Run as a daemon or service

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.

C

Most daemons have no stdout, but this draft task strangely wants to use stdout, so this program has a weird dup2() call. (Perhaps the task should remove its spurious stdout requirement.)

Library: BSD libc

<lang c>#include <err.h>

1. include <errno.h>
2. include <fcntl.h>
3. include <stdlib.h>
4. include <stdio.h>
5. include <string.h>
6. include <syslog.h>
7. include <time.h>
8. include <unistd.h>

int main(int argc, char **argv) { extern char *__progname; time_t clock; int fd;

if (argc != 2) { fprintf(stderr, "usage: %s file\n", __progname); exit(1); }

/* 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)); exit(1); } close(fd);

/* Dump clock. */ for (;;) { time(&clock); fputs(ctime(&clock), stdout); fflush(stdout); sleep(1); /* Can wake early or drift late. */ } }</lang>

$ 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
^C
$ pkill -x dumper
$ rm dump

Pike

__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".

<lang Pike>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;
   while(i--)
   {
       write(i+"\n");
       sleep(0.1);
   }

}</lang>