Disk and Execution Monitor (Daemon)

A disk and execution monitor (daemon) is a background process run in computer multitasking operating systems, usually at bootstrap time, to perform administrative changes or monitor services.

Common daemon processes include email handlers, print spoolers and other programs that perform OS administrative tasks. Daemons also perform specified operations at predefined times in response to events.

Unix daemon files generally have a "d" suffix. For example, "identd" refers to a daemon that provides the identity of a TCP connection. Microsoft OS daemons are referred to as terminate and stay resident (TSR) programs and are called "system agents" or "services" within the context of OS administration.

Mac OS X, a Unix-based system, also uses daemons, but it does not provide the same services as in the Microsoft OS.

Daemon parent processes are often the initialization process. A process becomes a daemon by forking a child process and exiting the parent process, causing initialization to adopt the child process.

Systems often start daemons at boot time, which to respond to network requests, hardware activity or programs that perform specified tasks. Daemons are also able to configure hardware and run scheduled tasks.

The common methods by which a process becomes a daemon include:

• Dissociating from the controlling tty
• Forming a session leader
• Forming a process group leader
• Staying in the background by forking and exiting
• Setting the root directory as the current working directory
• Setting the unmask to zero to allow open () and create () calls to provide their own permission masks
• Closing inherited files left open by the parent process at the time of execution
• Using the console, a log file or/dev/null as standard input, standard output and standard error