. In any case, there are no ill-effects if it is ignored.
#
# import signal # Set handlers for asynchronous events.
# signal.signal(signal.SIGHUP, signal.SIG_IGN)
try:
# Fork a second child and exit immediately to prevent zombies. This
# causes the second child process to be orphaned, making the init
# process responsible for its cleanup. And, since the first child is
# a session leader without a controlling terminal, it's possible for
# it to acquire one by opening a terminal in the future (System V-
# based systems). This second fork guarantees that the child is no
# longer a session leader, preventing the daemon from ever acquiring
# a controlling terminal.
pid = os.fork() # Fork a second child.
except OSError, e:
raise Exception, "%s [%d]" % (e.strerror, e.errno)
if (pid == 0): # The second child.
# Since the current working directory may be a mounted filesystem, we
# avoid the issue of not being able to unmount the filesystem at
# shutdown time by changing it to the root directory.
os.chdir(WORKDIR)
# We probably don't want the file mode creation mask inherited from
# the parent, so we give the child complete control over permissions.
os.umask(UMASK)
else:
# exit() or _exit()? See below.
os._exit(0) # Exit parent (the first child) of the second child.
else:
# exit() or _exit()?
# _exit is like exit(), but it doesn't call any functions registered
# with atexit (and on_exit) or any registered signal handlers. It also
# closes any open file descriptors. Using exit() may cause all stdio
# streams to be flushed twice and any temporary files may be unexpectedly
# removed. It's therefore recommended that child branches of a fork()
# and the parent branch(es) of a daemon use _exit().
os._exit(0) # Exit parent of the first child.
# Close all open file descriptors. This prevents the child from keeping
# open any file descriptors inherited from the parent. There is a variety
# of methods to accomplish this task. Three are listed below.
#
# Try the system configuration variable, SC_OPEN_MAX, to obtain the maximum
# number of open file descriptors to close. If it doesn't exists, use
# the default value (configurable).
#
# try:
# maxfd = os.sysconf("SC_OPEN_MAX")
# except (AttributeError, ValueError):
# maxfd = MAXFD
#
# OR
#
# if (os.sysconf_names.has_key("SC_OPEN_MAX")):
# maxfd = os.sysconf("SC_OPEN_MAX")
# else:
# maxfd = MAXFD
#
# OR
#
# Use the getrlimit method to retrieve the maximum file descriptor number
# that can be opened by this process. If there is not limit on the
# resource, use the default value.
#
import resource # Resource usage information.
maxfd = resource.getrlimit(resource.RLIMIT_NOFILE)[1]
if (maxfd == resource.RLIM_INFINITY):
maxfd = MAXFD
# Iterate through and close all file descriptors.
for fd in range(0, maxfd):
try:
os.close(fd)
except OSError: # ERROR, fd wasn't open to begin with (ignored)
pass
# Redirect the standard I/O file descriptors to the specified file. Since
# the daemon has no controlling terminal, most daemons redirect stdin,
# stdout, and stderr to /dev/null. This is done to prevent side-effects
# from reads and writes to the standard I/O file descriptors.
# This call to open is guaranteed to return the lowest file descriptor,
# which will be 0 (stdin), since it was closed above.
os.open(REDIRECT_TO, os.O_RDWR) # standard input (0)
# Duplicate standard input to standard output and standard error.
os.dup2(0, 1) # standard output (1)
os.dup2(0, 2) # standard error (2)
return(0)
if __name__ == "__main__":
retCode = createDaemon()
# The code, as is, will create a new file in the root directory, when
# executed with superuser privileges. The file will contain the following
# daemon related process parameters: return code, process ID, parent
# process group ID, session ID, user ID, effective user ID, real group ID,
# and the effective group ID. Notice the relationship between the daemon's
# process ID, process group ID, and its parent's process ID.
procParams = """
return code = %s
process ID = %s
parent process ID = %s
process group ID = %s
session ID = %s
user ID = %s
effective user ID = %s
real group ID = %s
effective group ID = %s
""" % (retCode, os.getpid(), os.getppid(), os.getpgrp(), os.getsid(0),
os.getuid(), os.geteuid(), os.getgid(), os.getegid())
open("createDaemon.log", "w").write(procParams + "\n")
sys.exit(retCode)
