THE CLIENT SERVER FRAMEWORK IN PRACTICE in Java

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} EXPORT_C TInt RHerculesSession::CaptureCeryneianHind(TInt& aCaptureCount) { const TAny* p[KMaxMessageArguments]; TPckg<TInt> countBuf(aCaptureCount); p[0]=(TAny*)&countBuf; return (SendReceive(ECaptureCeryneianHind,p)); } // The implementation of RHerculesSession::SlayErymanthianBoar() // is omitted here because it is discussed later in the section // Asynchronous request EXPORT_C void RHerculesSession::CleanAugeanStables(TRequestStatus& aStat) { SendReceive(ECleanAugeanStables, 0, aStat); } // Cancels the CleanAugeanStables() asynchronous request EXPORT_C void RHerculesSession::CancelCleanAugeanStables() { SendReceive(ECancelCleanAugeanStables, 0); } // Asynchronous request EXPORT_C void RHerculesSession::SlayStymphalianBirds(TInt aCount, TDes8& aData, TRequestStatus& aStatus) { const TAny* p[KMaxMessageArguments]; p[0] = (TAny*)aCount; p[1] = &aData; SendReceive(ESlayStymphalianBirds, p, aStatus); } // Cancels the SlayStymphalianBirds() asynchronous request EXPORT_C void RHerculesSession::CancelSlayStymphalianBirds () {// Every asynchronous request should have a cancellation method SendReceive(ECancelSlayStymphalianBirds, 0); }
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You ll understand now why I ve called it boilerplate code there s a lot of repetition in the methods In each case, you ll notice that, if any parameter data is passed to the server with the request, the method instantiates an array of 32-bit values of size KMaxMessageArguments (= 4, as de ned in e32stdh) If there are no accompanying request parameters (as in the case of the request with opcode ECleanAugeanStables or the cancellation methods), the array is not needed The array is used to hold either the request data itself (if it can be stored in the 32-bit elements) or a pointer to a descriptor that stores the client-side data It is the contents of this array that are stored in an RMessage on the server side, but there is no direct equivalent of RMessage for client-side code The array is passed to RSessionBase::SendReceive() and must always be of size KMaxMessageArguments even when fewer parameters are passed to the server
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It is important that the client-side data passed to an asynchronous request must not be stack-based This is because the server may not process the incoming request data until some arbitrary time after the client issued the request The parameters must remain in existence until that time so they cannot exist on the stack in case the client-side function which submitted the request returns, destroying the stack frame The client API differentiates between non-modi able descriptors passed to functions which pass constant data to the server and modi able descriptors used to retrieve data from it However, the client-side interface code simply passes a pointer to the descriptor as a message parameter Serverside, this will be used in a call to RThread::ReadL() to retrieve data from the client or in a call to RThread::WriteL() to write data to the client thread The RThread methods inspect the descriptor to which it points, to check that it appears to be a descriptor, and leave with KErrBadDescriptor if it does not SlayNemeanLion() and CaptureCeryneianHind() show how integer and descriptor data are passed to a server, but what about custom data What if it has variable length or does not just contain at data, but owns pointers to other objects, as is common for a C class object I ll show how to pass a CBase-derived object across the client server boundary in SlayErymanthianBoar() shortly, but rst, let s consider how to pass an object of a T class or a struct RHerculesSession::SlayHydra() passes an object of type THydraData which is a simple struct that contains only built-in types, de ned as follows:
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struct THydraData { TVersion iHydraVersion; TInt iHeadCount; };
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TVersion is a Symbian OS class de ned as follows in e32stdh:
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class TVersion { public: // Constructors omitted for clarity public: TInt8 iMajor; TInt8 iMinor; TInt16 iBuild; };
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A THydraData object is thus 64 bits in size, which is too large to be passed to the server as one of the 32-bit elements of the request data array It isn t enough to pass a pointer to the existing THydraData
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