End-of-file

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, file access and other I/O functions may 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 a system-dependent negative number, commonly -1, which is guaranteed to be unequal to any valid character code.

In UNIX and AmigaDOS, an end-of-file indication can be sent from an interactive shell (console) by typing Ctrl+D (conventional standard); a program reading from stdin would then read an EOF. In Microsoft's DOS and Windows it is sent by pressing Ctrl+Z.

In certain cases when dealing with text files or reading data from a "character device", MS-DOS programs will act like the file ends when it encounters a Ctrl+Z byte. This was done for compatibility with CP/M and MSDOS 1.0, which could only measure the lengths of files in 128-byte blocks, and thus used the Ctrl+Z to indicate where in the middle of a block the text actually ended. MS-DOS 2.0 and all later versions can store the exact length of files, and no C libraries have ever translated this byte to an EOF value; the interpretation is left to the program.

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. [1]. 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

• End-of-transmission character (ASCII no. 4), the UNIX EOF character produced by Ctrl+D
• Substitute character (ASCII no. 26 decimal, 1A hexadecimal), the DOS EOF character produced by Ctrl+Z

References

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