Editing SIGHUP (section)

==History==
[[Image:Acoustic coupler 20041015 175456 1.jpg|thumb|A ''hangup'' was often the result of a connected user physically hanging up the modem]]
Access to [[Mainframe computer|computer systems]] for many years consisted of connecting a [[Computer terminal|terminal]] to a mainframe system via a serial line and the [[RS-232]] [[Communications protocol|protocol]]. When a system of software [[interrupt]]s, called signals, was being developed, one of those signals was designated for use on hangup.

SIGHUP would be sent to [[Computer program|programs]] when the serial line was dropped, often because the connected user terminated the connection by hanging up the [[modem]]. The system would detect the line was dropped via the lost [[Data Carrier Detect]] (DCD) signal.

Signals have always been a convenient method of [[inter-process communication]] (IPC), but in early implementations there were no user-definable signals (such as the later additions of [[SIGUSR1 and SIGUSR2]]) that programs could intercept and interpret for their own purposes. For this reason, applications that did not require a controlling terminal, such as [[daemon (computing)|daemons]], would re-purpose SIGHUP as a signal to re-read [[configuration file]]s, or reinitialize. This convention survives to this day in packages such as [[Apache HTTP Server|Apache]] and [[Sendmail]].