[from the mythological
meaning, later rationalized as the acronym `Disk And Execution
MONitor'] A program that is not invoked explicitly, but lies
dormant waiting for some condition(s) to occur. The idea is that
the perpetrator of the condition need not be aware that a daemon is
lurking (though often a program will commit an action only because
it knows that it will implicitly invoke a daemon). For example,
under ITS writing a file on the LPT spooler's directory
would invoke the spooling daemon, which would then print the file.
The advantage is that programs wanting (in this example) files
printed need neither compete for access to nor understand any
idiosyncrasies of the LPT. They simply enter their implicit
requests and let the daemon decide what to do with them. Daemons
are usually spawned automatically by the system, and may either
live forever or be regenerated at intervals.
Daemon and demon are often used interchangeably, but seem to
have distinct connotations. The term `daemon' was introduced to
computing by CTSS people (who pronounced it /dee'mon/) and
used it to refer to what ITS called a dragon; the prototype
was a program called DAEMON that automatically made tape backups of
the file system. Although the meaning and the pronunciation have
drifted, we think this glossary reflects current (2000) usage.
