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<h2 id="details">Details</h2>
<p>In the <a href="/facts/C_standard_library/qdG1nW2c">C standard library</a>, the character-reading functions such as <a href="/facts/Getchar/m5n82Fqz">getchar</a> return a value equal to the symbolic value (macro) EOF to indicate that an end-of-file condition has occurred. The actual value of EOF is implementation-dependent and must be negative (it is commonly −1, such as in <a href="/facts/Glibc/AL6IYDkA">glibc</a><a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a>). 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 <a href="/facts/Errno/ahgJbtKC">errno</a> or dedicated function, such as ferror is required to determine which).
</p>
<h2 id="eof-character">EOF character</h2>
<p>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 <a href="/facts/Unix/0pNL2seI">UNIX</a>, 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 <a href="/facts/End-of-transmission_character/6kUye5G0">Control-D</a> 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 <a href="/facts/Control-V/Lgo6ETB8">Control-V</a>). <a href="/facts/AmigaDOS/D3AXdf2m">AmigaDOS</a> is similar but uses Control-\ instead of Control-D.
</p><p>In <a href="/facts/DOS/sWwDXNmX">DOS</a> and <a href="/facts/Microsoft_Windows/bwhAhuSL">Windows</a> (and in <a href="/facts/CP/M/n5rTgXdC">CP/M</a> and many <a href="/facts/Digital_Equipment_Corporation/ztxs6keA">DEC</a> operating systems such as the <a href="/facts/PDP-6/dlvBVlr8">PDP-6</a> monitor,<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> <a href="/facts/RT-11/AXaahzdB">RT-11</a>, <a href="/facts/OpenVMS/rAnHAgpD">VMS</a> or <a href="/facts/TOPS-10/FdKwlRK5">TOPS-10</a><a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a>), 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 <a href="/facts/ASCII/vGUI33Qu">ASCII</a> <a href="/facts/Substitute_character/9yHehSB0">Control-Z</a>, code 26. Some MS-DOS programs, including parts of the Microsoft MS-DOS shell (<a href="/facts/COMMAND.COM/06Sz5wle">COMMAND.COM</a>) and operating-system utility programs (such as <a href="/facts/EDLIN/7yxzQfR7">EDLIN</a>), 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:
</p>
<ul><li>Backward compatibility with <a href="/facts/CP/M/n5rTgXdC">CP/M</a>. The CP/M <a href="/facts/File_system/FdPm90uc">file system</a> (and also the original <a href="/facts/8-bit_FAT/uFvpmWxa">8-bit FAT</a> implemented in <a href="/facts/Microsoft_BASIC/SuctFIyc">Microsoft BASIC</a>) 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 <a href="/facts/FAT12/uFvpmWxa">FAT12</a> filesystem introduced with <a href="/facts/86-DOS/u0Hm0Ijv">86-DOS</a> and MS-DOS has always recorded the exact byte-length of files, so this was never necessary on DOS.</li>
<li>It allows programs to use the same code to read input from both a terminal and a text file.</li></ul>
<p>

In the ANSI X3.27-1969 <a href="/facts/Magnetic_tape_data_storage/qJUpJnk9">magnetic tape</a> standard, the end of file was indicated by a tape mark, which consisted of a gap of approximately 3.5 inches of tape followed by a single byte containing the character 0x13 (hex) for <a href="/facts/9-track_tape/5Eb6MjMr">nine-track tapes</a> and 017 (octal) for <a href="/facts/IBM_7-track/HSoVDxMh">seven-track tapes</a>.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> The end-of-tape, commonly abbreviated as EOT, was indicated by two tape marks. This was the standard used, for example, on <a href="/facts/IBM_360/7znvpy4K">IBM 360</a>. The <a href="/facts/9-track_tape/5Eb6MjMr">reflective strip</a> that was used to announce impending physical end of tape was also called an EOT marker.
</p>
<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/End-of-Transmission_character/6kUye5G0">End-of-Transmission character</a> (EOT)</li>
<li><a href="/facts/Substitute_character/9yHehSB0">Substitute character</a></li>
<li><a href="/facts/End_of_message/AAQmVjbr">End of message</a></li>
<li><a href="/facts/Here_document/7GJs16sB">Here document</a></li>
<li><a href="/facts/-30-/s4lGLVPw">-30-</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>Pollock, Wayne. "Shell Here Document Overview". hccfl.edu. Archived from the original on 2014-05-29. Retrieved 2014-05-28. <a href="https://web.archive.org/web/20140529084958/http://content.hccfl.edu/pollock/ShScript/HereDoc.htm" target="_blank">https://web.archive.org/web/20140529084958/http://content.hccfl.edu/pollock/ShScript/HereDoc.htm</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>"The GNU C Library". www.gnu.org. <a href="https://www.gnu.org/software/libc/manual/html_mono/libc.html#EOF-and-Errors" target="_blank">https://www.gnu.org/software/libc/manual/html_mono/libc.html#EOF-and-Errors</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>"Table of IO Device Characteristics - Console or Teletypewriters". PDP-6 Multiprogramming System Manual (PDF). Maynard, Massachusetts, USA: Digital Equipment Corporation (DEC). 1965. p. 43. DEC-6-0-EX-SYS-UM-IP-PRE00. Archived (PDF) from the original on 2014-07-14. Retrieved 2014-07-10. (1+84+10 pages) <a href="http://bitsavers.trailing-edge.com/pdf/dec/pdp6/DEC-6-0-EX-SYS-UM-IP-PRE00_Multiprogramming_System_Manual_1965.pdf" target="_blank">http://bitsavers.trailing-edge.com/pdf/dec/pdp6/DEC-6-0-EX-SYS-UM-IP-PRE00_Multiprogramming_System_Manual_1965.pdf</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
<li id="fn:4"><p>"5.1.1.1. Device Dependent Functions - Data Modes - Full-Duplex Software A(ASCII) and AL(ASCII Line)". PDP-10 Reference Handbook: Communicating with the Monitor - Time-Sharing Monitors (PDF). Vol. 3. Digital Equipment Corporation (DEC). 1969. pp. 5-3 – 5-6 [5-5 (431)]. Archived (PDF) from the original on 2011-11-15. Retrieved 2014-07-10. (207 pages) <a href="http://bitsavers.org/pdf/dec/pdp10/1970_PDP-10_Ref/1970PDP10Ref_Part3.pdf" target="_blank">http://bitsavers.org/pdf/dec/pdp10/1970_PDP-10_Ref/1970PDP10Ref_Part3.pdf</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li>
<li id="fn:5"><p>"Tape Transfer (Pre-1977): Exchange Media: MARC 21 Specifications for Record Structure, Character Sets, and Exchange Media (Library of Congress)". www.loc.gov. <a href="https://www.loc.gov/marc/specifications/specexchtape2.html#mark" target="_blank">https://www.loc.gov/marc/specifications/specexchtape2.html#mark</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li>
</ol>

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