In computing, a file descriptor is an identifier maintained by the operating system. It is a unique identifier identifying an open file. Once the file has been opened, the file descriptor can be used by different system calls to read or write it. Originally, the word computing was synonymous with counting and calculating, and a science that deals with the original sense of computing mathematical calculations. ... In computing, an operating system (OS) is the system software responsible for the direct control and management of hardware and basic system operations. ... A file in a computer system is a stream (sequence) of bits stored as a single unit, typically in a file system on disk or magnetic tape. ... In computing, a system call, or software interrupt is the mechanism used by an application program to request service from the operating system. ...

The term "file descriptor" is generally used in POSIX operating systems. In Microsoft Windows terminology, "file handle" is preferred. POSIX is the collective name of a family of related standards specified by the IEEE to define the application program interface for software designed to run on variants of the Unix OS. They are formally designated as IEEE 1003 and the international standard name is ISO/IEC 9945. ... In computing, an operating system (OS) is the system software responsible for the direct control and management of hardware and basic system operations. ... Microsoft Windows is a range of operating environments for personal computers and servers. ... Look up Handle in Wiktionary, the free dictionary The word handle may refer to any of several things: a device attached to a movable object, that is gripped to move or use the object; see handle (grip) (especially in citizens band radio and online) a pseudonym; see user name ans...

In POSIX, a file descriptor is an integer, specifically of the C type int. There are 3 standard POSIX file descriptors which presumably every process (save perhaps a daemon) should expect to have: POSIX is the collective name of a family of related standards specified by the IEEE to define the application program interface for software designed to run on variants of the Unix OS. They are formally designated as IEEE 1003 and the international standard name is ISO/IEC 9945. ... The integers consist of the positive natural numbers (1, 2, 3, …), their negatives (−1, −2, −3, ...) and the number zero. ... The term Daemon has several meanings: Daemon (computer software) Daemon (His Dark Materials) in the Philip Pullman trilogy of novels His Dark Materials Daemon (mythology) Daemon (Warhammer) Daemon Sadi (SaDiablo) is a character in the Black Jewels Trilogy by Anne Bishop. ...

Integer value	English Name
0	Standard Input (stdin)
1	Standard Output (stdout)
2	Standard Error (stderr)

Generally, a file descriptor is a key to a kernel-resident data structure containing the details of all open files. In POSIX this data structure is called a file descriptor table, and each process has its own. The user application passes the abstract key to the kernel through a system call, and the kernel will access the file on behalf of the application, based on the key. The application itself cannot read or write the file descriptor table directly. The word key has several uses: Look up Key on Wiktionary, the free dictionary // Instrument or Tool A key (instrument) comprises a moving part of a musical instrument. ... The word kernel has a a variety of meanings in a several fields. ... POSIX is the collective name of a family of related standards specified by the IEEE to define the application program interface for software designed to run on variants of the Unix OS. They are formally designated as IEEE 1003 and the international standard name is ISO/IEC 9945. ...

In Unix-like system, file descriptors can refer to files, directories, block or character devices (also called "special files"), sockets, FIFOs (also called named pipes), unnamed pipes, or symbolic links. It has been suggested that this article or section be merged with Unix. ... File has several meanings: Computer file File (tool) file (Unix), a program used to determine file types. ... In computing, a directory, catalog, or folder, is an entity in a file system which contains a group of files and other directories. ... A socket generally designates a cavity or region used for fitting and connecting some specific device. ... FIFO is an acronym for First In, First Out. ... In computing, a named pipe (also FIFO for its behaviour) is an extension to the classical pipe concept on UNIX and UNIX-like systems, and is one of the methods of interprocess communication. ... The word pipe can refer to: The basic cylindrical pipe shape a tubular man-made channel, generally round in cross section, in steel or concrete for transporting or guiding a fluid substance see plumbing and pipeline transport used in construction as column, truss element or space frame in mechanical engineering... A symbolic link (often symlink, especially in verb form, or soft link) is a special type of directory entry in modern Unix (or Unix-like) filesystems that allows almost transparent references to another directory entry, typically a file or a directory. ...