[John Fors]

Hi I think I need to semiphore protect some code in an object (vb6 class)
would like to get a pointer
about whats the best way to do this and get some resources to study. Thanks
in advance for you time
and help.

John

Archived from group: microsoft>public>vb>enterprise

[Sin]

> Hi I think I need to semiphore protect some code in an object (vb6 class)
> would like to get a pointer
> about whats the best way to do this and get some resources to study.
Thanks
> in advance for you time
> and help.


That would be a sem_A_phore John...

A semaphore is usually used to control access to something which can be
accessed by more than one thread at the same time. For example you have 10
threads, but only 3 objects to do the job... 3 use it, and the 7 others wait
for the semaphore to be released by one of the 3 threads that acquired it.

If you have ONE point which you want to limit access, a Mutex (Mutual
Exclusion) is more appropriate. Look at the following functions :

CreateMutex()
OpenMutex()
WaitForSingleObject()
ReleaseMutex()
CloseHandle()

If both threads are within the same app (that is, if you can share the
handle), an unnamed mutex will be fine. If the threads are different
applications, then a named mutex (with OpenMutex()) is the way.

If you only have one thread, then you're wasting your time, you do not need
a mutex or semaphore...

Alex.

[Tony Proctor]

I was just going to post his myself Alex. :

Anyway, since you beat me to it, I'll just add a note about critical
sections...

Normally, multi-threaded applications use CRITICAL_SECTION objects for
mutual exclusion between the threads. However, they rely on being able to
have a shared piece of memory containing the CRITICAL_SECTION object (which
I think is about 7 long integers in size). Unfortunately, VB6's limited
multi-threading capability prevents you from sharing any data at all between
your threads. If you tried defining a CRITICAL_SECTION datum in a VB6 Module
file then each thread would end up with its own copy, and they wouldn't be
synchronising against each other at all.

It is possible to forcible share a piece of new memory between the threads,
but it's much easier to use Mutex objects as Alex suggested. They're a
little slower than critical sections because they can also synchronise
between different applications, as well as different threads in the same
application. Another advantage is that it doesn't matter which thread
accesses the mutex first - the first one usually creates it automatically
and subsequent ones merely open it. With critical sections, one thread has
to be the master and initialise the CRITICAL_SECTION object before the
others can use it.

Tony Proctor

"Sin" wrote in message@TK2MSFTNGP09.phx.gbl...
> > Hi I think I need to semiphore protect some code in an object (vb6
class)
> > would like to get a pointer
> > about whats the best way to do this and get some resources to study.
> Thanks
> > in advance for you time
> > and help.
>
>
> That would be a sem_A_phore John...
>
> A semaphore is usually used to control access to something which can be
> accessed by more than one thread at the same time. For example you have 10
> threads, but only 3 objects to do the job... 3 use it, and the 7 others
wait
> for the semaphore to be released by one of the 3 threads that acquired it.
>
> If you have ONE point which you want to limit access, a Mutex (Mutual
> Exclusion) is more appropriate. Look at the following functions :
>
> CreateMutex()
> OpenMutex()
> WaitForSingleObject()
> ReleaseMutex()
> CloseHandle()
>
> If both threads are within the same app (that is, if you can share the
> handle), an unnamed mutex will be fine. If the threads are different
> applications, then a named mutex (with OpenMutex()) is the way.
>
> If you only have one thread, then you're wasting your time, you do not
need
> a mutex or semaphore...
>
> Alex.
>
>

[Jens Neuhalfen]

Hi group(s)

If anyone is interested, I recently posted a vb-wrapper for semaphores
to microsoft.public.vb.general.discussion (subject "Re: Previous
instance") and have one for mutexes lying around here (somewhere).

If you are interested I will post the code

Jens

PS:
>
> It is possible to forcible share a piece of new memory between the threads,
How? This would be most interesting to me (if it is without using
VirtualAlloc and a fixed adress).