4.3BSD - /usr/man/cat5/dir.0

DIR(5)                                                                  DIR(5)


NAME
       dir - format of directories

SYNOPSIS
       #include <sys/types.h>
       #include <sys/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(5).
       The structure of a directory entry as given in the include file is:

              /*
               * 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.
               */
              #ifdef KERNEL
              #define DIRBLKSIZ DEV_BSIZE
              #else
              #define DIRBLKSIZ 512
              #endif

              #define MAXNAMLEN 255

              /*
               * The 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 DIRSIZ
              #define DIRSIZ(dp) \
                  ((sizeof (struct direct) - (MAXNAMLEN+1)) + (((dp)->d_namlen+1 + 3) &~ 3))

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

              struct _dirdesc {
                      int       dd_fd;
                      long      dd_loc;
                      long      dd_size;
                      char      dd_buf[DIRBLKSIZ];
              };

       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 meaning of ‘..’ is modified for the root
       directory  of  the  master  file  system (“/”), where ‘..’ has the same
       meaning as ‘.’.

SEE ALSO
       fs(5)


4.2 Berkeley Distribution        May 15, 1985                           DIR(5)