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more Version 1 AT&T UNIX history:
a few cases that weren't altogether straightforward;
tweak and OK jmc@, OK sobrado@

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.\"     @(#)dir.5	8.4 (Berkeley) 5/3/95
.\"
.Dd $Mdocdate: September 7 2019 $
.Dt DIR 5
.Os
.Sh NAME
.Nm dir ,
.Nm dirent
.Nd directory file format
.Sh SYNOPSIS
.In dirent.h
.Sh DESCRIPTION
Directories provide a convenient hierarchical method of grouping
files while obscuring the underlying details of the storage medium.
A directory file is differentiated from a plain file by a flag in its
.Xr inode 5
entry.
It consists of records (directory entries) each of which contains
information about a file and a pointer to the file itself.
Directory entries may contain other directories as well as plain files;
such nested directories are referred to as subdirectories.
A hierarchy of directories and files is formed in this manner
and is called a file system (or referred to as a file system tree).
.\" An entry in this tree,
.\" nested or not nested,
.\" is a pathname.
.Pp
Each directory file contains two special directory entries; one is a pointer
to the directory itself called dot
.Pq Dq \&.
and the other a pointer to its parent directory called dot-dot
.Pq Dq \&.. .
Dot and dot-dot are valid pathnames, however, the system root directory
.Pq Dq / ,
has no parent and dot-dot points to itself like dot.
.Pp
File system nodes are ordinary directory files on which has
been grafted a file system object, such as a physical disk or a
partitioned area of such a disk (see
.Xr mount 8 ) .
.Pp
The directory entry format is defined in the file
.In dirent.h :
.Bd -literal
/*
 * A directory entry has a struct dirent at the front of it, containing
 * its inode number, the length of the entry, and the length of the name
 * contained in the entry.  These are followed by the name padded to some
 * alignment (currently 8 bytes) with NUL bytes.  All names are guaranteed
 * NUL terminated.  The maximum length of a name in a directory is MAXNAMLEN.
 */

struct dirent {
	ino_t		d_fileno;	/* file number of entry */
	off_t		d_off;		/* offset of next entry */
	u_int16_t	d_reclen;	/* length of this record */
	u_int8_t	d_type;		/* file type, see below */
	u_int8_t	d_namlen;	/* length of string in d_name */
#define MAXNAMLEN       255
	char    d_name[MAXNAMLEN + 1];  /* maximum name length */
};

#define	d_ino		d_fileno	/* backward compatibility */

/*
 * File types
 */
#define DT_UNKNOWN	0
#define DT_FIFO		1
#define DT_CHR		2
#define DT_DIR		4
#define DT_BLK		6
#define DT_REG		8
#define DT_LNK		10
#define DT_SOCK		12
.Ed
.Sh SEE ALSO
.Xr getdents 2 ,
.Xr fs 5 ,
.Xr inode 5
.Sh HISTORY
A
.Nm dir
file format appeared in
.At v1 .
A predecessor
.Vt struct direct
first appeared in
.At v7 .
The
.Vt dirent
structure first appeared in
.Bx 4.3 Reno .
The
.Fa d_off
member was added in
.Ox 5.5 .