Author: James CE Johnson

ACE Tutorial 021
Pooling your memories

Everything common the server & client is kept here. In particular, the Constants class where we keep the names & semaphore keys.

The Allocator class is just a thin wrapper around ACE_Malloc<> that moves some of the details out of the application logic.

    // page04.html,v 1.6 1999/04/03 21:58:06 jcej Exp

    #ifndef MPOOL_H
    #define MPOOL_H

    // Everything else we need is in this one header
    #include "ace/Malloc.h"

    #if !defined (ACE_LACKS_SYSV_SHMEM)

    /*
      With this we will abstract away some of the details of the memory
      pool.  Note that we don't treat this as a singleton because an
      application may need more than one pool.  Each would have a
      different name and be used for different purposes.
     */

    class Allocator
    {
    public:
      // The pool name will be used to create a unique semaphore to
      // keep this pool separate from others.
      Allocator (const char * _name = "MemoryPool");
      ~Allocator (void);

      typedef ACE_Malloc<ACE_MMAP_MEMORY_POOL, ACE_SV_Semaphore_Simple> pool_t;

      // Provide an accessor to the pool.  This will also allocate the
      // pool when first invoked.
      pool_t &pool (void);

    protected:

      // The name we gave to the pool
      char *name_;

      pool_t *pool_;
    };

    /*
      The client and server need to agree on a certain set of values.  By
      placing them in the Constants class we can eliminate a bit of confusion.
     */
    class Constants
    {
    public:
      // The semaphore keys are needed for the two semaphores that
      // synch access to the shared memory area.
      static const int SEM_KEY_1;
      static const int SEM_KEY_2;

      // How big the pool will be and what we'll put into it.  A real
      // app wouldn't need SHMDATA of course.
      static const int SHMSZ;
      static const char *SHMDATA;

      // The name assigned to the memory pool by the server is needed
      // by the client.  Without it, the pool cannot be found.
      // Likewise, the name the server will bind() to the region of the
      // pool must be available to the client.
      static const char *PoolName;
      static const char *RegionName;
    };

    #endif /* ACE_LACKS_SYSV_SHMEM */
    #endif /* MPOOL_H */

The really hard stuff is done by the ACE_Malloc<> template. This template takes two parameters.* The first is a memory pool class to use. ACE has several, I've choosen one that uses a memory-mapped file. The second parameter is a lock class of some sort. This is needed so that the ACE_Malloc<> can protect its internal data. Note that you still have to provide your own mutex around the data you put into the malloc'd area.

* Actually, some implementations may require a different number of parameters. That's why ACE uses those funky macros. ACE_MMAP_MEMORY_POOL for instance turns into ACE_MMAP_Memory_Pool on Linux but may do other things on your platform.

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