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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 949
918
CASE STUDY 2: WINDOWS 8
CHAP. 11
Semaphores are kernel-mode objects and thus have security descriptors and hand-
les. The handle for a semaphore can be duplicated using
DuplicateHandle
and pas-
sed to another process so that multiple processes can synchronize on the same sem-
aphore. A semaphore can also be given a name in the Win32 namespace and have
an ACL set to protect it.
Sometimes sharing a semaphore by name is more ap-
propriate than duplicating the handle.
Calls for
up
and
down
exist, although they have the somewhat odd names of
ReleaseSemaphore
(
up
) and
WaitForSingleObject
(
down
). It is also possible to
give
WaitForSingleObject
a timeout, so the calling thread can be released eventual-
ly, even if the semaphore remains at 0 (although timers reintroduce races).
Wait-
ForSingleObject
and
WaitForMultipleObjects
are the common interfaces used for
waiting on the dispatcher objects discussed in Sec. 11.3.
While it would have been
possible to wrap the single-object version of these APIs in a wrapper with a some-
what more semaphore-friendly name, many threads use the multiple-object version
which may include waiting for multiple flavors of synchronization objects as well
as other events like process or thread termination, I/O completion, and messages
being available on sockets and ports.
Mutexes are also kernel-mode objects used for synchronization, but simpler
than semaphores because they do not have counters. They are essentially locks,
with API functions for locking
WaitForSingleObject
and unlocking
ReleaseMutex
.
Like semaphore handles, mutex handles can be duplicated and passed between
processes so that threads in different processes can access the same mutex.
A third synchronization mechanism is called
critical sections
, which imple-
ment the concept of critical regions. These are similar to mutexes in Windows, ex-
cept local to the address space of the creating thread.
Because critical sections are
not kernel-mode objects, they do not have explicit handles or security descriptors
and cannot be passed between processes.
Locking and unlocking are done with
EnterCriticalSection
and
LeaveCriticalSection
, respectively.
Because these API
functions are performed initially in user space and make kernel calls only when
blocking is needed, they are much faster than mutexes. Critical sections are opti-
mized to combine spin locks (on multiprocessors) with the use of kernel synchroni-
zation only when necessary.
In many applications most critical sections are so
rarely contended or have such short hold times that it is never necessary to allocate
a kernel synchronization object.
This results in a very significant savings in kernel
memory.
Another synchronization mechanism we discuss uses kernel-mode objects call-
ed
events
.
As we have described previously, there are two kinds:
notification
events
and
synchronization events
.
An event can be in one of two states: signaled
or not-signaled.
A thread can wait for an event to be signaled with
WaitForSin-
gleObject
.
If another thread signals an event with
SetEvent
, what happens depends
on the type of event. With a notification event, all waiting threads are released and
the event stays set until manually cleared with
ResetEvent
.
With a synchroniza-
tion event, if one or more threads are waiting, exactly one thread is released and
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