creating a daemon?

[Harry George]

bmgx <bmgx_no_sp at mgleesonprop.co.za> writes:

> This is what I am trying to find out, instruction on how to create a
> simple daemon on unix systems(Linux), can't find any info on usual
> sources..
> 

1. The idea is to fork, clean up all the inheritable stuff like files
   and environment vars, and then fork again.  The resulting process
   can then be left running to do the daemon work.  I recall there is
   a daemon.py module somewhere which does this all for you.  Here are
   the basics:

def main(args):
    #---first fork---
    try:
        pid = os.fork()
        if pid > 0:
            #---first parent---
            sys.exit(0)
    except OSError,e:
        print >>sys.stderr, "fork #1 failed %d (%s)" % (e.errno,e.strerror)
        sys.exit(1)

    
    #---make a clean process---
    os.chdir('/')
    os.setsid()
    os.umask(000)
    child_in =open('/dev/null','r')
    child_out=open('/dev/null','w')
    os.dup2(child_in.fileno(), sys.stdin.fileno())
    os.dup2(child_out.fileno(), sys.stdout.fileno())
    os.dup2(child_out.fileno(), sys.stderr.fileno())

    #---second fork---
    try:
        pid = os.fork()
        if pid > 0:
            #---second parent---
            sys.exit(0)
    except OSError,e:
        print >>sys.stderr, "fork #2 failed %d (%s)" % (e.errno,e.strerror)
        sys.exit(1)
    

    #---in clean child---
    x=MyApp()
    x.run()


2. To talk to that daemon, you can use a) files at known locations, b)
   pipes (see popen), c) sockets with ad hoc protocols (see asyncchat
   and SocketServer), d) predefined protocols (see Internet Protocols
   and Services).  The various "server" modules have the setup code
   from "1" above builtin, so you can ignore that level of detail.
   ["see" references can be found in the normal python documentation's
   Library Reference"].

3. Once you have a working daemon, you need a way to start and stop
   it.  You could manually start it each time you need it and then
   "kill" it when done.  More commonly you would do it (in Linux and
   UNIX) via a boot-time initialization script (typically found in
   /etc/init.d).  Each *NIX has its own flavor of setting these up, so
   you need to look at existing services and maybe copy-and-edit a
   working script.  

4. If security is an issue (it probably is), you may also want to hide
   the service behind xinetd.  That is a whole setup story in its own
   right.  And if security is really troublesome, you may need to
   assure the python interpreter itself is safe.  That kind of
   consideration is way out of my league.

-- 
harry.g.george at boeing.com
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