GNU/Linux man pages
Livre :
Expressions régulières,
Syntaxe et mise en oeuvre :
Unix |
Unix v6 |
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fs(5) |
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fs format of file system volume Every file system
storage volume (e.g. RF disk, RK disk, RP disk, DECtape
reel) has a common format for certain vital information.
Every such volume is divided into a certain number of 256
word (512 byte) blocks. Block 0 is unused and is available
to contain a bootstrap program, pack label, or other
information. Block 1 is the Starting from its first word,
the format of a super-block is
struct {
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int |
isize; |
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int |
fsize; |
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int |
nfree; |
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int |
free[100]; |
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int |
ninode; |
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int |
inode[100]; |
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char |
flock; |
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char |
ilock; |
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char |
fmod; |
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int |
time[2]; |
};
is the number of blocks devoted to the i-list, which starts
just after the super-block, in block 2. is the first block
not potentially available for allocation to a file. These
numbers are used by the system to check for bad block
numbers; if an ‘‘impossible’’ block
number 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 corrupted free list. The free list for
each volume is maintained as follows. The array contains, in
up to 99 numbers of free blocks. is the block number of the
head of a chain of blocks constituting the free list. The
first word in each free-chain block is the number (up to
100) of free-block numbers listed in the next 100 words of
this chain member. The first of these 100 blocks is the link
to the next member of the chain. To allocate a block:
decrement and the new block is If the new block number is 0,
there are no blocks left, so give an error. If became 0,
read in the block named by the new block number, replace by
its first word, and copy the block numbers in the next 100
words into the array. To free a block, check if is 100; if
so, copy and the array into it, write it out, and set to 0.
In any event set to the freed block’s number and
increment is the number of free i-numbers in the array. To
allocate an i-node: if is greater than 0, decrement it and
return If it was 0, read the i-list and place the numbers of
all free inodes (up to 100) into the array, then try again.
To free an i-node, provided is less than 100, place its
number into and increment If is already 100, don’t
bother to enter the freed i-node into any table. This list
of i-nodes is only to speed up the allocation process; the
information as to whether the inode is really free or not is
maintained in the inode itself. and 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 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.
is the last time the super-block of the file system was
changed, and is a double-precision representation of the
number of seconds that have elapsed since 0000 Jan. 1 1970
(GMT). During a reboot, the of the super-block for the root
file system is used to set the system’s idea of the
time. I-numbers begin at 1, and the storage for i-nodes
begins in block 2. Also, i-nodes are 32 bytes long, so 16 of
them fit into a block. Therefore, i-node is located in block
(i + 31) / 16, and begins
32.((i + 31) (mod 16) bytes
from its start. I-node 1 is reserved for the root directory
of the file system, but no other i-number has a built-in
meaning. Each i-node represents one file. The format of an
i-node is as follows.
struct {
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int |
flags; |
/* +0: see below */ |
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char |
nlinks; |
/* +2: number of links to file */ |
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char |
uid; |
/* +3: user ID of owner */ |
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char |
gid; |
/* +4: group ID of owner */ |
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char |
size0; |
/* +5: high byte of 24-bit size */ |
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int |
size1; |
/* +6: low word of 24-bit size */ |
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int |
addr[8]; |
/* +8: block numbers or device number */ |
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int |
actime[2]; |
/* +24: time of last access */ |
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int |
modtime[2]; |
/* +28: time of last modification */ |
};
The flags are as follows: 100000 i-node is allocated 060000
2-bit file type: 000000 plain file 040000 directory 020000
character-type special file 060000 block-type special file.
010000 large file 004000 set user-ID on execution 002000 set
group-ID on execution 000400 read (owner) 000200 write
(owner) 000100 execute (owner) 000070 read, write, execute
(group) 000007 read, write, execute (others) Special files
are recognized by their flags and not by i-number. A
block-type special file is basically 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 high byte of the
first address word specifies the type of device; the low
byte specifies one of several devices of that type. The
device type numbers of block and character special files
overlap. The address words of ordinary files and directories
contain the numbers of the blocks in the file (if it is
small) or the numbers of indirect blocks (if the file is
large). Byte number of a file is accessed as follows. is
divided by 512 to find its logical block number (say ) in
the file. If the file is small (flag 010000 is 0), then must
be less than 8, and the physical block number is If the file
is large, is divided by 256 to yield If is less than 7, then
is the physical block number of the indirect block. The
remainder from the division yields the word in the indirect
block which contains the number of the block for the
sought-for byte. If is equal to 7, then the file has become
extra-large (huge), and is the address of a first indirect
block. Each word in this block is the number of a
second-level indirect block; each word in the second-level
indirect blocks points to a data block. Notice that
extra-large files are not marked by any mode bit, but only
by having non-zero; and that although this scheme allows for
more than 256256512 = 33,554,432 bytes per file, the length
of files is stored in 24 bits so in practice a file can be
at most 16,777,216 bytes long. For block in a file to exist,
it is not necessary that all blocks less than 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. icheck, dcheck (VIII)
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fs(5) | |