Editing End-of-file

{{short description|Offset that corresponds to the first byte beyond the length of a computer file}}
{{use dmy dates|date=November 2021|cs1-dates=y}}
{{use list-defined references|date=December 2021}}
In [[computing]], '''end-of-file''' ('''EOF''')<ref name="Pollock"/> is a condition in a computer [[operating system]] where no more data can be read from a data source. The data source is usually called a [[file (computing)|file]] or [[stream (computing)|stream]].

==Details==
In the [[C standard library]], the character-reading functions such as [[getchar]] return a value equal to the symbolic value (macro) <code>EOF</code> to indicate that an end-of-file condition has occurred. The actual value of <code>EOF</code> is implementation-dependent and must be negative (it is commonly −1, such as in [[glibc]]<ref name="GNU-C"/>). Block-reading functions return the number of bytes read, and if this is fewer than asked for, then the end of file was reached or an error occurred (checking of <code>[[errno]]</code> or dedicated function, such as <code>ferror</code> is required to determine which).

==EOF character==
Input from a terminal never really "ends" (unless the device is disconnected), but it is useful to enter more than one "file" into a terminal, so a key sequence is reserved to indicate end of input. In [[Unix|UNIX]], the translation of the keystroke to EOF is performed by the terminal driver, so a program does not need to distinguish terminals from other input files. By default, the driver converts a [[End-of-transmission character|Control-D]] character at the start of a line into an end-of-file indicator. To insert an actual Control-D (ASCII 04) character into the input stream, the user precedes it with a "quote" command character (usually [[Control-V]]). [[AmigaDOS]] is similar but uses Control-\ instead of Control-D.

In [[DOS]] and [[Microsoft Windows|Windows]] (and in [[CP/M]] and many [[Digital Equipment Corporation|DEC]] operating systems such as the [[PDP-6]] monitor,<ref name="DEC_1965_PDP-6"/> [[RT-11]], [[OpenVMS|VMS]] or [[TOPS-10]]<ref name="DEC_1970_PDP-10"/>), reading from the terminal will never produce an EOF. Instead, programs recognize that the source is a terminal (or other "character device") and interpret a given reserved character or sequence as an end-of-file indicator; most commonly, this is an [[ASCII]] [[Substitute character|Control-Z]], code 26. Some MS-DOS programs, including parts of the Microsoft MS-DOS shell ([[COMMAND.COM]]) and operating-system utility programs (such as [[EDLIN]]), treat a Control-Z in a text file as marking the end of meaningful data, and/or append a Control-Z to the end when writing a text file. This was done for two reasons:
* Backward compatibility with [[CP/M]]. The CP/M [[file system]] (and also the original [[8-bit FAT]] implemented in [[Microsoft BASIC]]) only recorded the lengths of files in multiples of 128-byte "records", so, by convention, a Control-Z character was used to mark the end of meaningful data if it ended in the middle of a record. The [[FAT12]] filesystem introduced with [[86-DOS]] and MS-DOS has always recorded the exact byte-length of files, so this was never necessary on DOS.
* It allows programs to use the same code to read input from both a terminal and a text file.

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{{anchor|EOT}}
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In the ANSI X3.27-1969 [[magnetic tape data storage|magnetic tape]] standard, the end of file was indicated by a '''tape mark''', which consisted of a gap of approximately 3.5&nbsp;inches of tape followed by a single byte containing the character <code>0x13</code> (hex) for [[9-track tape|nine-track tapes]] and <code>017</code> (octal) for [[IBM 7-track|seven-track tapes]].<ref name="MARC21_1977"/> The '''end-of-tape''', commonly abbreviated as '''EOT''', was indicated by two tape marks. This was the standard used, for example, on [[IBM 360]]. The [[9-track tape#Technical details|reflective strip]] that was used to announce impending physical end of tape was also called an '''EOT''' marker.

==See also==
* [[End-of-Transmission character]] (EOT)
* [[Substitute character]]
* [[End of message]]
* [[Here document]]
* [[-30-]]

==References==
{{reflist|refs=
<ref name="Pollock">{{cite web |title=Shell Here Document Overview |publisher=hccfl.edu |author-first=Wayne |author-last=Pollock |url=http://content.hccfl.edu/pollock/ShScript/HereDoc.htm |access-date=2014-05-28 |url-status=dead |archive-url=https://web.archive.org/web/20140529084958/http://content.hccfl.edu/pollock/ShScript/HereDoc.htm |archive-date=2014-05-29}}</ref>
<ref name="GNU-C">{{cite web |title=The GNU C Library |website=www.gnu.org |url=https://www.gnu.org/software/libc/manual/html_mono/libc.html#EOF-and-Errors}}</ref>
<ref name="DEC_1965_PDP-6">{{cite book |title=PDP-6 Multiprogramming System Manual |chapter=Table of IO Device Characteristics - Console or Teletypewriters |id=DEC-6-0-EX-SYS-UM-IP-PRE00 |publisher=[[Digital Equipment Corporation]] (DEC) |publication-place=Maynard, Massachusetts, USA |date=1965 |page=43 |url=http://bitsavers.trailing-edge.com/pdf/dec/pdp6/DEC-6-0-EX-SYS-UM-IP-PRE00_Multiprogramming_System_Manual_1965.pdf |access-date=2014-07-10 |url-status=live |archive-url=https://web.archive.org/web/20140714140253/http://bitsavers.trailing-edge.com/pdf/dec/pdp6/DEC-6-0-EX-SYS-UM-IP-PRE00_Multiprogramming_System_Manual_1965.pdf |archive-date=2014-07-14}} (1+84+10 pages)</ref>
<ref name="DEC_1970_PDP-10">{{cite book |title=PDP-10 Reference Handbook: Communicating with the Monitor - Time-Sharing Monitors |volume=3 |chapter=5.1.1.1. Device Dependent Functions - Data Modes - Full-Duplex Software A(ASCII) and AL(ASCII Line) |publisher=[[Digital Equipment Corporation]] (DEC) |date=1969 |pages=5-3 – 5-6 [5-5 (431)] |url=http://bitsavers.org/pdf/dec/pdp10/1970_PDP-10_Ref/1970PDP10Ref_Part3.pdf |access-date=2014-07-10 |url-status=live |archive-url=https://web.archive.org/web/20111115083418/http://www.bitsavers.org/pdf/dec/pdp10/1970_PDP-10_Ref/1970PDP10Ref_Part3.pdf |archive-date=2011-11-15}} (207 pages)</ref>
<ref name="MARC21_1977">{{cite web |title=Tape Transfer (Pre-1977): Exchange Media: MARC 21 Specifications for Record Structure, Character Sets, and Exchange Media (Library of Congress) |website=www.loc.gov |url=https://www.loc.gov/marc/specifications/specexchtape2.html#mark}}</ref>
}}

[[Category:C standard library]]
[[Category:Computer files]]