Author: James CE Johnson

ACE Tutorial 021
Pooling your memories

The key components for creating the memory pool are:

- Create and name the pool
- Allocate a chunk (region) of memory from the pool
- Name the allocated region

The rest of it is just critical sections and data manipulation.

    // page02.html,v 1.7 1999/04/03 21:58:06 jcej Exp

    /*
      I've hidden the details in an Allocator class declared in mpool.h
      We'll come to that a little later.
    */
    #include "mpool.h"

    #if defined(ACE_LACKS_SYSV_SHMEM)
    int
    main (int, char *[])
    {
      ACE_ERROR_RETURN ((LM_ERROR,
                         "System V Semaphores not available on this platform.\n"),100);
    }
    #else // ACE_LACKS_SYSV_SHMEM
    int
    main (int, char *[])
    {
      /*
        Construction of an Allocator will create the memory pool and
        provide it with a name.  The Constants class is also
        declared in mpool.h to keep server and client on the same
        page.  The name is used to generate a unique semaphore which
        prevents simultaneous access to the pools housekeeping
        information.  (Note that you still have to provide your own
        synch mechanisms for the data *you* put in the poo.)
        */
      Allocator allocator (Constants::PoolName);

      /*
        The Allocator class provides the pool() member so that you
        have access to the actual memory pool.  A more robust
        implementation would behave more as a bridge class but this
        is good enough for what we're doing here.
        Once you have a reference to the pool, the malloc() method
        can be used to get some bytes.  If successful, shm will
        point to the data.  Otherwise, it will be zero.
        */
      char *shm = (char *) allocator.pool ().malloc (27);

      ACE_ASSERT (shm != 0);

      /// FYI
      ACE_DEBUG ((LM_INFO,
                  "Shared memory is at 0x%x\n",
                  shm));

      /*
        Something that we can do with a memory pool is map a name to
        a region provided by malloc.  By doing this, we can
        communicate that name to the client as a rendezvous
        location.  Again, a member of Constants is used to keep the
        client and server coordinated.
        */
      if (allocator.pool ().bind(Constants::RegionName,shm) == -1)
        ACE_ERROR_RETURN ((LM_ERROR,
                           "Cannot bind the name '%s' to the pointer 0x%x\n",
                           Constants::RegionName,
                           shm),
                          100);

      /*
        One of the best ways to synch between different processes is
        through the use of semaphores.  ACE_SV_Semaphore_Complex
        hides the gory details and lets us use them rather easily.

        Here, we'll create two semaphores:  mutex and synch.  mutex
        will be used to provide mutually exclusive access to the
        shared region for writting/reading.  synch will be used to
        prevent the server from removing the memory pool before the
        client is done with it.

        Both semaphores are created in an initially locked state.
        */

      ACE_SV_Semaphore_Complex mutex;
      ACE_ASSERT (mutex.open (Constants::SEM_KEY_1,
                              ACE_SV_Semaphore_Complex::ACE_CREATE,
                              0) != -1);

      ACE_SV_Semaphore_Complex synch;
      ACE_ASSERT (synch.open (Constants::SEM_KEY_2,
                              ACE_SV_Semaphore_Complex::ACE_CREATE,
                              0) != -1);

      /*
        We know the mutex is locked because we created it that way.
        Take a moment to write some data into the shared region.
        */
      for (int i = 0; i < Constants::SHMSZ; i++)
        shm[i] = Constants::SHMDATA[i];

      /*
        The client will be blocking on an acquire() of mutex.  By
        releasing it here, the client can go look at the shared data.
        */
      if (mutex.release () == -1)
        ACE_ERROR ((LM_ERROR,
                    "(%P) %p",
                    "server mutex.release"));
      /*
        Even though we created the synch semaphore in a locked
        state, if we attempt to acquire() it, we will block.  Our
        design requires that the client release() synch when it is
        OK for us to remove the shared memory.
        */
      else if (synch.acquire () == -1)
        ACE_ERROR ((LM_ERROR,
                    "(%P) %p",
                    "server synch.acquire"));
      /*
        This will remove all of the memory pool's resources.  In the
        case where a memory mapped file is used, the physical file
        will also be removed.
        */
      if (allocator.pool ().remove () == -1)
        ACE_ERROR ((LM_ERROR,
                    "(%P) %p\n",
                    "server allocator.remove"));
      /*
        We now have to cleanup the semaphores we created.  Use the
        ipcs command to see that they did, indeed, go away after the
        server exits.
        */

      if (mutex.remove () == -1)
        ACE_ERROR ((LM_ERROR,
                    "(%P) %p\n",
                    "server mutex.remove"));
      else if (synch.remove () == -1)
        ACE_ERROR ((LM_ERROR,
                    "(%P) %p\n",
                    "server synch.remove"));
      return 0;
    }

    /*
      This tutorial was created by shamelessly modifying one of the ACE
      examples.  Someone there had already created the necessary explicit
      template instantiations & I don't want them to go to waste...
     */
    #if defined (ACE_HAS_EXPLICIT_TEMPLATE_INSTANTIATION)
    template class ACE_Malloc<ACE_MMAP_MEMORY_POOL, ACE_SV_Semaphore_Simple>;
    template class ACE_Guard<ACE_SV_Semaphore_Simple>;
    template class ACE_Write_Guard<ACE_SV_Semaphore_Simple>;
    template class ACE_Read_Guard<ACE_SV_Semaphore_Simple>;
    #elif defined (ACE_HAS_TEMPLATE_INSTANTIATION_PRAGMA)
    #pragma instantiate ACE_Malloc<ACE_MMAP_MEMORY_POOL, ACE_SV_Semaphore_Simple>
    #pragma instantiate ACE_Guard<ACE_SV_Semaphore_Simple>
    #pragma instantiate ACE_Write_Guard<ACE_SV_Semaphore_Simple>
    #pragma instantiate ACE_Read_Guard<ACE_SV_Semaphore_Simple>
    #endif /* ACE_HAS_EXPLICIT_TEMPLATE_INSTANTIATION */

    #endif /* ACE_LACKS_SYSV_SHMEM */

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