BSD 2.11 - man page for dir (bsd section 5)

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DIR(5)											   DIR(5)

       dir - format of directories

       #include <sys/types.h>
       #include <sys/dir.h>

       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) \
		  ((((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 meaning
       of `..' is modified for the root directory of the master file system ("/"), where `..' has
       the same meaning as `.'.


       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 arith-
       metic in the kernel.

4.2 Berkeley Distribution		   May 15, 1985 				   DIR(5)
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