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).
Read more about this topic: Daemon (computing)
Famous quotes containing the word creation:
“Like witches they flew along rows
Keeping creation at ease;
With a tendril for needle
They sewed up the air with a stem;”
—Theodore Roethke (1908–1963)
“There’s something wonderfully exciting about the quiet sing song of an aeroplane overhead with all the guns in creation lighting out at it, and searchlights feeling their way across the sky like antennae, and the earth shaking snort of the bombs and the whimper of shrapnel pieces when they come down to patter on the roof.”
—John Dos Passos (1896–1970)
“As the truest society approaches always nearer to solitude, so the most excellent speech finally falls into Silence. Silence is audible to all men, at all times, and in all places. She is when we hear inwardly, sound when we hear outwardly. Creation has not displaced her, but is her visible framework and foil. All sounds are her servants, and purveyors, proclaiming not only that their mistress is, but is a rare mistress, and earnestly to be sought after.”
—Henry David Thoreau (1817–1862)