DIR(4)DIR(4)NAMEdir - format of EFS directories
SYNOPSIS
#include <sys/fs/efs_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 [see inode(4)]. The EFS
directory format supports variable length names of up to 255 characters.
DIRECTORY BLOCKS
Each EFS directory is segmented into directory blocks defined by the
following data structure:
#define EFS_DIRBLK_HEADERSIZE 4
struct efs_dirblk {
/* begin header */
ushort magic;
unchar firstused;
unchar slots;
/* end header */
/* rest is space for efs_dent's */
unchar space[EFS_DIRBSIZE - EFS_DIRBLK_HEADERSIZE];
};
Each directory block is subdivided into three separate areas: a header,
an array of entry offsets and an array of directory entries. The system
restricts directory entries to short boundaries and stores offsets in the
directory block compacted by shifting them right by one.
The header area contains a magic number to identify the block as being a
directory block. If the magic number is incorrect, the operating system
will refuse to manipulate the directory, thus avoiding further
corruption.
The array of entry offsets immediately follows the header and is sized
according to the directories contents and contains compacted offsets
which point to each directory entry. The number of entry offsets
available is kept in slots. The firstused field contains a compacted
offset which positions the first byte of the directory entries.
The space between the end of the entry array and the beginning of the
directory entries (firstused) is free space which the system uses for
allocating new directory entries and entry offsets. The system keeps the
free space in a directory block compacted by coalescing holes created by
entry removal. When a directory entry is removed, the system adjusts the
entry offsets for all entries that move. Also, the entry offset for the
removed entry is zeroed. If the removed entry was the last in the entry
offset array, the number of slots is reduced. Directory entries never
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DIR(4)DIR(4)
change which entry offset they use.
DIRECTORY ENTRIES
Directory entries have the following structure:
struct efs_dent {
union {
ulong l;
ushorts[2];
} ud_inum;
unchar d_namelen;
char d_name[3];
};
The d_name field is actually of variable size, depending upon the value
contained in d_namelen. The system pads out the directory entry to
insure that it begins on a short boundary in the directory block. The
ud_inum field contains the entries inode number.
SEE ALSOfs(4), inode(4).
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