Incomputing,end-of-file (EOF)[1] is a condition in a computeroperating system where no more data can be read from a data source. The data source is usually called afile orstream.
In theC standard library, the character-reading functions such asgetchar return a value equal to the symbolic value (macro)EOF to indicate that an end-of-file condition has occurred. The actual value ofEOF is implementation-dependent and must be negative (it is commonly −1, such as inglibc[2]). Block-reading functions return the number of bytes read, and if this is fewer than asked for, then the end of file was reached or an error occurred (checking oferrno or dedicated function, such asferror is required to determine which).
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. InUNIX, 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 aControl-D character at the start of a line 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 (usuallyControl-V).AmigaDOS is similar but uses Control-\ instead of Control-D.
InDOS andWindows (and inCP/M and manyDEC operating systems such as thePDP-6 monitor,[3]RT-11,VMS orTOPS-10[4]), 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 anASCIIControl-Z, code 26. Some MS-DOS programs, including parts of the Microsoft MS-DOS shell (COMMAND.COM) and operating-system utility programs (such asEDLIN), 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:
In the ANSI X3.27-1969magnetic tape standard, the end of file was indicated by atape mark, which consisted of a gap of approximately 3.5 inches of tape followed by a single byte containing the character0x13 (hex) fornine-track tapes and017 (octal) forseven-track tapes.[5] Theend-of-tape, commonly abbreviated asEOT, was indicated by two tape marks. This was the standard used, for example, onIBM 360. Thereflective strip that was used to announce impending physical end of tape was also called anEOT marker.