Hardware Reference
In-Depth Information
System call
Meaning
mkdir(name, mode)
Create a new directory
rmdir(name)
Delete an empty directory
opendir(name)
Open a directory for reading
readdir(dirpointer)
Read the next entry in a directory
closedir(dirpointer)
Close a directory
chdir(dirname)
Change working directory to
dirname
link(name1, name2)
Create a directory entry
name2
pointing to
name1
unlink(name)
Remove
name
from its directory
Figure 6-38.
The principal
UNIX
directory-management calls.
only one link, the file is deleted. If it has two or more links, it is kept. It does not
matter whether a removed link is the original or a copy made later. Once a link is
made, it is a first-class citizen, indistinguishable from the original. The call
unlink(
′′
/usr/ast/bin/game3
′′
)
makes
game3
accessible only via the path
/usr/jim/jotto
henceforth.
Link
and
unlink
can be used in this way to ''move'' files from one directory to another.
Associated with every file (including directories, because they are also files) is
a bit map telling who may access the file. The map contains three RWX fields, the
first controlling the Read, Write, eXecute permissions for the owner, the second for
others in the owner's group, and the third for everybody else. Thus RWX R-X --X
means that the owner can read the file, write the file, and execute the file (ob-
viously, it is an executable program, or execute would be off), whereas others in his
group can read or execute it and strangers can only execute it. With these permis-
sions, strangers can use the program but not steal (copy) it because they do not
have read permission. The assignment of users to groups is done by the system
administrator, usually called the
superuser
. The superuser also has the power to
override the protection mechanism and read, write, or execute any file.
Let us now briefly examine how files and directories are implemented in
UNIX
. For a more complete treatment, see Vahalia (1996). Associated with each
file (and each directory, because a directory is also a file) is a 64-byte block of
information called an
i-node
. The i-node tells who owns the file, what the permis-
sions are, where to find the data, and similar things. The i-nodes for the files on
each disk are located either in numerical sequence at the beginning of the disk or,
if the disk is split up into groups of cylinders, at the start of a cylinder group.
Thus, given an i-node number, the
UNIX
system can locate the i-node by simply
calculating its disk address.
A directory entry consists of two parts: a file name and an i-node number.
When a program executes
open(
′′
foo.c
′′
,0)
