dir

DIR(5)                        File Formats Manual                       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(2) and mount(8).)

     The directory entry format is defined in the file <sys/dirent.h> (which
     should not be included directly by applications):

     #ifndef _SYS_DIRENT_H_
     #define _SYS_DIRENT_H_

     #include <machine/ansi.h>

     /*
      * The dirent structure defines the format of directory entries returned by
      * the getdirentries(2) system call.
      *
      * 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 8
      * byte boundary with null bytes.  All names are guaranteed null terminated.
      * The maximum length of a name in a directory is MAXNAMLEN.
      * Explicit pad is added between the last member of the header and
      * d_name, to avoid having the ABI padding in the end of dirent on
      * LP64 arches.  There is code depending on d_name being last.  Also,
      * keeping this pad for ILP32 architectures simplifies compat32 layer.
      */

     struct dirent {
             ino_t      d_fileno;            /* file number of entry */
             off_t      d_off;               /* directory offset 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 */
             __uint32_t d_pad0;
     #if __BSD_VISIBLE
     #define MAXNAMLEN       255
             char    d_name[MAXNAMLEN + 1];  /* name must be no longer than this */
     #else
             char    d_name[255 + 1];        /* name must be no longer than this */
     #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_LNK          10
     #define DT_SOCK         12
     #define DT_WHT          14

     /*
      * Convert between stat structure types and directory types.
      */
     #define IFTODT(mode)    (((mode) & 0170000) >> 12)
     #define DTTOIF(dirtype) ((dirtype) << 12)

     /*
      * The _GENERIC_DIRSIZ macro gives the minimum record length which will hold
      * the directory entry.  This returns the amount of space in struct direct
      * without the d_name field, plus enough space for the name with a terminating
      * null byte (dp->d_namlen+1), rounded up to a 8 byte boundary.
      *
      * XXX although this macro is in the implementation namespace, it requires
      * a manifest constant that is not.
      */
     #define _GENERIC_DIRLEN(namlen)                                         ((__offsetof(struct dirent, d_name) + (namlen) + 1 + 7) & ~7)
     #define _GENERIC_DIRSIZ(dp)     _GENERIC_DIRLEN((dp)->d_namlen)
     #endif /* __BSD_VISIBLE */

     #ifdef _KERNEL
     #define GENERIC_DIRSIZ(dp)      _GENERIC_DIRSIZ(dp)
     #endif

     #endif /* !_SYS_DIRENT_H_ */

SEE ALSO
     fs(5), inode(5)

HISTORY
     A dir file format appeared in Version 7 AT&T UNIX.

BUGS
     The usage of the member d_type of struct dirent is unportable as it is
     FreeBSD-specific.  It also may fail on certain file systems, for example
     the cd9660 file system.

                                 June 20, 2018