DIR(5)

NAME
dir, dirent - directory file format

SYNOPSIS
#include <sys/types.h>
#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(2) and mount(8).)

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

    #ifndef _DIRENT_H_
    #define _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 a 4
    * byte boundary with null bytes.  All names are guaranteed null terminated.
    * The maximum length of a name in a directory is MAXNAMLEN.
    */
    struct dirent {
    	u_long	d_fileno;	/* file number of entry */
    	u_short	d_reclen;	/* length of this record */
    	u_char  d_type;		/* file type, see below */
    	u_char	d_namlen;	/* length of string in d_name */
    #ifdef _POSIX_SOURCE
    	char	d_name[255 + 1];	/* maximum name length */
    #else
    #define MAXNAMLEN       255
    	char    d_name[MAXNAMLEN + 1];  /* maximum name length */
    #endif
    };
    /*
     * 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_LKN		10
    #define DT_SOCK		12
    #define DT_WHT		14
    #ifdef _POSIX_SOURCE
    typedef void *	DIR;
    #else
    #define	d_ino		d_fileno	/* backward compatibility */
    /* definitions for library routines operating on directories. */
    #define	DIRBLKSIZ	1024
    /* structure describing an open directory. */
    typedef struct _dirdesc {
    	int	dd_fd;    /* file descriptor associated with directory */
    	long	dd_loc;	  /* offset in current buffer */
    	long	dd_size;  /* amount of data returned by getdirentries */
    	char	*dd_buf;  /* data buffer */
    	int	dd_len;   /* size of data buffer */
    	long	dd_seek;  /* magic cookie returned by getdirentries */
    	long	dd_rewind;/* magic cookie for rewinding */
    } DIR;
    #define	dirfd(dirp)	((dirp)->dd_fd)
    #ifndef NULL
    #define	NULL	0
    #endif
    #endif /* _POSIX_SOURCE */
    #ifndef KERNEL
    #include <sys/cdefs.h>
    #endif /* !KERNEL */
    #endif /* !_DIRENT_H_ */

SEE ALSO
fs(5), inode(5)

HISTORY
A dir file format appeared in AT&T UNIX.