On Monday 24 July 2006 10:21, Graeme Geldenhuys wrote:
> procedure TtiPool.CreatePoolSemaphore;
> begin
> {$IFDEF MSWINDOWS}
> if FSemaphore <> 0 then
> CloseHandle( FSemaphore ) ;
> FSemaphore := CreateSemaphore( nil, FiMaxPoolSize, FiMaxPoolSize,
> nil ); {$ENDIF MSWINDOWS}
> {$IFDEF LINUX}
> sem_destroy( FSemaphore );
>
> if sem_init(FSemaphore, 0, 1) <> 0 then
> raise Exception.Create('Failed to create the semaphore');
> {$ENDIF LINUX}
> end;
What is FiMaxPoolSize? I assume, it's something like a number of request
being allowed inside the semaphore-protected region at once?
So in the linux version only one thread is allowed being there.
I suppose you wrote something like
|for i := 0 to 9 do
|begin
| Lock;
| WriteLn ('Ping');
|end {for};
|
|for i := 0 to 9 do
|begin
| WriteLn ('Pong');
| Unlock;
|end {for};
In that case you're experiencing the lock-up when the semaphore is being
"entered"/locked the second time inside your loop. The counter is zero
after the first sem_wait and remains that way, because there's nobody
there to call the sem_post() operation. Well, it may seen as a kind of
a classical deadlock situation, although there's nothing classical in
here. ;)
That's what you're experiencing. Although it might appear similar, a
semaphore is *not* a mutex.
What I don't understand is why you don't use the "SyncObjs" unit where
almost all the stuff someone ever needs is nicely laid out in a
portable and object-oriented way.
> > if sem_wait(FSemaphore) <> 0 then
> > raise
> > EtiOPFInternalException.Create(cErrorTimedOutWaitingForSemaphore);
BTW, sem_wait() never returns anything else than zero, so checking is
sort of useless here. There's sem_trywait() for that, if you really
need to go so low-level.
To the question, what are semaphores. It's quite easily explained:
Atomic counters, you can only get past if they're non-zero and in that
case the counter is decremented.
Vinzent.
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