Hardware Reference
In-Depth Information
The call to
read
has three parameters: a file descriptor, a buffer, and a byte
count. The call tries to read the desired number of bytes from the indicated file
into the buffer. The number of bytes actually read is returned in
count
, which will
be smaller than
bytes
if the file was too short. The
write
call deposits the newly
read bytes on the output file. The loop continues until the input file has been com-
pletely read, at which time the loop terminates and both files are closed.
File descriptors in
UNIX
are small integers (usually below 20). File descriptors
0, 1, and 2 are special and correspond to
standard input
,
standard output
, and
standard error
, respectively. Normally, these refer to the keyboard, the display,
and the display, respectively, but they can be redirected to files by the user. Many
UNIX
programs get their input from standard input and write the processed output
on standard output. Such programs are often called
filters
.
Closely related to the file system is the directory system. Each user may have
multiple directories, with each directory containing both files and subdirectories.
UNIX
systems normally are configured with a main directory, called the
root
directory
, containing subdirectories
bin
(for frequently executed programs),
dev
(for the special I/O device files),
lib
(for libraries), and
usr
(for user directories), as
shown in Fig. 6-37. In this example, the
usr
directory contains subdirectories for
ast
and
jim
. The
ast
directory contains two files,
data
and
foo.c
, and a subdirec-
tory,
bin
, containing four games.
When multiple disks or disk partitions are present, they can be mounted in the
naming tree so that all files on all disks appear in the same directory hierarchy, all
reachable from the root directory.
Files can be named by giving their
path
from the root directory. A path con-
tains a list of all the directories traversed from the root to the file, with directory
names separated by slashes. For example, the absolute path name of
game2
is
/usr/ast/bin/game2
. A path starting at the root is called an
absolute path
.
At every instant, each running program has a
working directory
. Path names
may also be relative to the working directory, in which case they do not begin with
a slash, to distinguish them from absolute path names. Such paths are called
rela-
tive paths
. When
/usr/ast
is the working directory,
game3
can be accessed using
the path
bin/game3
. A user may create a
link
to someone else's file using the
link
system call. In the above example,
/usr/ast/bin/game3
and
/usr/jim/jotto
both ac-
cess the same file. To prevent cycles in the directory system, links are not permit-
ted to directories. The calls
open
and
creat
take either absolute or relative path
names as arguments.
The major directory-management system calls in
UNIX
are listed in Fig. 6-38.
Mkdir
creates a new directory and
rmdir
deletes an existing (empty) directory. The
next three calls are used to read directory entries. The first one opens the directory,
the next one reads entries from it, and the last one closes the directory.
Chdir
changes the working directory.
Link
makes a new directory entry with the new entry pointing to an existing
file. For example, the entry
/usr/jim/jotto
might have been created by the call
