HOWTO: Use RPC Callback Functions

ID: Q96781

The information in this article applies to:

SUMMARY

The standard remote procedure call (RPC) model has a server containing one or more exported function calls, and a client, which calls the server's exported functions. However, Microsoft's implementation of RPC defines callbacks as a special interface definition language (IDL) attribute allowing a server to call a client function.

Callbacks can be used only in the context of a server call. Thus, a server may call a client's callback function only when the server is performing a client's remote procedure call (before it returns from processing). For example: 

                CLIENT                SERVER
                ------                ------

       Client makes RPC call. --->
                                <--- Server calls callback procedure.
Client returns from callback. ---> 
                                <--- Server calls callback procedure.
Client returns from callback. ---> 
                                <--- Server returns from original RPC call.

MORE INFORMATION

Callbacks are declared in the RPC .IDL file and defined in the source of the client. The following demonstrates how callbacks are declared and defined: 

   [ SAMPLE.IDL ]
   [
      uuid(9FEE4F51-0396-101A-AE4F-08002B2D0065),
      version(1.0),
      pointer_default( unique )
   ]

   {
    void RPCProc( [in, string] unsigned char *pszStr );
    [callback] void CallbackProc([in,string] unsigned char *pszStr);
   }

   [ SAMPLEC.C (Client)]
   /*
      Callback RPC call (initiated from server, executed on client).
   */ 
   void CallbackProc( unsigned char *pszString )
   {
     printf("Call from server, printed on client: %s", pszStr );
   }

   [ SAMPLES.C (Server)]
   /*
      "Standard" RPC call (initiated from client, executed on server).
      Makes a call to client callback procedure, CallbackProc().
   */ 
   void RPCProc( unsigned char *pszStr )
   {
     printf("About to call Callback() client function.."
     CallbackProc( pszStr );
     printf("Called callback function.");
   }
In the makefile for the sample, the "-ms_ext" switch must be used for the MIDL compile. For example: 

   midl -ms_ext -cpp_cmd $(cc) -cpp_opt "-E" sample.idl
Additional query words: 
Keywords          : kbnetwork kbAPI kbNTOS310 kbNTOS350 kbNTOS351 kbNTOS400 kbRPC kbSDKPlatform kbWinOS310 kbWinOS95 kbGrpNet 
Issue type        : kbhowto

Last Reviewed: July 31, 1998