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<TITLE>ACE Tutorial 019</TITLE>
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<CENTER><B><FONT SIZE=+2>ACE Tutorial 019</FONT></B></CENTER>
<CENTER><B><FONT SIZE=+2>Sharing your Memories</FONT></B></CENTER>
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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.
<P>
There are a few caveats to putting objects into shared memory. The
most important ones all deal with pointers:
<ul>
<li>Be sure your pointers point into the shared memory and not
local process memory.
<li>Only in very special cases will objects with virtual methods
work (because of the VTable pointers).
</ul>
<P>
That's not to say you shouldn't try... Just try carefully and test a lot!
<HR>
<HR width=50%><P><center>server2.cpp</center><HR width=50%>
<PRE>
<font color=red>// page04.html,v 1.7 2000/11/27 17:56:44 othman Exp</font>
<font color=blue>#include</font> "<font color=green>shmem.h</font>"
<font color=blue>#include</font> "<A HREF="../../../ace/Log_Msg.h">ace/Log_Msg.h</A>"
<font color=blue>#if defined</font> (<font color=purple>ACE_LACKS_SYSV_SHMEM</font>)
int
main (int, char *[])
{
ACE_ERROR_RETURN ((LM_ERROR,
"<font color=green>System V Shared Memory not available on this platform\n</font>"),
100);
}
#else <font color=red>// ACE_LACKS_SYSV_SHMEM</font>
int
main (int, char *argv[])
{
<font color=red>// Be sure the segment is sized to hold our object.</font>
ACE_Shared_Memory_SV shm_server (SHM_KEY,
sizeof (SharedData),
<font color=#008888>ACE_Shared_Memory_SV::ACE_CREATE</font>);
char *shm = (char *) shm_server.malloc ();
if (shm == 0)
ACE_ERROR_RETURN ((LM_ERROR,
"<font color=green>%p\n\t(%P|%t) Cannot create shared memory segment.\n</font>"
"<font color=green>\tUse 'ipcs' to see if it already exists\n</font>",
argv[0]),
100);
ACE_DEBUG ((LM_INFO,
"<font color=green>(%P|%t) Shared Memory is at 0x%x\n</font>",
shm ));
<font color=red>/*
Use the placement new syntax to stuff the object into the
correct location. I think they generally reserve this for
the advanced class...
*/</font>
SharedData *sd = new (shm) SharedData;
<font color=red>// Use the set() method to put some data into the object</font>
sd->set ();
<font color=red>// Set the 'available' flag to zero so that we can wait on it</font>
sd->available (0);
<font color=red>/*
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.
*/</font>
while (sd->available () == 0)
<font color=#008888>ACE_OS::sleep</font> (1);
<font color=red>// Show the user what's in the segment</font>
sd->show ();
<font color=red>// All done.</font>
if (shm_server.remove () < 0)
ACE_ERROR ((LM_ERROR,
"<font color=green>%p\n</font>",
"<font color=green>remove</font>"));
return 0;
}
<font color=blue>#endif</font> <font color=red>/* ACE_LACKS_SYSV_SHMEM */</font>
</PRE>
<HR width=50%><P><center>client2.cpp</center><HR width=50%>
<PRE>
<font color=red>// page04.html,v 1.7 2000/11/27 17:56:44 othman Exp</font>
<font color=blue>#include</font> "<font color=green>shmem.h</font>"
<font color=blue>#include</font> "<A HREF="../../../ace/Log_Msg.h">ace/Log_Msg.h</A>"
<font color=blue>#if defined</font>(<font color=purple>ACE_LACKS_SYSV_SHMEM</font>)
int
main (int, char *[])
{
ACE_ERROR_RETURN ((LM_ERROR,
"<font color=green>System V Shared Memory not available on this platform\n</font>"),
100);
}
#else <font color=red>// ACE_LACKS_SYSV_SHMEM</font>
int
main (int, char *[])
{
ACE_Shared_Memory_SV shm_client (SHM_KEY,
sizeof (SharedData));
char *shm = (char *) shm_client.malloc ();
ACE_DEBUG ((LM_INFO,
"<font color=green>(%P|%t) Shared Memory is at 0x%x\n</font>",
shm));
<font color=red>/*
More placement new. The constructor parameter prevents
clobbering what the server may have written with it's show()
method.
*/</font>
SharedData *sd = new (shm) SharedData (0);
<font color=red>// Show it</font>
sd->show ();
<font color=red>// Change it</font>
sd->set ();
<font color=red>// Advertise it</font>
sd->available (1);
shm_client.close ();
return 0;
}
<font color=blue>#endif</font> <font color=red>/* ACE_LACKS_SYSV_SHMEM */</font>
</PRE>
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