Problem understanding Synclock

  • Thread starter Thread starter HONOREDANCESTOR
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HONOREDANCESTOR

I have a buffer that needs to be locked sometimes, because 2 processes
update it.
So I made the buffer into a class and whenever there is code that
affects it, I sandwich the code between

Synclock MyBufferClass
...........
End Synclock

My question is, what is Synlock protecting? Is it just protecting
MyBufferClass and all the members of MyBufferClass? Or could you have
code inside the sandwich that is totally unrelated like:

Synclock MyBufferClass
unrelatedvariable = "foozleberg"
End Synclock

Would "unrelatedvariable" be also protected from other processes
updating it?

Thanks,
HA
 
I have a buffer that needs to be locked sometimes, because 2 processes
update it.
Click to expand...

Synclock (and the related Monitor.Enter) apply to multiple threads in one
process. If you are really dealing with two processes, you need to use a
mutex. If you are dealing with multiple threads within one process, then
read on.
My question is, what is Synlock protecting? Is it just protecting
MyBufferClass and all the members of MyBufferClass? Or could you have
code inside the sandwich that is totally unrelated like...
Click to expand...

Synclock (and Monotor.Enter) assure that only one thread will be executing
at a time within Synclock'ed section(s) of code. When thread1 is executing
between Synclock xxx and its matching End Synclock, thread2 will block when
it attempts to execute Synclock xxx. Thread2's block will release when
thread1 executes End Synclock. What you do within the sections of code is up
to you, so the Synclock is not really protecting anything beyond enforcing
only one Synclock 'owner' of xxx. This notion is the same as with mutexes -
all they do is assure one 'owner' of the mutex at a time.

Imagine two subs that add and remove entries from some object of yours. No
problem if your program is single threaded. If it has multiple threads, all
will be well if you Synclock appropriately, eg

Dim MyObject As New SomeObjectClassYouWrote

Sub Add(whatever)
Synclock MyObject
whatever it takes to do an add goes here
End Synclock
End sub

Sub Remove(whatever)
Synclock MyObject
whatever it takes to do a remove goes here
End Synclock
End sub

The object being Synclock'ed could be the implied object (Me) if sub Add is
a method of class SomeObjectClassYouWrote, eg

Sub Add(whatever)
Synclock Me
whatever it takes to do an add goes here
End Synclock
End sub

In this case, if you have two object instances of SomeObjectClassYouWrote,
say o1 and o2, then calling o1.Add on one thread and calling o2.Add on
another thread would proceed without any blocking. On the other hand, if
both threads called o1.Add nearly simultaneously, then one would enter the
synclock, and the other would block.
Would "unrelatedvariable" be also protected from other processes
updating it?
Click to expand...

No, if you followed the above. All mutexes and Synclocks do is enforce
taking turns. Remember, mutexes are for processes, and Synclocks are for
threads within processes.
 
I have a buffer that needs to be locked sometimes, because 2 processes
update it.
So I made the buffer into a class and whenever there is code that
affects it, I sandwich the code between

Synclock MyBufferClass
..........
End Synclock

My question is, what is Synlock protecting? Is it just protecting
MyBufferClass and all the members of MyBufferClass? Or could you have
code inside the sandwich that is totally unrelated like:

Synclock MyBufferClass
unrelatedvariable = "foozleberg"
End Synclock

Would "unrelatedvariable" be also protected from other processes
updating it?

Thanks,
HA
Click to expand...

Hi,

SyncLock will not synchronize anything between processes. SyncLock is
based on the Monitor class which usually operates on AppDomain
specific objects. You could use domain neutral objects as targets for
the Monitor to synchronize across domains, but that's pretty rare.

The Monitor.Enter method (which SyncLock calls for you) accepts an
object that will be used as a monitor. It's important to understand
that it isn't the monitor object that's being synchronized. The
monitor object just acts as guard. Everything in the SyncLock block
is protected from other threads that use a SyncLock on the *same*
monitor object.

If you're really needing to synchronize access to a buffer that is
shared between processes then you could use a Mutex.

Brian
 
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