<!-- page02.html,v 1.7 2000/11/27 17:56:43 othman Exp --> <HTML> <HEAD> <META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1"> <META NAME="Author" CONTENT="James CE Johnson"> <TITLE>ACE Tutorial 019</TITLE> </HEAD> <BODY TEXT="#000000" BGCOLOR="#FFFFFF" LINK="#000FFF" VLINK="#FF0F0F"> <CENTER><B><FONT SIZE=+2>ACE Tutorial 019</FONT></B></CENTER> <CENTER><B><FONT SIZE=+2>Sharing your Memories</FONT></B></CENTER> <P> <HR WIDTH="100%"> We'll first take a look at the server side. As usual with co-operating applications, you need the server up and running first. In the case of shared memory applications, the server will create the shared memory segment. In this example, it will also remove the segment when done. It is important to realize though that the segment can be created external to your application and can persist beyond it's lifetime. In fact, you can use shared memory to create a layer of persistence between application instances (at least, until the machine comes down.) <HR> <PRE> <font color=red>// page02.html,v 1.7 2000/11/27 17:56:43 othman Exp</font> <font color=red>/* The client and server both need to know the shared memory key and size. To prevent headaches, I've put those into a header they both can share. */</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>/* You can use the ACE_Malloc template to create memory pools from various shared memory strategies. It's really cool. We're not going to use it. Instead, I want to get to the roots of it all and directly use ACE_Shared_Memory_SV. Like many ACE objects, this is a wrapper around OS services. With this constructor we create a shared memory area to use. The ACE_CREATE flag will cause it to be created if it doesn't already exist. The SHM_KEY value (from shmem.h) uniquely identifies the segment and allows other apps to attach to the same segment. Execute 'ipcs -m' before and after starting this app to see that the segment is created. (I can't for the life of me correlate the SHM_KEY value back to the key/id reported by ipcs though.) */</font> ACE_Shared_Memory_SV shm_server (SHM_KEY, SHMSZ, <font color=#008888>ACE_Shared_Memory_SV::ACE_CREATE</font>); <font color=red>/* The constructor created the segment for us but we still need to map the segment into our address space. (Note that you can pass a value to malloc() but it will be silently igored.) The void* (cast to char*) that is returned will point to the beginning of the shared segment. */</font> char *shm = (char *) shm_server.malloc (); <font color=red>/* Since we're asking to create the segment, we will fail if it already exists. We could fall back and simply attach to it like the client but I'd rather not assume it was a previous instance of this app that left the segment around. */</font> 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); <font color=red>/* This second pointer will be used to walk through the block of memory... */</font> char *s = shm; <font color=red>/* Out of curiosity, I added this output message. The tests I've done so far show me the same address for client and server. What does your OS tell you? */</font> ACE_DEBUG ((LM_INFO, "<font color=green>(%P|%t) Shared Memory is at 0x%x\n</font>", shm )); <font color=red>/* At this point, our application can use the pointer just like any other given to us by new or malloc. For our purposes, we'll copy in the alpabet as a null-terminated string. */</font> for (char c = 'a'; c <= 'z'; c++) *s++ = c; *s = '\0'; <font color=red>/* Using a simple not-too-busy loop, we'll wait for the client (or anyone else) to change the first byte in the shared area to a '*' character. This is where you would rather use semaphores or some similar "<font color=green>resource light</font>" approach. */</font> while (*shm != '*') <font color=#008888>ACE_OS::sleep</font> (1); <font color=red>/* Let's see what the client did to the segment... */</font> for (s = shm; *s != '\0'; s++) putchar (*s); putchar ('\n'); <font color=red>/* If you're done with the segment and ready for it to be removed from the system, use the remove() method. Once the program exits, do 'ipcs -m' again and you'll see that the segment is gone. If you just want to terminate your use of the segment but leave it around for other apps, use the close() method instead. The free() method may be tempting but it doesn't actually do anything. If your app is *really* done with the shared memory then use either close() or remove(). */</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> <P><HR WIDTH="100%"> <CENTER>[<A HREF="../online-tutorials.html">Tutorial Index</A>] [<A HREF="page03.html">Continue This Tutorial</A>]</CENTER>