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Date: Tue, 2 Jan 2007 08:35:10 +0100
Lines: 90
Subject: [PHP-DEV] Semaphores - feature request
From: wojtekm86@konto.pl ("Wojciech Malota")

It's impossible to correct remove semaphores with sem_remove function
when I use them to provide execution of concurrent processes.
When the last process releases the semaphore I should be able to remove
it. But I don't know if another process haven't acquired the semaphore.
For safety reasons I don't remove released semaphores from system.
Now, If I get 128 semaphores and I execute sem_get one more time I will
get warning.
So, I can't remove semaphores (because I don't know if another process
is using it) and I can't get next semaphore.
I will try to tell about my sugestion.

I assume that I have a function my_sem_remove($sem_id):

function my_sem_remove($id) {
  if(Semaphore $id isn't acquired and its FIFO is empty)
    sem_remove($id);
}
None of instructions in this function can be interrupted.
So, now the problem is simple to resolve:

$id = sem_get(ID);
sem_acquire($id);

CRITICAL REGION

sem_release($id);
my_sem_remove($id);

But now, two instructions: sem_release and my_sem_remove are not needed.
It's simple to replace them by one function my_sem_release:

function my_sem_release($id) {
  sem_release($id);
  my_sem_remove($id);
}

For reasons which about I will write later those instructions can't be
interrupted.

So, my example look like this:

$id = sem_get(ID);
sem_acquire($id);

CRITICAL REGION

my_sem_release($id);

But let's imagine two concurrent processes executing code above:

PROC1: $id = sem_get(ID);
<---  interrupt
PROC2: $id = sem_get(ID);
       sem_acquire($id);
       CRITICAL REGION
       my_sem_release($id);
<--- interrupt
PROC1: sem_acquire($id); <--- Uuups!! Error: semaphore doesn't exists,
PROC2 has removed it.

My solution:
Instructions sem_get and sem_acquire can't be interrupted.
So, I assume that there is a function my_sem_acquire:

function my_sem_acquire($key, $max_acquire = 1, $perm = 0666,
$auto_release = true) {

  $id = sem_get($key, $max_acquire, $perm, $auto_release);
  sem_acquire();
  return $id;
}

None of instructions in function above can be interrupted.

And finally correct example:

$id = my_sem_acquire(SEMKEY);

CRITICAL REGION

my_sem_release($id);

It's the best solution, without danger of operating on not existing
semaphores. Also, unused semaphores are automaticcally removed. The
problem is, that functions my_sem_acquire and my_sem_release can't be
witten in PHP so it must be done in C inside source of PHP.

Wojciech Malota