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. 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 a 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 ‘.’.