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Tape mark

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Tape mark

In computing, end of file (commonly abbreviated EOF) 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 or stream.

In the C Standard Library, the character reading functions such as getchar 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 system-dependent (but is commonly -1, such as in glibc[1]) and is unequal to any valid character code. Block-reading functions return the number of bytes read, and if this is fewer than asked for, then the end of file was reached.

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 and AmigaDOS, 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 Control-D character at the start of a line or Control-\ for AmigaDOS (Control-D is used as a breaking character) 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).

In Microsoft's DOS and Windows (and in CP/M and many DEC operating systems), 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 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 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 MS-DOS filesystem has always recorded the exact byte-length of files, so this was never necessary on MS-DOS.
  • It allows programs to use the same code to read input from both a terminal and a text file.

In the ANSI X3.27-1969 magnetic tape 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 13 (hex) for nine-track tapes and 17 (octal) for seven-track tapes.[2] 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 reflective strip used to announce impending physical end of tape was also called an EOT marker.

See also

Computer science portal

References

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