Writing a service in Linux Post: 30661

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Top Forums UNIX for Advanced & Expert Users Writing a service in Linux Post 30661 by lusername on Thursday 24th of October 2002 10:02:30 PM

10-24-2002

Registered User

If your program is written in C, your program should use the daemon() library call to make your program a daemon. This will cause your program to close its standard input, output, and error, and fork() into the background.

You should then add a command to one of your boot scripts to cause your program to run. The location of the script to change differs between Linux distributions. If you are running Red Hat, you should edit /etc/rc.d/rc.local. The script you need to edit is started directly or indirectly from /etc/inittab.

my-daemon.c:

Code:

#include <unistd.h>
#include <err.h>

int main()
{
   if(daemon(0,0) == -1)
       err(1, NULL);

   while(1)
   {
      /* do stuff */
   }
}

/etc/rc.d/rc.local:

Code:

#!/bin/sh
#
# This script will be executed *after* all the other init scripts.
# You can put your own initialization stuff in here if you don't
# want to do the full Sys V style init stuff.

# Run my-daemon in the background
/usr/sbin/my-daemon

Your daemon will run until the system shuts down, or until it crashes. Also, if your daemon has a memory leak, the kernel will kill it when it uses too much RAM.

If you make your program a non-daemon that closes its standard input/output/err, you can start it directly from inittab, with the action field set to "respawn". In this case, if your program crashes, init will restart it.

my-service.c:

Code:

#include <unistd.h>

int main()
{
   close(0);
   close(1);
   close(2);
   chdir("/");

   while(1)
   {
      /* do stuff */
   }
}

/etc/inittab:

Code:

#
# inittab       This file describes how the INIT process should set up
#               the system in a certain run-level.
#

# [ snip for brevity ]

# Run only in runlevel 5. Init will kill my-service if
# another runlevel is selected. If it crashes quickly
# and repeatedly, init will stop restarting it for 5 minutes.

mine:5:respawn:/usr/sbin/my-service

lusername

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initscript

INITSCRIPT(5)						Linux System Administrator's Manual					     INITSCRIPT(5)

NAME

       initscript - script that executes inittab commands.

SYNOPSIS

       /bin/sh /etc/initscript id runlevels action process

DESCRIPTION

       When  the  shell  script /etc/initscript is present, init will use it to execute the commands from inittab.  This script can be used to set
       things like ulimit and umask default values for every process.

EXAMPLES

       This is a sample initscript, which might be installed on your system as /etc/initscript.sample.

	      #
	      # initscript   Executed by init(8) for every program it
	      # 	     wants to spawn like this:
	      #
	      # 	     /bin/sh /etc/initscript <id> <level> <action> <process>
	      #

		# Set umask to safe level, and enable core dumps.
		umask 022
		ulimit -c 2097151
		PATH=/bin:/sbin:/usr/bin:/usr/sbin
		export PATH

		# Increase the hard file descriptor limit for all processes
		# to 8192.  The soft limit is still 1024, but any unprivileged
		# process can increase its soft limit up to the hard limit
		# with "ulimit -Sn xxx" (needs a 2.2.13 or later Linux kernel).
		ulimit -Hn 8192

		# Execute the program.
		eval exec "$4"

NOTES

       This script is not meant as startup script for daemons or somesuch.  It has nothing to do with a rc.local style script. It's just a handler
       for things executed from /etc/inittab. Experimenting with this can make your system un(re)bootable.

FILES

       /etc/inittab, /etc/initscript.

AUTHOR

       Miquel van Smoorenburg ,<miquels@cistron.nl>

SEE ALSO

       init(8), inittab(5).

								   July 10, 2003						     INITSCRIPT(5)

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