[Omp] Shared Variable Problem

[Yuan Lin]

The value of a shared variable updated by one thread is not guaranteed 
to be seen by another thread unless flush (implicit or explicit) is 
used. The memory model is described in Spec 2.5. In your sample, the 
value of i may always be read from a register.

Try the following code and see if it works as you wanted.

i = 0;

#pragma omp parallel private(tid)
{
   tid = omp_get_thread_num();
   while(1)
   {
	if (!tid)
	{
                 #pragma omp flush(i)
		printf("master: i = %d\n", i);
		fflush(stdout);
	}
	else
	{
		printf("worker: i = %d\n", i);
		fflush(stdout);
		i ++;
                 #pragma omp flush(i)
	}
   }
}


Also depending on how the threads are scheduled, you may see thousands 
of master: i=0 before you see  worker: i=1, 2, 3, ...., then you may see 
master: i= some big number. Do not expect to see the value of i printed 
from the master be to consecutive. There is no synchronization between 
the master thread and the slave thread in your code.

Hope it helps.

-- Yuan

----------------------------------------
http://blogs.sun.com/roller/page/yuanlin
----------------------------------------


KaveH Aasaraai wrote:
> Hey guys,
> 
> I'm new to this OpenMP thing. I'm facing this problem
> of using a shared value, and trying to synchronize two
> threads with this shared variable. Any help would
> really be appreciated. Here is the problem:
> 
> i = 0;
> 
> #pragma omp parallel private(tid)
> tid = omp_get_thread_num();
> while(1)
> {
> 	if (!tid)
> 	{
> 		printf("master: i = %d\n", i);
> 		fflush(stdout);
> 	}
> 	else
> 	{
> 		printf("worker: i = %d\n", i);
> 		fflush(stdout);
> 		i ++;
> 	}
> }
> 
> The "0" thread doesn't see the changes to the shared
> variable, i. If I remove the while(1) thing, then the
> value printed for "i" is correct. Here is a sample
> (partial) output:
> 
> 
> master: i = 0
> master: i = 0
> master: i = 0
> worker: i = 2445
> worker: i = 2446
> worker: i = 2447
> worker: i = 2448
> worker: i = 2449
> worker: i = 2450
> worker: i = 2451
> worker: i = 2452
> 
> 
> 
> Thanks to you guys in advance,
> 
> Kaveh
> 
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