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 an end-of-file indication can be sent from an interactive shell (console) by typing Ctrl+D (conventional standard). 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", the Microsoft MS-DOS shell (COMMAND.COM) or operating-system utility programs would historically append an ASCII control-Z character to the end of a disk file (though the basic kernel MSDOS.SYS file write calls never appended a control-Z). This was done for backward compatibility with some of the peculiarities of CP/M, since the CP/M file system only recorded the lengths of files in terms of how many 128-byte "records" were allocated. The MS-DOS filesystem has always recorded the exact byte-length of files from its very first version.
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. 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.