dir(5)dir(5)Namedir - 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 direc‐
tory 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 increas‐
ing 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 termi‐
nating 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 Alsofs(5)dir(5)