DIR(4)                                                     DIR(4)



     NAME
          dir - format of EFS directories

     SYNOPSIS
          #include <sys/fs/efsdir.h>

     DESCRIPTION
          A directory behaves exactly like an ordinary file, save that
          no user may write into a directory.  The fact that a file is
          a directory is indicated by a bit in the flag word of its
          i-node entry [see efsinode(4)].  The EFS directory format
          supports variable length names of up to 255 characters.

     DIRECTORY BLOCKS
          Each EFS directory is segmented into directory blocks
          defined by the following data structure:

               #define EFS_DIRBLK_HEADERSIZE 4
               struct  efs_dirblk {
                       /* begin header */
                       ushort      magic;
                       unchar      firstused;
                       unchar      slots;
                       /* end header */

                       /* rest is space for efs_dent's */
                       unchar      space[EFS_DIRBSIZE - EFS_DIRBLK_HEADERSIZE];
               };

          Each directory block is subdivided into three separate
          areas: a header, an array of entry offsets and an array of
          directory entries.  The system restricts directory entries
          to short boundaries and stores offsets in the directory
          block compacted by shifting them right by one.

          The header area contains a magic number to identify the
          block as being a directory block.  If the magic number is
          incorrect, the operating system will refuse to manipulate
          the directory, thus avoiding further corruption.

          The array of entry offsets immediately follows the header
          and is sized according to the directories contents and
          contains compacted offsets which point to each directory
          entry.  The number of entry offsets available is kept in
          slots.  The firstused field contains a compacted offset
          which positions the first byte of the directory entries.

          The space between the end of the entry array and the
          beginning of the directory entries (firstused) is free space
          which the system uses for allocating new directory entries
          and entry offsets.  The system keeps the free space in a
          directory block compacted by coalescing holes created by



     Page 1                                        (last mod. 8/20/87)

     DIR(4)                                                     DIR(4)



          entry removal.  When a directory entry is removed, the
          system adjusts the entry offsets for all entries that move.
          Also, the entry offset for the removed entry is zeroed.  If
          the removed entry was the last in the entry offset array,
          the number of slots is reduced.  Directory entries never
          change which entry offset they use.

     DIRECTORY ENTRIES
          Directory entries have the following structure:

               struct  efs_dent {
                       union {
                             ulong l;
                             ushorts[2];
                       } ud_inum;
                       unchar      d_namelen;
                       char  d_name[3];
               };

          The d_name field is actually of variable size, depending
          upon the value contained in d_namelen.  The system pads out
          the directory entry to insure that it begins on a short
          boundary in the directory block.  The ud_inum field contains
          the entries inode number.

     SEE ALSO
          fs(4), inode(4).

     ORIGIN
          Silicon Graphics, Inc.

























     Page 2                                        (last mod. 8/20/87)