Author: James CE Johnson

ACE Tutorial 019
Sharing your Memories

Before we move on to shmem.h, I want to show a different approach. In this new client/server pair, I use placement new to stuff an object (instead of a blob of bytes) into the shared memory segment.

There are a few caveats to putting objects into shared memory. The most important ones all deal with pointers:

- Be sure your pointers point into the shared memory and not local process memory.
- Only in very special cases will objects with virtual methods work (because of the VTable pointers).

That's not to say you shouldn't try... Just try carefully and test a lot!

server2.cpp

    // page04.html,v 1.4 1999/04/03 21:56:33 jcej Exp

    #include "shmem.h"

    #if defined (ACE_LACKS_SYSV_SHMEM)
    int
    main (int, char *[])
    {
      ACE_ERROR_RETURN ((LM_ERROR,
                         "System V Shared Memory not available on this platform\n"),
                        100);
    }
    #else // ACE_LACKS_SYSV_SHMEM
    int
    main (int, char *argv[])
    {
      // Be sure the segment is sized to hold our object.
      ACE_Shared_Memory_SV shm_server (SHM_KEY,
                                       sizeof (SharedData),
                                       ACE_Shared_Memory_SV::ACE_CREATE);
      char *shm = (char *) shm_server.malloc ();

      if (shm == 0)
        ACE_ERROR_RETURN ((LM_ERROR,
                           "%p\n\t(%P|%t) Cannot create shared memory segment.\n"
                           "\tUse 'ipcs' to see if it already exists\n",
                           argv[0]),
                          100);

      ACE_DEBUG ((LM_INFO,
                  "(%P|%t) Shared Memory is at 0x%x\n",
                  shm ));

      /*
        Use the placement new syntax to stuff the object into the
        correct location.  I think they generally reserve this for
        the advanced class...
        */
      SharedData *sd = new (shm) SharedData;

      // Use the set() method to put some data into the object
      sd->set ();

      // Set the 'available' flag to zero so that we can wait on it
      sd->available (0);

      /*
        Another cheesy busy loop while we wait for the object to
        become available.  The cool way would be to hide a semaphore
        or two behind this method call & eliminate the sleep.
        */
      while (sd->available () == 0)
        ACE_OS::sleep (1);

      // Show the user what's in the segment
      sd->show ();

      // All done.
      if (shm_server.remove () < 0)
        ACE_ERROR ((LM_ERROR,
                    "%p\n",
                    "remove"));
      return 0;
    }

    #endif /* ACE_LACKS_SYSV_SHMEM */

client2.cpp

    // page04.html,v 1.4 1999/04/03 21:56:33 jcej Exp

    #include "shmem.h"

    #if defined(ACE_LACKS_SYSV_SHMEM)
    int
    main (int, char *[])
    {
      ACE_ERROR_RETURN ((LM_ERROR,
                         "System V Shared Memory not available on this platform\n"),
                        100);
    }
    #else // ACE_LACKS_SYSV_SHMEM
    int
    main (int, char *[])
    {
      ACE_Shared_Memory_SV shm_client (SHM_KEY,
                                       sizeof (SharedData));

      char *shm = (char *) shm_client.malloc ();

      ACE_DEBUG ((LM_INFO,
                  "(%P|%t) Shared Memory is at 0x%x\n",
                  shm));

      /*
        More placement new.  The constructor parameter prevents
        clobbering what the server may have written with it's show()
        method.
        */
      SharedData *sd = new (shm) SharedData (0);

      // Show it
      sd->show ();

      // Change it
      sd->set ();

      // Advertise it
      sd->available (1);

      shm_client.close ();

      return 0;
    }

    #endif /* ACE_LACKS_SYSV_SHMEM */

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