CS162B: DaemonizationJacob T.Chan

Foreground Process

Has input/output capabilities These require users at the terminal

Lives as long as the terminal that started it

Associated with the caller (user or process executing it) or a controlling terminal

Background Process

Runs by itself Responding to events aside from I/O Example: SSH (Secure Shell) connection requests

Spawned by system upon boot up

Has same access privileges as root

Usually handles hardare

Can be associated with caller, but not under direct control Lack of controlling terminal Termination of parent process

Daemons

Background process in Unix

These do not die usually or will require restarting

They spawn other processes to handle further events Example: upon receiving connection request, server daemon will spawn another

process to establish a MORE SPECIFIC connection

They write to a certain system log file (found in /var/log) instead of console Reason: they lack controlling terminals

Daemons

#include <unistd.h>

#include <sys/stat.h>

if(fork() != 0)

exit(0);

// only the child process gets this far – daemonize now!

setsid(); // make this process a leader of its own session

chdir("/"); // prevents process from hogging a directory

// the chdir call prevents "directory in use" issue

Daemons

// files created must have rwx permissions for the owner of this process (usually root)and rx permissions for the group

umask(027); //remember this?

// close stdin, stdout, stderr (where applicable)

close(0); // stdin

close(1); // stdout

close(2); // stderr

Printing out System Logs

#include <syslog.h>

openlog("Daemon Log Test", LOG_PID | LOG_CONS, LOG_USER);

Configure system log so that:1) Whenever this process writes to it, the string "Daemon Log Test" is appended at

the beginning.2) The process ID is logged with every message (LOG_PID) and the messages will be

written to the console if the system log is unavailable (LOG_CONS).3) Messages are assumed to be generated from an arbitrary process (LOG_USER).

Printing out System Logs

generate a system log message "AIEEEEE" that will be marked as an error message generated by an arbitrary process

syslog(LOG_ERR | LOG_USER, "AIEEEEE"); syslog acts like printf! remember: it (usually) writes to /var/log

int i = 5 + 5;

syslog(LOG_ERR | LOG_USER, "5+5=%d", i);

closelog();

close file descriptors that were opened to gain access to the system logs by both openlog and syslog (in short, so that it is accessible)