ultrix man page for dir

dir(5)								File Formats Manual							    dir(5)

Name
       dir - format of directories

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

Description
       A  directory behaves exactly like an ordinary file, except 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.  For further information, see The structure of a directory entry is given  in  the
       include file.

       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 (for example, 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  action
       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 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 that 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 dot (.) and dot dot (..).  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 dot
       dot has the same meaning as dot.

See Also
       fs(5)

																	    dir(5)