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DIR(5) File Formats Manual DIR(5)

dir, direntdirectory file format

#include <dirent.h>

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 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).

Each directory file contains two special directory entries; one is a pointer to the directory itself called dot (“.”) and the other a pointer to its parent directory called dot-dot (“..”). Dot and dot-dot are valid pathnames, however, the system root directory (“/”), has no parent and dot-dot points to itself like dot.

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 mount(8)).

The directory entry format is defined in the file <dirent.h>:

 * 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

getdents(2), fs(5), inode(5)

A dir file format appeared in Version 7 AT&T UNIX. The d_off member was added in OpenBSD 5.5.

September 10, 2015 OpenBSD-6.4