1DIR(5) File Formats Manual DIR(5)
2
6 dir - format of directories
7
9 #include <sys/types.h>
10 #include <sys/dir.h>
11
13 A directory behaves exactly like an ordinary file, save that no user
14 may write into a directory. The fact that a file is a directory is
15 indicated by a bit in the flag word of its i-node entry; see fs(5).
16 The structure of a directory entry as given in the include file is:
17 /*
19 * A directory consists of some number of blocks of DIRBLKSIZ
20 * bytes, where DIRBLKSIZ is chosen such that it can be transferred
21 * to disk in a single atomic operation (e.g. 512 bytes on most machines).
22 *
23 * Each DIRBLKSIZ byte block contains some number of directory entry
24 * structures, which are of variable length. Each directory entry has
25 * a struct direct at the front of it, containing its inode number,
26 * the length of the entry, and the length of the name contained in
27 * the entry. These are followed by the name padded to a 4 byte boundary
28 * with null bytes. All names are guaranteed null terminated.
29 * The maximum length of a name in a directory is MAXNAMLEN.
30 *
31 * The macro DIRSIZ(dp) gives the amount of space required to represent
32 * a directory entry. Free space in a directory is represented by
33 * entries which have dp->d_reclen > DIRSIZ(dp). All DIRBLKSIZ bytes
34 * in a directory block are claimed by the directory entries. This
35 * usually results in the last entry in a directory having a large
36 * dp->d_reclen. When entries are deleted from a directory, the
37 * space is returned to the previous entry in the same directory
38 * block by increasing its dp->d_reclen. If the first entry of
39 * a directory block is free, then its dp->d_ino is set to 0.
40 * Entries other than the first in a directory do not normally have
41 * dp->d_ino set to 0.
42 */
43 #define DIRBLKSIZ 512
45 #define MAXNAMLEN 63
47 /*
49 * The DIRSIZ macro gives the minimum record length which will hold
50 * the directory entry. This requires the amount of space in struct direct
51 * without the d_name field, plus enough space for the name with a terminating
52 * null byte (dp->d_namlen+1), rounded up to a 4 byte boundary.
53 */
54 #undef DIRSIZ
55 #define DIRSIZ(dp) \
56 ((((sizeof (struct direct) - (MAXNAMLEN+1)) + (dp)->d_namlen+1) + 3) &~ 3)
57 struct direct {
59 ino_t d_ino;
60 short d_reclen;
61 short d_namlen;
62 char d_name[MAXNAMLEN + 1];
63 /* typically shorter */
64 };
65 struct _dirdesc {
67 int dd_fd;
68 long dd_loc;
69 long dd_size;
70 char dd_buf[DIRBLKSIZ];
71 };
72 By convention, the first two entries in each directory are for `.' and
74 `..'. The first is an entry for the directory itself. The second is
75 for the parent directory. The meaning of `..' is modified for the root
76 directory of the master file system (“/”), where `..' has the same
77 meaning as `.'.
78
80 fs(5)
81
83 The 63 character MAXNAMLEN value is shorter than the 255 characters
84 allowed by 4BSD. This could lead to file name portability problems in
85 unusual circumstances.
86 The disk format of directories is only slightly different from the 4BSD
88 directory format, the inode number is of type ino_t rather than u_long
89 to reduce the amount of 32 bit arithmetic in the kernel.
904.2 Berkeley Distribution May 15, 1985 DIR(5)