Substitute character: Difference between revisions

In the [[ASCII]] and [[Unicode]] [[character set]]s, this character is encoded by the number 26 (<tt>{{mono|1A</tt>}} [[hexadecimal|hex]]). Standard [[computer keyboard|keyboards]] transmit this code when the [[Controlcontrol key|{{keypress|Ctrl}}]] and {{keypress|[[Z]]}} keys are pressed simultaneously ({{mono|Ctrl+Z}}, often documented by convention often described as ''{{mono|^Z}})''.<ref name="Microsoft SupportMicrosoft_126449"/>{{cite web|title=Keyboard[[Unicode]] shortcutsinherits forthis Windows|url=http://support.microsoft.com/kb/126449|work=Microsoftcharacter Support|publisher=Microsoft|accessdate=2from JuneASCII, 2012}}</ref>but recommends that the [[replacement character]] (�, U+FFFD) be used instead to represent un-decodable inputs, when the output encoding is compatible with it.

Historically, under [[PDP-6]] monitor,<ref name="DEC_1965_PDP-6"/> [[RT-11]], [[OpenVMS|VMS]], and [[TOPS-10]],<ref name="DEC_1969_PDP-10"/> and in early PC [[CP/M]] 1 and 2 [[operating system]]s (and derivatives like [[MP/M]])<!-- and possibly 86-DOS before 0.42, 0.56 or 1.00 as well -- needs testing --> it was necessary to explicitly mark the [[end -of -file|end of a file]] (EOF) because the CP/Mnative [[filesystem]] could not record the exact file size by itself; files were allocated in extents (records) of a fixed size, typically leaving some allocated but unused space at the end of each file.<ref name="Elliott_1998_CPM14"/>John<ref Elliott (1998). ''CPname="Elliott_1998_CPM22"/M 1.4 disc formats''. ([http://www.seasip.info/Cpm/format14.html])</ref><ref name="Elliott_1998_CPM22DRI_1979_CPM20-IG"/>John<ref Elliott (1998). ''CPname="Hogan_1982_CP/M 2.2 disc formats''. ([http:"//www.seasip.info/Cpm/format22.html])</ref> This extra space was filled with {{mono|1A}}<ttsub style="font-size:6px">1A16</ttsub> ([[hexadecimal|hex]]) characters under CP/M. The extended CP/M filesystems used by CP/M 3<!-- definitely 3.1 and 4.1, but probably since 3.0 --> and higher (and derivatives like [[Concurrent CP/M]], [[Concurrent DOS]], and [[DOS Plus]]) did support byte-granular files,<ref name="Elliott_1998_CPM31">John Elliott (1998). ''CP/M 3.1 disc formats''. ([http://www.seasip.info/Cpm/format31.html])</ref><ref name="Elliott_1998_DOSPLUS">John Elliott (1998). ''CP/M 4.1 disc formats''. ([http://www.seasip.info/Cpm/format41.html])</ref> so this was no longer a physical requirement, but ait remained mereas a convention (especially for [[text file]]s) in order to ensure backward compatibility.

In [[CP/M]], [[86-DOS]], [[MS-DOS]], [[PC &nbsp;DOS]], [[DR-DOS]], and their various derivatives, the SUB character was also used to indicate the end of a character stream,{{citation needed|date=October 2023}} and thereby used to terminate user input in an interactive [[command line]] window (and as such, often used to finish console input redirection, e.g. as instigated by <tt>the command {{code|COPY CON: TYPEDTXT.TXT</tt>|dosbatch}}).

While no longer technically required to indicate the end of a file, as of 2017, many text editors{{which|date=October 2023}} and program languages still<!--as of 2017--> support this convention, or can be configured to insert this character at the end of a file when editing, or at least properly cope with them in text files.{{citation needed|date=October 2021}} In such cases, it is often termed a "soft" EOF, as it does not necessarily represent the physical end of the file, but is more a marker indicating that "there is no useful data beyond this point". In reality, more data may exist beyond this character up to the actual end of the data in the file system, thus it can be used to hide file content when the file is entered at the console or opened in editors. Many file format standards (e.g. [[Portable Network Graphics|PNG]] or [[GIF]]) include the SUB character in their headers to perform precisely this function. Some modern text file formats (e.g. [[CSV-1203]]<ref>[http://www.mastpoint.com/csv-1203 CSVname="Mastpoint_2016_CSV-1203 format specification]<"/ref>) still recommend a trailing EOF character to be appended as the last character in the file. However, typing {{key presskeypress|Control|Z}} does not embed an EOF character into a file in either [[MS-DOS]] or [[Microsoft Windows|Windows]], nor do the [[Applicationapplication programming interface|APIs]] of those systems use the character to denote the actual end of a file.

Some programming languages (e.g. [[Visual Basic]]) will not read past a "soft" EOF when using the built-in text file reading primitives (INPUT, LINE INPUT etc.),{{citation needed|date=October 2023}} and alternate methods must be adopted, e.g. opening the file in binary mode or using the File System Object to progress beyond it.

Character 26 was used to mark "End of file" even ifthough the [[ASCII]] calls itthis character Substitute, and has other characters forto thisindicate "End of file". Number 28 which is called "[[File Separator]]" has also been used for similar purposes.

In [[Unix]]-like operating systems, this character is typically used in [[Shell (computing)|shell]]s as a way for the user to [[SIGTSTP|suspend]] the currently executing interactive process.<ref name="U of WashingtonUW_Unix">{{cite web|title=Quick Reference: Unix Commands|url=http://www.washington.edu/computing/unix/unixqr.html|work=IT Connect|publisher=University of Washington|accessdate=2 June 2012}}</ref> The suspended process can then be resumed in ''foreground'' (interactive) mode, or be made to resume execution in ''[[background process|background]]'' mode, or be [[exit (operatingsystem systemcall)|terminated]]. When entered by a user at their [[computer terminal]], the currently running foreground process is sent a "terminal stop" ([[SIGTSTP]]) signal, which generally causes the process to suspend its execution. The user can later continue the process execution by using the "foreground" command (<code>[[fg (Unix)|fg]]</code>) or the "[[Backgroundbackground (computer software)process|background]]" command (<code>[[bg (Unix)|bg]]</code>).

The [http://unicode.org/reports/tr36/#Text_Comparison Unicode Security Considerations report]<ref name="Unicode_USC"/> recommends this character as a safe replacement for unmappable characters during character set conversion.

In many GUIs and applications, {{key presskeypress|Control|Z}} ({{key presskeypress|Command|Z}} on Mac OS[[macOS]]) can be used to [[undo]] the last action. In many applications, earlier actions than the last one can also be undone by pressing {{key presskeypress|Control|Z}} multiple times. {{key presskeypress|Control|Z}} was one of a handful of [[computer keyboard|keyboard]] sequences chosen by the program designers at [[Xerox PARC]] to control [[text editor|text editing]]. Presumably these particular [[keystroke]]s were chosen because of their location on a standard [[QWERTY keyboard]], since the Z (undo), [[control-X|X]] (cut), [[control-C|C]] (copy), and [[control-V|V]] (paste) keys are located together at the left end of the bottom row of the standard QWERTY keyboard.

* [[ASCII]] and [[Unicode]] representation of "substitute":

*: Octal code: 32

*: Decimal code: 26

*: Hexadecimal code: 1A, U+001A

*: Mnemonic symbol: SUB

*: Binary value: 11010

* [[U+FFFD]] (Unicode replacement character �)