Linux Network Programming, Part 2
init's primary role is to create processes from information stored in the /etc/inittab file. It is either directly or indirectly responsible for all the user-created processes running on a system. It can respawn processes it starts if they die.
The respawning capabilities of init will get quite confused if the daemon forks as per the code in Listing 1. The original daemon process will immediately exit (with a child daemon continuing to run), and init will take this to mean the daemon has died. A simple solution is to add a command-line switch to the daemon (perhaps -init) to inform it to avoid the forking code. A better solution is to start the daemon from /etc/rc scripts rather than from the /etc/inittab.
The System V layout of /etc/rc is used in the popular Red Hat and Debian distributions of Linux. In this system, each daemon that must be started/stopped has a script in /etc/rc/init.d for Red Hat and in /etc/init.d for Debian. This script is invoked with a single command-line argument start to start the daemon, and a single command-line argument stop to stop the daemon. The script is typically named after the daemon.
If you want to start the daemon in a particular run level, you will need a link from the run-level directory to the appropriate script in /etc/rc/init.d. You must name this start link Sxxfoobar, where foobar is the name of the daemon and xx is a two digit number. The number is used to arrange the order in which the scripts are run.
Similarly, if you want the daemon to die when changing out of a particular run level, you will need a corresponding link from the run-level directory to the /etc/rc/init.d script. The kill link must be named Kxxfoobar, following the same naming convention as the start link.
Allowing system administrators to start/stop daemons (by calling the appropriate script from /etc/rc/init.d, with the appropriate command- line argument) is one of the nicer advantages of the SysV structure as well as its greater flexibility over the previous BSD-style /etc/rc.d layout.
The shell script in Listing 6 shows a typical Red Hat style example in /etc/rc/init.d for a daemon called foobar.
It is often useful for a daemon to log its activities for debugging and system administration/maintenance purposes. It does this by opening a file and writing events to this file as they happen. Many Linux daemons use the syslog() call to log daemon status information etc. The syslog is a client-server logging facility, originating from BSD 4.2. I am not aware of any SVR4 or POSIX equivalent. Messages to the syslog service are generally sent to text files described in /etc/syslog.conf, but may be sent to remote machines running a syslogd daemon.
Using the Linux syslog interface is quite simple. Three function calls are prototyped in /usr/include/syslog.h (see the syslog.3 man page):
void openlog(char *ident, int option, int facility); void syslog(int priority, char *format, ...); void closelog(void);
openlog() creates a connection to the system logger. The ident string is added to each message logged and is generally the name of the daemon. The option parameter allows for logging to the console in case of error, logging to stderr as well as the console, logging of the PID and so on. The facility argument classifies the type of program or daemon logging the message and this defaults to LOG_USER.
The syslog() call does the actual logging. Values for format and the variable arguments are similar to printf(), with the exception that %m will be replaced by the error message corresponding to the current value of errno. The priority parameter indicates the type and relative importance of the message being logged.
To break the connection with the system logger and close any associated file descriptor or socket, use closelog(). The use of openlog() and closelog() is optional. More detailed information on these functions is available in the syslog (3) man page.
Dr. John Nelson is a senior lecturer in Computer Engineering at the University of Limerick. His interests include mobile communications, intelligent networks, Software Engineering and VLSI design. His e-mail address is email@example.com.
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