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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 820
SEC. 10.6
THE LINUX FILE SYSTEM
789
The problem is as follows. Associated with every file descriptor is a file position
that tells at which byte the next read (or write) will start. Where should it go?
One
possibility is to put it in the i-node table. However, this approach fails if two or
more unrelated processes happen to open the same file at the same time because
each one has its own file position.
A second possibility is to put the file position in the file-descriptor table.
In
that way, every process that opens a file gets its own private file position. Unfortun-
ately this scheme fails too, but the reasoning is more subtle and has to do with the
nature of file sharing in Linux. Consider a shell script,
s
, consisting of two com-
mands,
p1
and
p2
, to be run in order.
If the shell script is called by the command
s >x
it is expected that
p1
will write its output to
x
, and then
p2
will write its output to
x
also, starting at the place where
p1
stopped.
When the shell forks off
p1
,
x
is initially empty, so
p1
just starts writing at file
position 0.
However, when
p1
finishes, some mechanism is needed to make sure
that the initial file position that
p2
sees is not 0 (which it would be if the file posi-
tion were kept in the file-descriptor table), but the value
p1
ended with.
The way this is achieved is shown in Fig. 10-34. The trick is to introduce a
new table, the
open-file-description table
, between the file descriptor table and
the i-node table, and put the file position (and read/write bit) there.
In this figure,
the parent is the shell and the child is first
p1
and later
p2
.
When the shell forks off
p1
, its user structure (including the file-descriptor table) is an exact copy of the
shell’s, so both of them point to the same open-file-description table entry. When
p1
finishes, the shell’s file descriptor is still pointing to the open-file description
containing
p1
’s file position. When the shell now forks off
p2
, the new child auto-
matically inherits the file position, without either it or the shell even having to
know what that position is.
However, if an unrelated process opens the file, it gets its own open-file-de-
scription entry, with its own file position, which is precisely what is needed. Thus
the whole point of the open-file-description table is to allow a parent and child to
share a file position, but to provide unrelated processes with their own values.
Getting back to the problem of doing the
read
, we have now shown how the
file position and i-node are located. The i-node contains the disk addresses of the
first 12 blocks of the file. If the file position falls in the first 12 blocks, the block is
read and the data are copied to the user. For files longer than 12 blocks, a field in
the i-node contains the disk address of a
single indirect block
, as shown in
Fig. 10-34.
This block contains the disk addresses of more disk blocks. For ex-
ample, if a block is 1 KB and a disk address is 4 bytes, the single indirect block
can hold 256 disk addresses. Thus this scheme works for files of up to 268 KB.
Beyond that, a
double indirect block
is used.
It contains the addresses of 256
single indirect blocks, each of which holds the addresses of 256 data blocks. This
mechanism is sufficient to handle files up to 10
+
2
16
blocks (67,119,104 bytes).
If
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