How do I get my program to act like a daemon?


A daemon process is usually defined as a background process that
does not belong to a terminal session. Many system services are performed
by daemons; network services, printing etc.



Simply invoking a program in the background isn't really adequate for these
long-running programs; that does not correctly detach the process from the
terminal session that started it. Also, the conventional way of starting
daemons is simply to issue the command manually or from an rc script; the
daemon is expected to put itself into the background.



Here are the steps to become a daemon:






  1. fork() so the parent can exit, this returns control to the
    command line or shell invoking your program. This step is required so
    that the new process is guaranteed not to be a process group leader. The
    next step, setsid(), fails if you're a process group leader.



  2. setsid() to become a process group and session group
    leader. Since a controlling terminal is associated with a session, and
    this new session has not yet acquired a controlling terminal our process
    now has no controlling terminal, which is a Good Thing for daemons.



  3. fork() again so the parent, (the session group leader), can exit.
    This means that we, as a non-session group leader, can never regain a
    controlling terminal.



  4. chdir("/") to ensure that our process doesn't keep any directory
    in use. Failure to do this could make it so that an administrator
    couldn't unmount a filesystem, because it was our current directory.

    [Equivalently, we could change to any directory containing files
    important to the daemon's operation.]



  5. umask(0) so that we have complete control over the permissions of
    anything we write. We don't know what umask we may have inherited.

    [This step is optional]



  6. close() fds 0, 1, and 2. This releases the standard in, out, and
    error we inherited from our parent process. We have no way of knowing
    where these fds might have been redirected to. Note that many daemons
    use sysconf() to determine the limit _SC_OPEN_MAX.
    _SC_OPEN_MAX tells you the maximun open files/process. Then in a
    loop, the daemon can close all possible file descriptors. You have to
    decide if you need to do this or not. If you think that there might be
    file-descriptors open you should close them, since there's a limit on
    number of concurrent file descriptors.



  7. Establish new open descriptors for stdin, stdout and stderr. Even if you
    don't plan to use them, it is still a good idea to have them open. The
    precise handling of these is a matter of taste; if you have a logfile,
    for example, you might wish to open it as stdout or stderr, and open
    `/dev/null' as stdin; alternatively, you could open
    `/dev/console' as stderr and/or stdout, and `/dev/null' as
    stdin, or any other combination that makes sense for your particular
    daemon.



Almost none of this is necessary (or advisable) if your daemon is being
started by inetd. In that case, stdin, stdout and stderr are all
set up for you to refer to the network connection, and the
fork()s and session manipulation should not be done (to
avoid confusing inetd). Only the chdir() and
umask() steps remain as useful.






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