DIR(5)

NAME
dir, dirent - directory file format

SYNOPSIS
#include <dirent.h>

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 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

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

HISTORY
A dir file format appeared in AT&T UNIX. A predecessor struct direct first appeared in AT&T UNIX. The dirent structure first appeared in 4.3BSD Reno. The d_off member was added in OpenBSD 5.5.