CS162B: Daemonization Jacob 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 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 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)