You must create atoms for your application-defined window messages if other applications are likely to recognize those messages. For example, your application might communicate with another application by using an agreed-upon message that is not defined by the system. Both applications must use the same string identifier for the shared message type-for example, OUR_LINK_MESSAGE. Each time the applications run, they add this string to the system atom table and receive an atom in return. Both applications register the same string in the system atom table, so they both receive the same atom. Then, this atom can be used to identify the message without conflicting with other system-wide message identifiers. A consequence of using atoms to identify a window message is that the message cannot be decoded as a C-language case statement, as usually done, because the value of the atom cannot be known until run time. Instead, you must add a default case that checks the value of the message against the value of the atoms you have registered. The following sample code fragment shows how to add an application-defined message string to the system atom table, then use the resultant atom to broadcast and receive the message:

#define IDM_BROADCAST 25

HATOMTBL hatomtblSystem;              /* System atom table handle     */
ATOM atomLinkMessage;                 /* Atom message                 */

/* Message text */
UCHAR szLinkMessage[] = "OUR_LINK_MESSAGE";

MRESULT EXPENTRY ClientWndProc(HWND hwnd,ULONG msg,
  MPARAM mp1,MPARAM mp2)
{

 /* At create time obtain atom for text message */
 switch (msg)
 {
   case WM_CREATE:
     hatomtblSystem  = WinQuerySystemAtomTable();
     atomLinkMessage = WinAddAtom(hatomtblSystem, szLinkMessage);
     return FALSE;

   /* Broadcast text message */
   case WM_COMMAND:
     if (SHORT1FROMMP(mp1) == IDM_BROADCAST)
     {
         WinBroadcastMsg(HWND_DESKTOP, atomLinkMessage,
           (MPARAM) NULL, (MPARAM) NULL,
            BMSG_DESCENDANTS | BMSG_POSTQUEUE);
     }
     return FALSE;

   default;

     /* Check for the atom representing "OUR_LINK_MESSAGE" */
     if (msg == atomLinkMessage) return DoOurMessage(...);
     break;
 }

 /* Execute default window procedure */
 return WinDefWindowProc(hwnd,msg,mp1,mp2);
}