File Descriptor

In computer programming, a file descriptor (FD) is an abstract indicator for accessing a file. The term is generally used in POSIX operating systems. In Microsoft Windows terminology and in the context of the C standard I/O library, file handle is preferred, though the latter case is technically a different object (see below).

In POSIX, a file descriptor is an integer, specifically of the C type int. There are three standard POSIX file descriptors, corresponding to the three standard streams, which presumably every process (save perhaps a daemon) should expect to have:

Generally, a file descriptor is an index for an entry in a kernel-resident array data structure containing the details of open files. In POSIX this data structure is called a file descriptor table, and each process has its own file descriptor table. The process passes the file descriptor to the kernel through a system call, and the kernel will access the file on behalf of the process. The process itself cannot read or write the file descriptor table directly.

On Linux, the set of file descriptors open in a process can be accessed under the path /proc/PID/fd/, where PID is the process identifier.

In Unix-like systems, file descriptors can refer to any Unix file type named in a file system. As well as regular files, this includes directories, block and character devices (also called "special files"), Unix domain sockets, and named pipes. File descriptors can also refer to other objects that do not normally exist in the file system, such as anonymous pipes and network sockets.

The FILE data structure in the C standard I/O library usually includes a low level file descriptor for the object in question on Unix-like systems. Since file handle refers to this additional layer, it is not interchangeable with file descriptor.

Microsoft Windows also uses the term file handle to refer to the more low-level construct, akin to POSIX's file descriptors. Microsoft's C libraries also provide compatibility functions which "wrap" these native handles to support the POSIX-like convention of integer file descriptors as detailed above.