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:
“The private detective of fiction is a fantastic creation who acts and speaks like a real man. He can be completely realistic in every sense but one, that one sense being that in life as we know it such a man would not be a private detective.”
—Raymond Chandler (1888–1959)
“Language ought to be the joint creation of poets and manual workers.”
—George Orwell (1903–1950)
“Poetry, at all times, exercises two distinct functions: it may reveal, it may unveil to every eye, the ideal aspects of common things ... or it may actually add to the number of motives poetic and uncommon in themselves, by the imaginative creation of things that are ideal from their very birth.”
—Walter Pater (1839–1894)