Daemon (computing) - Creation

Creation

In a strictly technical sense, a Unix-like system process is a daemon when its parent process terminates and the daemon is assigned the init process (process number 1) as its parent process and has no controlling terminal. However, more commonly, a daemon may be any background process, whether a child of the init process or not.

On a Unix-like system, the common method for a process to become a daemon, when the process is started from the command line or from a startup script such as an init script or a SystemStarter script, involves:

• Dissociating from the controlling tty
• Becoming a session leader
• Becoming a process group leader
• Executing as a background task by forking and exiting (once or twice). This is required sometimes for the process to become a session leader. It also allows the parent process to continue its normal execution.
• Setting the root directory (/) as the current working directory so that the process does not keep any directory in use that may be on a mounted file system (allowing it to be unmounted).
• Changing the umask to 0 to allow open, creat, et al. operating system calls to provide their own permission masks and not to depend on the umask of the caller
• Closing all inherited files at the time of execution that are left open by the parent process, including file descriptors 0, 1 and 2 (stdin, stdout, stderr). Required files will be opened later.
• Using a logfile, the console, or /dev/null as stdin, stdout, and stderr

If the process is started by a super-server daemon, such as inetd, launchd, or systemd, the super-server daemon will perform those functions for the process (except for old-style daemons not converted to run under systemd and specified as Type=forking and "multi-threaded" datagram servers under inetd).