Using RPC Callback Functions

• Microsoft Win32 Software Development Kit (SDK), versions 3.1, 3.5, 3.51, and 4.0

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.

                                <--- Server calls callback procedure.

                                <--- 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