DIR(5)

NAME
dir, dirent - directory file format

SYNOPSIS
#include <sys/types.h>
#include <sys/dir.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 refered 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(1) and mount(8).)

The directory entry format is defined in the file <sys/dirent.h> and further in the file <dirent.h>. When the macro _DARWIN_FEATURE_64_BIT_INODE is not defined (see stat(2) for more information on this macro), the dirent structure is defined as:

    /*** Excerpt from <sys/dirent.h> ***/
    /*
     * The dirent structure defines the format of directory entries.
     *
     * 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 255.
     */
    struct dirent { /* when _DARWIN_FEATURE_64_BIT_INODE is NOT defined */
            ino_t      d_ino;                /* file number of entry */
            __uint16_t d_reclen;             /* length of this record */
            __uint8_t  d_type;               /* file type, see below */
            __uint8_t  d_namlen;             /* length of string in d_name */
            char    d_name[255 + 1];   /* name must be no longer than this */
    };

However, when the macro _DARWIN_FEATURE_64_BIT_INODE is defined, the dirent structure is defined as:

    /*
     * The dirent structure defines the format of directory entries.
     *
     * 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 1023.
     */
    struct dirent { /* when _DARWIN_FEATURE_64_BIT_INODE is defined */
            ino_t      d_fileno;     /* file number of entry */
            __uint16_t d_seekoff;    /* seek offset (optional, used by servers) */
            __uint16_t d_reclen;     /* length of this record */
            __uint16_t d_namlen;     /* length of string in d_name */
            __uint8_t  d_type;       /* file type, see below */
            char    d_name[1024];    /* name must be no longer than this */
    };

In addition:

    /*
     * 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
    #define DT_WHT          14

-----------------------------------------

    /*** Excerpt from <dirent.h> ***/
    #define d_fileno        d_ino        /* backward compatibility */
    /* definitions for library routines operating on directories. */
    #define DIRBLKSIZ       1024
    struct _telldir;                /* see telldir.h */
    /* 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 */
            int     __dd_flags;   /* flags for readdir */
            pthread_mutex_t __dd_lock; /* for thread locking */
            struct _telldir *__dd_td; /* telldir position recording */
    } DIR;
    #define dirfd(dirp)     ((dirp)->dd_fd)
    /* flags for opendir2 */
    #define DTF_HIDEW       0x0001  /* hide whiteout entries */
    #define DTF_NODUP       0x0002  /* don't return duplicate names */
    #define DTF_REWIND      0x0004  /* rewind after reading union stack */
    #define __DTF_READALL   0x0008  /* everything has been read */

SEE ALSO
fs(5), inode(5)

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