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(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 a 4
     * byte boundary with null bytes.  All names are guaranteed NUL terminated.
     * The maximum length of a name in a directory is MAXNAMLEN.
     */
    struct dirent {
    	u_int32_t	d_fileno;	/* file number of 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 */
    };
    /*
     * 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
    /*
     * Convert between stat structure types and directory types.
     */
    #define IFTODT(mode)	(((mode) & 0170000) >> 12)
    #define DTTOIF(dirtype)	((dirtype) << 12)
    #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 */
    	off_t	dd_seek;   /* magic cookie returned by getdirentries */
    	off_t	dd_rewind; /* magic cookie for rewinding */
    	int	dd_unused; /* was flags for readdir */
    	struct _telldir *dd_td; /* telldir position recording */
    	void	*dd_lock;  /* mutex to protect struct */
    } DIR;
    #define	dirfd(dirp)	((dirp)->dd_fd)
    #endif /* _POSIX_SOURCE */

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

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