2.11BSD#431 - /usr/man/cat5/dir.0

DIR(5)		    UNIX Programmer's Manual		   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.
	   */

	  #define DIRBLKSIZ 512

	  #define MAXNAMLEN 63

	  /*
	   * 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) \


Printed 11/26/99	  May 15, 1985				1


DIR(5)		    UNIX Programmer's Manual		   DIR(5)


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

	  struct  direct {
		  ino_t     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 mean-
     ing of `..' is modified for the root directory of the master
     file system ("/"), where `..' has the same meaning as `.'.

SEE ALSO
     fs(5)

BUGS
     The 63 character MAXNAMLEN value is shorter than the 255
     characters allowed by 4BSD.  This could lead to file name
     portability problems in unusual circumstances.

     The disk format of directories is only slightly different
     from the 4BSD directory format, the inode number is of type
     ino_t rather than u_long to reduce the amount of 32 bit
     arithmetic in the kernel.


Printed 11/26/99	  May 15, 1985				2