chickadee » daemon

daemon

Description

Create processes to do your dirty work in the background.

Based on section 1.7 of http://www.faqs.org/faqs/unix-faq/programmer/faq, quoted next for convenience:

1.7 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.

The highest overview possible is that the calling process (P) forks to create a child (C1), and this child will create another child (C2) that will run the thunk.

This egg is not thoroughly tested, but I've been using it for a while for some simple use cases, and it's behaved well enough so far.

Help, bug reports, suggestions, &c are all much appreciated!

Author

siiky

Repository

https://git.sr.ht/~siiky/daemon

Requirements

No dependencies.

API

Details extracted from the source code documentation.

daemon thunk #!key (cwd /) (killothers? #f) (stderr /dev/null-w) (stdin /dev/null-r) (stdout /dev/null-w) (want-pid? #f)procedure

The one and only procedure exported from the daemon module. Creates a background process to run the given thunk.

If want-pid? is #f returns #f; otherwise, and if everything goes well, returns the PID of the created process, as a fixnum.

As with all OS things, the devil is in the details -- implementation details do matter a great deal!

thunk

The thunk to run in the created daemon process.

cwd

The working directory of the created process. Defaults to "/". If #f is given, doesn't change the working directory (which will be inherited from the parent process P). If the given value is a string, changes to that directory. Otherwise, uses the default value.

killothers?

Check chicken.process.process-fork for details. Defaults to #f.

stderr, stdin, stdout

These change the (current-error-port), (current-input-port), and (current-output-port) available to the thunk. All three default to /dev/null. Legal values for these parameters are input/output ports (whichever is appropriate); a string designating the path to a file (absolute, or relative to cwd); a fixnum designating a file descriptor; a promise that may be forced or a thunk that may be called, that evaluate to an input/output port.

want-pid?

In some circumstances it is useful to know the PID of the created process. If want-pid? is given and not #f, then the calling process P waits for C2 to be created and for C1 to report back C2's PID; otherwise, the procedure returns #f immediately. The implementation is a hack: P opens a one-way pipe from C1; when C1 forks for C2, C1 writes C2's PID to the pipe; finally P reads the PID from the pipe. Lots could go wrong here, but it's hard to know beforehand of all the possible failure causes.

Example

Create a file stdin.txt with some text and run the following:

(import chicken.io chicken.port)
(import srfi-42)
(import daemon)

(print "daemon starting")
(let ((stdin "stdin.txt") ; Let each process read the file
      ; Share the output files.
      (stdout (open-output-file "stdout.txt"))
      (stderr (open-output-file "stderr.txt")))
  (let ((pids
          (list-ec (:range i 0 10)
                   (daemon
                     (lambda ()
                       (print "started[" i "]")
                       (do-ec (:port ln (current-input-port) read-line)
                              (begin
                                (with-output-to-port (current-error-port)
                                                     (cute print "stderr[" i "]: " ln))
                                (print "stdout[" i "]: " ln)
                                (sleep 1))))
                     #:cwd #f
                     #:stdin stdin
                     #:stdout stdout
                     #:stderr stderr
                     #:want-pid? #t))))

    (print "daemons started " pids)))

You should be able to see on your terminal the lines daemon starting, daemons started (list of 10 PIDs), and some lines started[X], where X is the "ID" of a daemon (the i variable).

Additionally, you should expect stdin.txt to be untouched, and stdout.txt and stderr.txt to have the contents of stdin.txt with lines like stdout[X]: LINE and stderr[X]: LINE, respectively -- where LINE is a line read from stdin.txt.

Important points to note here:

• stdin.txt is "private"/"local" to each daemon process because it is opened by each daemon process -- stdin, stdout, and stderr are opened inside C1 when given as a string (path) or fixnum (file descriptor).
• stdout and stderr are "shared"/"global" to both daemon processes because the ports were created by P, and therefore there's only one port for each.
• The files are relative to the CWD of the shell where you ran the program.
• Although forking is relatively cheap and fast (to a human), you should expect the example (P) to finish faster if you give #:want-pids? #f.

License

This is free and unencumbered software released into the public domain.

Anyone is free to copy, modify, publish, use, compile, sell, or
distribute this software, either in source code form or as a compiled
binary, for any purpose, commercial or non-commercial, and by any
means.

In jurisdictions that recognize copyright laws, the author or authors
of this software dedicate any and all copyright interest in the
software to the public domain. We make this dedication for the benefit
of the public at large and to the detriment of our heirs and
successors. We intend this dedication to be an overt act of
relinquishment in perpetuity of all present and future rights to this
software under copyright law.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
OTHER DEALINGS IN THE SOFTWARE.

For more information, please refer to <http://unlicense.org>

Version History

0.0.1 (2022/01/08)

Initial release with documentation and all.