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DIR(5) File Formats Manual DIR(5)

dir, dirent
directory 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-current