Developing an Echo Socket Server in Visual Studio .NET

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8.5 Developing an Echo Socket Server
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In this section we will create an echo socket server that will accept a single type of command with accompanying data and return the same data as the response. We will build it from the bottom up by creating the command classes, then the factory, and nally the socket client and server. This is the rst place in this book where we will meet EKA2. The Symbian OS kernel developed in ER5 is designed for embedded devices it is not a hard real-time kernel. This means that it is not possible to
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guarantee a response time. As smartphones incorporate multimedia and other increasingly demanding functions there is more need for such a kernel. From Symbian OS v8.0 onwards Symbian supports such a kernel the name EKA2 stands for EPOC Kernel Architecture 2 as an alternative to the existing kernel. The smartphone manufacturers choose which kernel to use and, at the time of writing, have not released details of which kernel will be used on their smartphones. Apart from the internal changes, EKA2 has changes to the thread and process model, and the detailed APIs are slightly different in these areas. In this chapter the difference lies in the boiler-plate used to create an E32Main().
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8.5.1 Command Classes
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To deal with the incoming commands, we need classes for the echo command itself and for our header. We will also add a base class for commands. In this case the base command class is not strictly necessary for such a simple server, but it is useful when we have more commands, and we will include some code to handle unrecognized messages. When we de ne the header class, we de ne at least part of our protocol, because the header class is responsible for unpacking and packing the message header. In this case we will use some fairly common parts. We will have a four-byte command identi er, a four-byte transaction identi er (so we can tell which command a response is linked to) and a four-byte length for the data trailer. In fact, we could use two-byte values for all practical purposes. In this example, I will use the conn namespace explicitly. This is the class declaration for our header class, with the members derived from MHeader and then our speci c members.
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class CHeader : public CBase, public conn::MHeader { public: /** The length of the message header */ enum { KHeaderLength = 12 }; /** The maximum length of the message data. */ enum { KMaxMessageLength = 0xFFFF }; public: //From MHeader const TDes8& ReadPtr() const; TBool ReadL(TUint aReadBytes); void PrepareToWrite(); const TDes8& WritePtr() const; void Reset(); inline void SetBodyLength(TInt aLength); inline TInt BodyLength() const; inline TInt MaxBodyLength() const;
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private: TUint32 iMessageId; // Message ID TUint32 iTransactionId; // Transaction ID TUint32 iLength; // Length of data trailer TBuf8<KHeaderLength> iData; // Buffer for header data public: inline TUint32& Id(); inline const TUint32& Id() const; inline TUint32& TransactionId(); inline const TUint32& TransactionId() const; }; // INLINE IMPLEMENTATION void CHeader::SetBodyLength(TInt aLength) {iLength = STATIC_CAST(TUint32, aLength);} TInt CHeader::BodyLength() const {return iLength;} TInt CHeader::MaxBodyLength() const {return KMaxMessageLength;} TUint32& CHeader::Id() {return iMessageId;} const TUint32& CHeader::Id() const {return iMessageId;} TUint32& CHeader::TransactionId() {return iTransactionId;} const TUint32& CHeader::TransactionId() const {return iTransactionId;}
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Then the methods derived from MHeader are straightforward. For reading and writing we provide access to the iData member.
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const TDes8& CHeader::ReadPtr() const { return iData; } const TDes8& CHeader::WritePtr() const { return iData; } void CHeader::Reset() { iData.SetLength(0); iLength = 0; }
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When our header data has been received, we need to unpack it. We use methods from the Serialise module and set up our data members.
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TBool CHeader::ReadL(TUint aReadBytes) { iData.SetLength(aReadBytes + iData.Length()); if (iData.Length() == KHeaderLength) { conn::UnpackL(iLength, iData, conn::ELittle); conn::UnpackL(iMessageId, iData, conn::ELittle); conn::UnpackL(iTransactionId, iData, conn::ELittle); return ETrue; } return EFalse; }
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We will use the same header for sending a response, so we need a corresponding method to put the data members back into the header part of a message.
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void CHeader::PrepareToWrite() { iData.Zero(); conn::Pack(iLength, iData, conn::ELittle); conn::Pack(iMessageId, iData, conn::ELittle); conn::Pack(iTransactionId, iData, conn::ELittle); }
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Once we have our header class, we can de ne our base command. This has two purposes: 1. 2. It provides implementations of methods that will be common to all commands, such as sending an error response. It can be used by the factory class as a default command if it cannot recognize a received message. For this purpose, the ExecuteLD() method returns an error message to the client.
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class CBaseCommand : public conn::CCommand { // Construction methods required for factory public: static conn::CCommand* NewL(conn::CClientSocket* aSocket); CBaseCommand(conn::CClientSocket* aSocket); // From MCommand public: virtual void ExecuteLD(); protected: virtual void SendErrorMessage(TUint aErrorCode); // Default routine to send a response
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