term% cat index.txt FILSYS(5) File Formats Manual FILSYS(5)
NAME
filsys, flblk, ino - format of file system volume
SYNOPSIS
#include <sys/types.h>
#include <sys/fblk.h>
#include <sys/filsys.h>
#include <sys/ino.h>
DESCRIPTION
Every file system is divided into a certain number of blocks of 1K or
4K bytes, as determined by the predicate BITFS() applied to the minor
device number where the file system is mounted. Block 0 is unused and
is available to contain a bootstrap program, pack label, or other in‐
formation.
Block 1 is the super block. The layout of the super block as defined
by the include file <sys/filsys.h> is:
S_isize is the address of the first block after the i-list, which
starts just after the super-block, in block 2. Thus the i-list is
s_isize-2 blocks long. S_fsize is the address of the first block not
potentially available for allocation to a file. These numbers are used
by the system to check for bad block addresses; if an ‘impossible'
block address is allocated from the free list or is freed, a diagnostic
is written on the on-line console. Moreover, the free array is
cleared, so as to prevent further allocation from a presumably cor‐
rupted free list.
The free list for a 1K file system is maintained as follows. The
s_free array contains, in s_free[1], ... , s_free[s_nfree-1], up to
NICFREE free block numbers. NICFREE is a configuration constant.
S_free[0] is the block address of the head of a chain of blocks consti‐
tuting the free list. The layout of each block of the free chain as
defined in the include file <sys/fblk.h> is:
The fields df_nfree and df_free in a free block are used exactly like
s_nfree and s_free in the super block. To allocate a block: decrement
s_nfree, and the new block number is s_free[s_nfree]. If the new block
address is 0, there are no blocks left, so give an error. If s_nfree
became 0, read the new block into s_nfree and s_free. To free a block,
check if s_nfree is NICFREE; if so, copy s_nfree and the s_free array
into it, write it out, and set s_nfree to 0. In any event set
s_free[s_nfree] to the freed block's address and increment s_nfree.
The free list for a 4K file system is a bitmap. The bit
(s_bfree[i/w]>>(i%w))&1, where w is the bit size of a long, is nonzero
if the ith data block is free. The bitmap of a mounted file system is
maintained only in main memory; the bitmap on the medium is marked in‐
valid by setting s_valid to zero. Unmounting updates the medium copy
and sets s_valid to 1. A file system with invalid bitmap may be
mounted read-only; its bitmap can be corrected by fsck(8).
S_ninode is the number of free i-numbers in the s_inode array. To al‐
locate an i-node: if s_ninode is greater than 0, decrement it and re‐
turn s_inode[s_ninode]. If it was 0, read the i-list and place the
numbers of all free inodes (up to NICINOD) into the s_inode array, then
try again. To free an i-node, provided s_ninode is less than NICINODE,
place its number into s_inode[s_ninode] and increment s_ninode. If
s_ninode is already NICINODE, don't bother to enter the freed i-node
into any table. This list of i-nodes is only to speed up the alloca‐
tion process; the information as to whether the inode is really free or
not is maintained in the inode itself.
The fields s_lasti and s_nbehind are used to avoid searching the inode
list from the beginning each time the system runs out of inodes.
S_lasti gives the base of the block of inodes last searched on the
filesystem when inodes ran out, and s_nbehind gives the number of in‐
odes, whose numbers were less than s_lasti when they were freed with
s_ninode already NICINODE. Thus s_ninode is the number of free inodes
before s_lasti. The system will search forward for free inodes from
s_lasti for more inodes unless s_nbehind is sufficiently large, in
which case it will search the file system inode list from the begin‐
ning. This mechanism avoids quadratic behavior in allocating inodes.
S_flock and s_ilock are flags maintained in the core copy of the file
system while it is mounted and their values on disk are immaterial.
The value of s_fmod on disk is likewise immaterial; it is used as a
flag to indicate that the super-block has changed and should be copied
to the disk during the next periodic update of file system information.
S_ronly is a write-protection indicator; its disk value is also immate‐
rial.
S_time is the last time the super-block of the file system was changed.
During a reboot, s_time of the super-block for the root file system is
used to set the system's idea of the time.
The fields s_tfree, s_tinode, s_fname and s_fpack are not currently
maintained.
I-numbers begin at 1, and the storage for i-nodes begins in block 2.
I-nodes are 64 bytes long, so 16 of them fit into a block. I-node 2 is
reserved for the root directory of the file system, but no other i-num‐
ber has a built-in meaning. Each i-node represents one file. The for‐
mat of an i-node as given in the include file <sys/ino.h> is:
Di_mode tells the kind of file; it is encoded identically to the
st_mode field of stat(2). Di_nlink is the number of directory entries
(links) that refer to this i-node. Di_uid and di_gid are the owner's
user and group IDs. Size is the number of bytes in the file. Di_atime
and di_mtime are the times of last access and modification of the file
contents (read, write or create) (see times(2)); Di_ctime records the
time of last modification to the inode or to the file, and is used to
determine whether it should be dumped.
Special files are recognized by their modes and not by i-number. A
block-type special file is one which can potentially be mounted as a
file system; a character-type special file cannot, though it is not
necessarily character-oriented. For special files, the di_addr field
is occupied by the device code (see types(5)). The device codes of
block and character special files overlap.
A symbolic link is, aside from mode, a plain file whose sole content is
the name of the file linked to.
Disk addresses of plain files and directories are kept in the array
di_addr packed into 3 bytes each. The first 10 addresses specify de‐
vice blocks directly. The last 3 addresses are singly, doubly, and
triply indirect and point to blocks of 256 block pointers. Pointers in
indirect blocks have the type daddr_t (see types(5)).
For block b in a file to exist, it is not necessary that all blocks
less than b exist. A zero block number either in the address words of
the i-node or in an indirect block indicates that the corresponding
block has never been allocated. Such a missing block reads as if it
contained all zero words.
SEE ALSO
fsck(8), icheck(8), dcheck(8), dir(5), mount(8), stat(2), types(5)
FILSYS(5)