--- /dev/null
+"""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)
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