End-of-Transmission character: Difference between revisions

Dated {{Which?}}. (Build p609)
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An EOT is often used to initiate other functions, such as releasing circuits, disconnecting terminals, or placing receive terminals in a [[standby]] condition.<ref name=1037c1996/> Its most common use today is to cause a [[Unix]] [[tty]] driver to signal [[end of file]] and thus exit programs that are awaiting input.
In [[ASCII]] and [[Unicode]], the character with the value 4 is EOT. It can be referred to as control-D, <tt>^D</tt> in [[caret notation]].{{FactCitation needed|date=March 2009}}
== Meaning in Unix ==
In Unix the end-of-file character (by default EOT) causes the terminal driver to make available all characters its input buffer immediately; normally the driver would collect characters until it sees an end-of-line character. If the input buffer is empty (because no characters have been typed since the last end-of-line or end-of-file), a program reading from the terminal reads a count of zero bytes. In Unix, such a condition is understood as having reached the end of the file.
This can be demonstrated with the <tt>[[cat (Unix)|cat]]</tt> program on [[Unix]]-based operating systems such as [[Linux]]: Run the <code>cat</code> command with no arguments, so that it accepts its input from the keyboard and prints output to the screen. Type a few characters without pressing {{keypress|Enter}}, then type {{keypress|Ctrl}}+{{keypress|D}}. The characters you have typed to that point are sent to cat, which then writes them to the screen. If you type {{keypress|Ctrl}}+{{keypress|D}} without typing any characters first, you terminate the input stream and the program ends. You may be able to get an actual EOT by typing {{keypress|Ctrl}}+{{keypress|V}} and then {{keypress|Ctrl}}+{{keypress|D}}. On some systems{{whichWhich?|date=April 2011}}, typing {{keypress|Ctrl}}+{{keypress|D}} twice generates the EOT.<!-- dubious, as the first ^D would be the eof. Perhaps only on systems where ^D is not EOF, but those are irrelevant to this discussion -->
If the terminal driver is in '''raw''' mode it no longer interprets control characters, and the EOT character is sent unchanged to the program, which is free to interpret it any way it likes. A program may then decide to handle the EOT byte as an indication that it should end the text, this would then be similar to how Control-Z is handled by DOS programs.