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A Reference Implementation of a Daemon
Default behaviors are as follows, but they can be modified:
- Ignore SIGHUP signals.
- Default current working directory to the "/" directory.
- Set the current file creation mode mask to 0.
- Close all open files (0 to [SC_OPEN_MAX or 256]).
- Redirect standard I/O streams to "/dev/null".
Failed fork() calls will return a tuple:
Again, this behavior can be modified to meet your program's needs.
- Advanced Programming in the Unix Environment: W. Richard Stevens
- Unix Network Programming (Volume 1): W. Richard Stevens
#!/usr/bin/env python """Disk And Execution MONitor (Daemon) Default daemon behaviors (they can be modified): 1.) Ignore SIGHUP signals. 2.) Default current working directory to the "/" directory. 3.) Set the current file creation mode mask to 0. 4.) Close all open files (0 to [SC_OPEN_MAX or 256]). 5.) Redirect standard I/O streams to "/dev/null". Failed fork() calls will return a tuple: (errno, strerror). This behavior can be modified to meet your program's needs. Resources: Advanced Programming in the Unix Environment: W. Richard Stevens Unix Network Programming (Volume 1): W. Richard Stevens http://www.erlenstar.demon.co.uk/unix/faq_2.html#SEC16 """ __author__ = "Chad J. Schroeder" __version__ = "$Id$" import os # Miscellaneous OS interfaces. import sys # System-specific parameters and functions. import signal # Set handlers for asynchronous events. 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 will return control # to the command line or shell. This is required so that the new process # is guaranteed not to be a process group leader. We have this guarantee # because the process GID of the parent is inherited by the child, but # the child gets a new PID, making it impossible for its PID to equal its # PGID. pid = os.fork() except OSError, e: return((e.errno, e.strerror)) # ERROR (return a tuple) if (pid == 0): # The first child. # Next we call os.setsid() to become the session leader of this new # session. The process also becomes the process group leader of the # new process group. Since a controlling terminal is associated with a # seesion, and this new session has not yet acquired a controlling # terminal our process now has no controlling terminal. This shouldn't # fail, since we're guaranteed that the child is not a process group # leader. os.setsid() # When the first child terminates, all processes in the second child # are sent a SIGHUP, so it's ignored. signal.signal(signal.SIGHUP, signal.SIG_IGN) try: # Fork a second child to prevent zombies. 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. This second # fork guarantees that the child is no longer a session leader, thus # preventing the daemon from ever acquiring a controlling terminal. pid = os.fork() # Fork a second child. except OSError, e: return((e.errno, e.strerror)) # ERROR (return a tuple) if (pid == 0): # The second child. # Ensure that the daemon doesn't keep any directory in use. Failure # to do this could make a filesystem unmountable. os.chdir("/") # Give the child complete control over permissions. os.umask(0) else: os._exit(0) # Exit parent (the first child) of the second child. else: os._exit(0) # Exit parent of the first child. # Close all 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 # the default value (configurable). if (os.sysconf_names.has_key("SC_OPEN_MAX")): maxfd = os.sysconf("SC_OPEN_MAX") else: maxfd = 256 # default maximum for fd in range(0, maxfd): try: os.close(fd) except OSError: # ERROR (ignore) pass # Redirect the standard file descriptors to /dev/null. os.open("/dev/null", os.O_RDONLY) # standard input (0) os.open("/dev/null", os.O_RDWR) # standard output (1) os.open("/dev/null", os.O_RDWR) # standard error (2) return(0) if __name__ == "__main__": # Self-test. retCode = createDaemon() # If executed with superuser privilages, there should be a new file in the # "/" directory. It should contain the function's return code, the daemon's # PID, PPID, and PGRP. Its PID should not equal its PGRP, and its PPID # should equal 1. If it's executed without superuser privilages, the file # won't be created and no errors will be reported. open("createDaemon.log", "w").write("rc: %s; pid: %d; ppid: %d; pgrp: %d" %\ (retCode, os.getpid(), os.getppid(), os.getpgrp())) sys.exit(0)