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Concept
Substitute character
Résumé
In computer data, a substitute character (␚) is a control character that is used to pad transmitted data in order to send it in blocks of fixed size, or to stand in place of a character that is recognized to be invalid, erroneous or unrepresentable on a given device. It is also used as an escape sequence in some programming languages.
In the ASCII character set, this character is encoded by the number 26 ( hex). Standard keyboards transmit this code when the and keys are pressed simultaneously (, often documented by convention as ). Unicode inherits this character from ASCII, but recommends that the replacement character (�, U+FFFD) be used instead to represent un-decodable inputs, when the output encoding is compatible with it.
End-of-file
Historically, under PDP-6 monitor, RT-11, VMS, and TOPS-10, and in early PC CP/M 1 and 2 operating systems (and derivatives like MP/M) it was necessary to explicitly mark the (EOF) because the native 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. This extra space was filled with 16 (hex) characters under CP/M. The extended CP/M filesystems used by CP/M 3 and higher (and derivatives like Concurrent CP/M, Concurrent DOS, and DOS Plus) did support byte-granular files, so this was no longer a requirement, but it remained as a convention (especially for s) in order to ensure backward compatibility.
In CP/M, 86-DOS, MS-DOS, PC DOS, DR-DOS, and their various derivatives, the SUB character was also used to indicate the end of a character stream, 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 the command ).
While no longer technically required to indicate the end of a file, as of 2017 many text editors and program languages still 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.
Source officielle
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