Editing File descriptor (section)

==Overview==
[[File:File table and inode table.svg|thumb|300px|File descriptors for a single process, file table and [[inode]] table. Note that multiple file descriptors can refer to the same file table entry (e.g., as a result of the [[dup (system call)|dup]] system call{{r|bach}}{{rp|104}}) and that multiple file table entries can in turn refer to the same inode (if it has been opened multiple times; the table is still simplified because it represents inodes by file names, even though an inode can have [[Hard link|multiple names]]). File descriptor 3 does not refer to anything in the file table, signifying that it has been closed.]]
In the traditional implementation of Unix, file descriptors index into a per-process '''{{visible anchor|file descriptor table}}''' maintained by the kernel, that in turn indexes into a system-wide table of files opened by all processes, called the '''{{visible anchor|file table}}'''. This table records the ''mode'' with which the file (or other resource) has been opened: for reading, writing, appending, and possibly other modes.  It also indexes into a third table called the [[Inode|inode table]] that describes the actual underlying files.<ref name="bach">{{cite book
| title=The Design of the UNIX Operating System
| first=Maurice J.
| last=Bach
| year=1986
| edition=8
| publisher=[[Prentice-Hall]]
| pages=[https://archive.org/details/designofunixoper00bach/page/92 92–96]
| isbn=9780132017992
| url-access=registration
| url=https://archive.org/details/designofunixoper00bach/page/92
}}</ref>  To perform input or output, 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 does not have direct access to the file or inode tables.

On [[Linux]], the set of file descriptors open in a process can be accessed under the path <code>/proc/PID/fd/</code>, where PID is the [[process identifier]]. File descriptor <code>/proc/PID/fd/0</code> is <code>stdin</code>, <code>/proc/PID/fd/1</code> is <code>stdout</code>, and <code>/proc/PID/fd/2</code> is <code>stderr</code>. As a shortcut to these, any running process can also access ''its own'' file descriptors through the folders <code>/proc/self/fd</code> and <code>/dev/fd</code>.<ref>{{Cite web|url=https://unix.stackexchange.com/questions/676683/what-does-the-output-of-ll-proc-self-fd-from-ll-dev-fd-mean|title = Devices - What does the output of 'll /Proc/Self/Fd/' (From 'll /Dev/Fd') mean?}}</ref>

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 [[directory (file systems)|directories]], [[block device|block]] and [[character device]]s (also called "special files"), [[Unix domain socket]]s, and [[named pipe]]s. File descriptors can also refer to other objects that do not normally exist in the file system, such as [[anonymous pipe]]s and [[network socket]]s.

The FILE data structure in the [[stdio|C standard I/O library]] usually includes a low level file descriptor for the object in question on Unix-like systems. The overall data structure provides additional abstraction and is instead known as a ''file [[Handle (computing)|handle]].''