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Modern Operating Systems by Herbert Bos and Andrew...
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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 929
CHAP. 11
A: Handle-table entries [512]
B: Handle-table pointers [1024]
C:Handle-table entries [512]
D: Handle-table pointers [32]
E: Handle-table pointers [1024]
F:Handle-table entries [512]
Table pointer
Figure 11-17.
Handle-table data structures for a maximal table of up to 16 mil-
lion handles.
at the time the object is created or opened.
If a process has only read permission to
an object, all the other rights bits in the mask will be 0s, giving the operating sys-
tem the ability to reject any operation on the object other than reads.
The Object Namespace
Processes can share objects by having one process duplicate a handle to the ob-
ject into the others.
But this requires that the duplicating process have handles to
the other processes, and is thus impractical in many situations, such as when the
processes sharing an object are unrelated, or are protected from each other.
other cases it is important that objects persist even when they are not being used by
any process, such as device objects representing physical devices, or mounted vol-
umes, or the objects used to implement the object manager and the NT namespace
itself. To address general sharing and persistence requirements, the object man-
ager allows arbitrary objects to be given names in the NT namespace when they are
created. However, it is up to the executive component that manipulates objects of a
particular type to provide interfaces that support use of the object manager’s na-
ming facilities.
The NT namespace is hierarchical, with the object manager implementing di-
rectories and symbolic links.
The namespace is also extensible, allowing any ob-
ject type to specify extensions of the namespace by specifying a
routine is one of the procedures that can be supplied for each object type
when it is created, as shown in Fig. 11-18.
procedure is rarely used because the default object-manager behav-
ior is usually what is needed and so the procedure is specified as NULL for almost
all object types.

Page 930
SEC. 11.3
When called
For every new handle
Rarely used
For object types that extend the namespace
Used for files and registry keys
At last handle close
Clean up visible side effects
At last pointer dereference
Object is about to be deleted
Get or set object’s security descriptor
Get object’s name
Rarely used outside kernel
Figure 11-18.
Object procedures supplied when specifying a new object type.
procedures represent different phases of being done with
an object.
When the last handle for an object is closed, there may be actions neces-
sary to clean up the state and these are performed by the
procedure. When
the final pointer reference is removed from the object, the
procedure is call-
ed so that the object can be prepared to be deleted and have its memory reused.
With file objects, both of these procedures are implemented as callbacks into the
I/O manager, which is the component that declared the file object type.
The ob-
ject-manager operations result in I/O operations that are sent down the device stack
associated with the file object; the file system does most of the work.
procedure is used to open or create objects, like files and registry
keys, that extend the NT namespace.
When the object manager is attempting to
open an object by name and encounters a leaf node in the part of the namespace it
manages, it checks to see if the type for the leaf-node object has specified a
procedure. If so, it invokes the procedure, passing it any unused part of the path
name. Again using file objects as an example, the leaf node is a device object
representing a particular file-system volume. The
procedure is implemented
by the I/O manager, and results in an I/O operation to the file system to fill in a file
object to refer to an open instance of the file that the path name refers to on the
volume. We will explore this particular example step-by-step below.
procedure is used to look up the name associated with an ob-
ject. The
procedure is used to get, set, or delete the security descriptors
on an object.
For most object types this procedure is supplied as a standard entry
point in the executive’s security reference monitor component.
Note that the procedures in Fig. 11-18 do not perform the most useful opera-
tions for each type of object, such as read or write on files (or down and up on
semaphores). Rather, the object manager procedures supply the functions needed
to correctly set up access to objects and then clean up when the system is finished
with them.
The objects are made useful by the APIs that operate on the data struc-
tures the objects contain.
System calls, like
, use the
process’ handle table created by the object manager to translate a handle into a ref-
erenced pointer on the underlying object, such as a file object, which contains the
data that is needed to implement the system calls.

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