CloudFlare suffered a massive security issue affecting all of its customers, including Rosetta Code. All passwords not changed since February 19th 2017 have been expired, and session cookie longevity will be reduced until late March.--Michael Mol (talk) 05:15, 25 February 2017 (UTC)

Run as a daemon or service

From Rosetta Code
Run as a daemon or service is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

A daemon is a service that runs in the background independent of a users login session.

Demonstrate how a program disconnects from the terminal to run as a daemon in the background.

Write a small program that writes a message roughly once a second to its stdout which should be redirected to a file.

Note that in some language implementations it may not be possible to disconnect from the terminal, and instead the process needs to be started with stdout (and stdin) redirected to files before program start. If that is the case then a helper program to set up this redirection should be written in the language itself. A shell wrapper, as would be the usual solution on Unix systems, is not appropriate.


Library: BSD libc

BSD provides a convenient daemon(3) function. GNU libc also provides daemon(3), but POSIX omits it, so it is not portable. Other BSDisms in this program are __progname and <err.h>.

The task also wants to redirect stdout. This program does so with dup2(2). Had we wanted to directly write to a file, we could open the file with file = fopen(argv[1], "a"), and write to file instead of stdout.

#include <err.h>
#include <errno.h>
#include <fcntl.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <syslog.h>
#include <time.h>
#include <unistd.h>
main(int argc, char **argv)
extern char *__progname;
time_t clock;
int fd;
if (argc != 2) {
fprintf(stderr, "usage: %s file\n", __progname);
/* Open the file before becoming a daemon. */
fd = open(argv[1], O_WRONLY | O_APPEND | O_CREAT, 0666);
if (fd < 0)
err(1, argv[1]);
* Become a daemon. Lose terminal, current working directory,
* stdin, stdout, stderr.

if (daemon(0, 0) < 0)
err(1, "daemon");
/* Redirect stdout. */
if (dup2(fd, STDOUT_FILENO) < 0) {
syslog(LOG_ERR, "dup2: %s", strerror(errno));
/* Dump clock. */
for (;;) {
fputs(ctime(&clock), stdout);
if (fflush(stdout) == EOF) {
syslog(LOG_ERR, "%s: %s", argv[1], strerror(errno));
sleep(1); /* Can wake early or drift late. */
$ make dumper
cc -O2 -pipe    -o dumper dumper.c 
$ ./dumper dump
$ tail -f dump
Fri Nov 18 13:50:41 2011
Fri Nov 18 13:50:42 2011
Fri Nov 18 13:50:43 2011
Fri Nov 18 13:50:44 2011
Fri Nov 18 13:50:45 2011
$ pkill -x dumper
$ rm dump


GP scripts cannot run in this fashion directly, but can be compiled into PARI code with gp2c. PARI code, whether from gp2c or not, can be run as a daemon just as C would be.


(unless (fork)
(out "file.log"
(println *Pid) # First write the daemon's PID to the file
(for N 3600 # Write count for about one hour (if not killed)
(wait 1000)
(println N)
(flush) ) )
(bye) ) # Child terminates after one hour
(bye) # Parent terminates immediately


__FILE__ is a preprocessor definition that contains the current filename. if the first argument is "daemon" the program will be restarted with stdout redirected to "foo".

int main(int argc, array argv)
if (sizeof(argv)>1 && argv[1] == "daemon")
Stdio.File newout = Stdio.File("foo", "wc");
Process.spawn_pike(({ __FILE__ }), ([ "stdout":newout ]));
return 1;
int i = 100;


#lang racket
(require ffi/unsafe)
((get-ffi-obj 'daemon #f (_fun _int _int -> _int)) 0 0)
(with-output-to-file "/tmp/foo"
(λ() (for ([i 10]) (displayln (random 1000)) (flush-output) (sleep 1))))


Tcl doesn't come with tools for converting the process into a daemon, but can build them easily enough. Here's the BSD daemon function mapped into a Tcl command in a package.

Library: Critcl
package provide daemon 1
package require critcl
critcl::ccode {
#include <stdlib.h>
critcl::cproc daemon {Tcl_Interp* interp} ok {
if (daemon(0, 0) < 0) {
Tcl_AppendResult(interp, "cannot switch to daemon operation: ",
Tcl_PosixError(interp), NULL);
return TCL_ERROR;
return TCL_OK;

These tools can then be used to solve this task:

### Command line argument parsing
if {$argc < 1} {
puts "usage: $argv0 file ?message...?"
exit 1
} elseif {$argc == 1} {
set filename [lindex $argv 0]
set message "Hi there!"
} else {
set message [join [lassign $argv filename]]
### Daemonize
package require daemon
close stdout; open $filename ;# Redirects stdout!
### Print the message to the file every second until killed
proc every {ms body} {eval $body; after $ms [info level 0]}
every 1000 {puts "[clock format [clock seconds]]: $message"}
vwait forever

On Windows, there is a commercial extension to Tcl which allows a script to be installed as a service. Such a script would be much like the one above, but without the daemonization section as that has become a property of the runtime.