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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 813
782
CASE STUDY 1: UNIX, LINUX, AND ANDROID
CHAP. 10
pointer that indicates the current position in the file. When reading (or writing) se-
quentially, it normally points to the next byte to be read (written).
If the pointer is
at, say, 4096, before 1024 bytes are read, it will automatically be moved to 5120
after a successful
read
system call. The
lseek
call changes the value of the position
pointer, so that subsequent calls to
read
or
write
can begin anywhere in the file, or
even beyond the end of it.
It is called
lseek
to avoid conflicting with
seek
,
a
now-
obsolete call that was formerly used on 16-bit computers for seeking.
Lseek
has three parameters: the first one is the file descriptor for the file; the
second is a file position; the third tells whether the file position is relative to the be-
ginning of the file, the current position, or the end of the file. The value returned by
lseek
is the absolute position in the file after the file pointer is changed. Slightly
ironically,
lseek
is the only file system call that never causes a real disk seek be-
cause all it does is update the current file position, which is a number in memory.
For each file, Linux keeps track of the file mode (regular, directory, special
file), size, time of last modification, and other information. Programs can ask to see
this information via the
stat
system call. The first parameter is the file name.
The
second is a pointer to a structure where the information requested is to be put. The
fields in the structure are shown in Fig. 10-28. The
fstat
call is the same as
stat
ex-
cept that it operates on an open file (whose name may not be known) rather than on
a path name.
Device the file is on
I-node number (which file on the device)
File mode (includes protection information)
Number of links to the file
Identity of the file’s owner
Group the file belongs to
File size (in bytes)
Creation time
Time of last access
Time of last modification
Figure 10-28.
The fields returned by the
stat
system call.
The
pipe
system call is used to create shell pipelines.
It creates a kind of
pseudofile, which buffers the data between the pipeline components, and returns
file descriptors for both reading and writing the buffer.
In a pipeline such as
sort <in | head –30
file descriptor 1 (standard output) in the process running
sort
would be set (by the
shell) to write to the pipe, and file descriptor 0 (standard input) in the process run-
ning
head
would be set to read from the pipe.
In this way,
sort
just reads from file
descriptor 0 (set to the file
in
) and writes to file descriptor 1 (the pipe) without even
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