Microsoft KB Archive/96781

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HOWTO: Use RPC Callback Functions

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Q96781

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The information in this article applies to:


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

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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(&quot;Call from server, printed on client: %s&quot;, pszStr ); }

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

  midl -ms_ext -cpp_cmd $(cc) -cpp_opt &quot;-E&quot; sample.idl Additional query words:

Keywords : kbnetwork kbAPI kbOSWinNT310 kbOSWinNT350 kbOSWinNT351 kbOSWinNT400 kbRPC kbSDKPlatform kbOSWin310 kbOSWin95 kbGrpDSNet

Issue type : kbhowto

Technology : kbWin32SDKSearch kbAudDeveloper kbSDKSearch kbWin32sSearch kbWin32SDK400 kbWin32SDK310 kbWin32SDK350 kbWin32SDK351