Daemon Processes

1

• Chapter 12.
• Daemon Processes
• and inetd Superserver

2
12.1 Introduction

• A daemon is a process that runs in the background
  and is independent of control from all terminals.
• There are numerous ways to start a daemon
• 1. the system initialization scripts (
  /etc/rc )
• 2. the inetd superserver
• 3. cron deamon
• 4. the at command
• 5. from user terminals
• Since a daemon does not have a controlling
  terminal, it needs some way to output message
  when something happens, either normal
  informational messages, or emergency messages
  that need to be handled by an administrator.

3
12.2 syslogd daemon

• Berkeley-derived implementation of syslogd
  perform the following actions upon startup.
• 1. The configuration file is read, specifying
  what to do with each type of log message that the
  daemon can receive.
• 2. A Unix domain socket is created and bound to
  the pathname /var/run/log ( /dev/log on some
  system).
• 3. A UDP socket is created and bound to port 514
• 4. The pathname /dev/klog is opened. Any error
  messages from within the kernel appear as input
  on this device.
• We could send log messages to the syslogd daemon
  from our daemons by creating a Unix domain
  datagram socket and sending our messages to the
  pathname that the daemon has bound, but an easier
  interface is the syslog function.

4
12. 3 syslog function

• the priority argument is a combination of a level
  and a facility.
• The message is like a format string to printf,
  with the addition of a m specification, which is
  replaced with the error message corresponding to
  the current value of errno.
• Ex) Syslog(LOG_INFOLOG_LOCAL2, rename(s, s)
  m,file1,file2)

5
12. 3 syslog function

• Log message have a level between 0 and 7.

6
12. 3 syslog function

• A facility to identify the type of process
  sending the message.

7
12. 3 syslog function

• Openlog and closelog
• openlog can be called before the first call to
  syslog and closelog can be called when the
  application is finished sending is finished log
  messages.

8
12.4 daemon_init Function

• include "unp.h"
• include ltsyslog.hgt
• define MAXFD 64
• extern int daemon_proc / defined in error.c /
• void daemon_init(const char pname, int facility)
• int i
• pid_t pid
• if ( (pid Fork()) ! 0)
• exit(0) / parent terminates /
• / 1st child continues /
• setsid() / become session leader /
• Signal(SIGHUP, SIG_IGN)
• if ( (pid Fork()) ! 0) exit(0) / 1st child
  terminates /
• / 2nd child continues /
• daemon_proc 1 / for our err_XXX() functions
  /
• chdir("/") / change working directory /

9
12.5 inetd Daemon

• A typical Unix systems problems
• 1. All these daemons contained nearly identical
  startup code.
• 2. Each daemon took a slot in the process table,
  but each daemon was asleep most of the time.
• inetd daemon fixes the two problems.
• 1. It simplifies writing daemon processes, since
  most of the startup details are handled by inetd.
• 2. It allow a single process(inetd) to be waiting
  for incoming client requests for multiple
  services, instead of one process for each service.

10
12.5 inetd daemon

• Figure 12.7

11
12.6 daemon_inetd Function

• Figure 12.11

include "unp.h" include
ltsyslog.hgt extern int daemon_proc /
defined in error.c / void daemon_inetd(const
char pname, int facility) daemon_proc
1 / for our err_XXX()
functions / openlog(pname, LOG_PID,
facility)
12
12.6 daemon_inetd Function

• Figure 12.12

include "unp.h" include
lttime.hgt int main(int argc, char argv)
socklen_t len struct sockaddr
cliaddr char
buffMAXLINE time_t
ticks daemon_inetd(argv0, 0)
cliaddr Malloc(MAXSOCKADDR) len
MAXSOCKADDR Getpeername(0, cliaddr, len)
err_msg("connection from s",
Sock_ntop(cliaddr, len)) ticks
time(NULL) snprintf(buff, sizeof(buff),
".24s\r\n", ctime(ticks)) Write(0, buff,
strlen(buff)) Close(0) / close TCP
connection / exit(0)