Thread Synchronization #2


  I have a shared resource, say a file which is to be accessed between 
  threads.  There are say 20 Read threads and 1 Write thread. When the Write 
  thread is writing into the file, the Read threads should be blocked, and 
when even 
  1 read thread reads, the Write thread should be blocked. The Read threads 
  read simultaneously, but only say 5 at a time. What kind of synchronization 
  mechanism can I use for this?

SD (74)
5/31/2005 8:50:08 AM
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You can create a mutex a adquire it when a read operation start, check it 
before start any write operation.
You will need another mutex for check where there is a write running. You 
must check this second mutex before start any read operation.

You can use WaitForSingleObject functions for wait to read or write.

In order to ensure no more than five readers at a time you can user a 

Un saludo
Rodrigo Corral Gonz�lez [MVP]

FAQ de

5/31/2005 9:29:54 AM
In that case, assume a Read thread acquires a mutex and starts reading,
and I use a semaphore to allow 5 Readers. If the Read thread that acquired
the mutex releases it, and the other Readers are not done yet, the Writer
will get the impression that the Readers are done, and start writing, and 
this will
lead back to the original problem.

"Rodrigo Corral [MVP]" wrote:

> You can create a mutex a adquire it when a read operation start, check it 
> before start any write operation.
> You will need another mutex for check where there is a write running. You 
> must check this second mutex before start any read operation.
> You can use WaitForSingleObject functions for wait to read or write.
> In order to ensure no more than five readers at a time you can user a 
> semaphore.
> -- 
> Un saludo
> Rodrigo Corral González [MVP]
> FAQ de
SD (74)
5/31/2005 10:07:08 AM
A rough solution:
Prepare 20 critical sections for the 20 readers and the writer:
look at this pseudocode:
UINT writer(..)
   // the others

   // writing process

   // the others

UINT Reaer1(..) {
   // ..
// other readers..


For 5 readers managing use a semaphore.

armancho_x1 (249)
5/31/2005 10:34:01 AM
You should check the semaphore when you are going to read. The state of a 
semaphore is set to signaled when its count is greater than zero, and 
nonsignaled when its count is zero. You can check the samphore status using 

if (WaitForSingleObject(hSemaphore, 0) == WAIT_TIMEOUT)
    //Semaphore not signaled, you can write.

Un saludo
Rodrigo Corral Gonz�lez [MVP]

FAQ de

Un saludo
Rodrigo Corral Gonz�lez [MVP]

FAQ de

5/31/2005 11:07:10 AM

Un saludo
Rodrigo Corral Gonz�lez [MVP]

FAQ de

5/31/2005 11:11:56 AM

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