DIR.FFS(4-SysV)     RISC/os Reference Manual      DIR.FFS(4-SysV)



NAME
     dir.ffs - format of directories

SYNOPSIS
     #include <sys/types.h>
     #include <sys/fs/ffs_dir.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 fs.ffs(4).  The structure of a directory
     entry for an FFS file system is as follows:

          /*
           * A directory consists of some number of blocks of DIRBLKSIZ
           * bytes, where DIRBLKSIZ is chosen such that it can be transferred
           * to disk in a single atomic operation (e.g. 512 bytes on most machines).
           *
           * Each DIRBLKSIZ byte block contains some number of directory entry
           * structures, which are of variable length.  Each directory entry has
           * a struct direct 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.
           *
           * The macro DIRSIZ(dp) gives the amount of space required to represent
           * a directory entry.  Free space in a directory is represented by
           * entries which have dp->d_reclen > DIRSIZ(dp).  All DIRBLKSIZ bytes
           * in a directory block are claimed by the directory entries.  This
           * usually results in the last entry in a directory having a large
           * dp->d_reclen.  When entries are deleted from a directory, the
           * space is returned to the previous entry in the same directory
           * block by increasing its dp->d_reclen.  If the first entry of
           * a directory block is free, then its dp->d_ino is set to 0.
           * Entries other than the first in a directory do not normally have
           * dp->d_ino set to 0.
           */
          #define BFS_DIRBLKSIZ 512
          #endif

          #define BFS_MAXNAMLEN 255

          /*
           * The BFS_DIRSIZ macro gives the minimum record length which will hold
           * the directory entry.  This requires 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 4 byte boundary.
           */
          #undef BFS_DIRSIZ
          #define BFS_DIRSIZ(dp) \



                         Printed 1/15/91                   Page 1

DIR.FFS(4-SysV)     RISC/os Reference Manual      DIR.FFS(4-SysV)



              ((sizeof (struct bfs_direct) - (MAXNAMLEN+1)) + (((dp)->d_namlen+1 + 3) &~ 3))

          struct  bfs_direct {
                  u_long    d_ino;
                  short     d_reclen;
                  short     d_namlen;
                  char      d_name[MAXNAMLEN + 1];
                  /* typically shorter */
          };


     By convention, the first two entries in each directory are
     for `.' and `..'.  The first is an entry for the directory
     itself.  The second is for the parent directory.  The mean-
     ing of `..' is modified for the root directory of the master
     file system ("/"), where `..' has the same meaning as `.'.

SEE ALSO
     fs.ffs(4)




































 Page 2                  Printed 1/15/91