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We also have an issue with using a xed port number. The PC client needs to know which port number to connect on in order to access a server. A simple approach would be to use a xed port number for each server, but this has some drawbacks. One is that it is frowned upon in the Internet community to use xed port numbers unless they are registered (actually, any PC Connectivity services running on a Symbian OS phone may be specialized, and clashes with other services might not matter in practice). The other drawback is that it introduces a risk for third-party developers. Any third-party PC Connectivity developer has to create their server without knowing which other servers are under development. Therefore, there is a risk of two developers independently choosing the same port numbers (given the range of port numbers, the risk is small, but de nitely non-zero). The sockets library supports opening a random port number for listening, but the server then needs some way to communicate the port number to the PC client. In Symbian OS v8.0 the problems of starting a server and communicating a port number are solved by using a Service Broker. This is a server that listens on a xed socket (with the port number set by the IANA) and supports commands that will start a server by name and then return the port number on which it is listening. In this way, the Service Broker needs to be active whenever the PC is connected and needs to have a xed port number, but the same is not true of any other Connectivity Socket Server. Later in this chapter we will see how a socket server can register its name with the Service Broker and provide its port number, and how the PC client can use the Service Broker to make a connection to a Connectivity Socket Server. It is possible to create a Connectivity Socket Server just by using the standard sockets API and the Service Broker, but some additional classes are provided that handle some of the tasks common to many Connectivity Socket Servers. Bearing in mind that these classes have been debugged and are already present on the Symbian OS phone, using them is likely to reduce your debugging time and reduce the ROM footprint of your service. One side effect of using standard sockets is that a socket server could be accessed over a data connection other than from the PC (known as OTA or Over The Air). This has interesting possibilities (as well as raising some possible security issues), but, at the time of writing, there is no support for starting services in this way.
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8.2 An Introduction to the Server Socket Classes
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The major generic tasks that any Connectivity Socket Server will have are: open a socket for listening and inform the Service Broker of the port number
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AN INTRODUCTION TO THE SERVER SOCKET CLASSES
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wait on a socket for a connection from the PC and create a new connection on request when a message is received on a connection, decode it to decide what the command is, act on it and compose a response to send back to the PC. In order to combine the generic behavior with specialized behavior, a set of base classes are used that encapsulate the standard functionality, along with a factory pattern and specialized classes to encapsulate nonstandard functionality.
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8.2.1 The Server and Client Classes
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The CServerSocket class is used to set up the listening socket and interact with the Service Broker. When a client connects to the listening socket, a CClientSocket-derived object is created and assigned to the connection. CClientSocket uses the CMessageclass to send and receive data over the connection. The client socket stays connected until either the remote client disconnects or a write or read error occurs. When designing a socket server, starting up can be delegated to the Service Broker, but the developer needs to consider closing the server. The Service Broker and the server socket classes make no rules about when a server should terminate. The server should not keep running inde nitely, otherwise it will absorb resources when the mobile phone is not connected to a PC, which would be wasteful. The most common approach to closedown is to exit when the connection between the mobile phone and the PC is broken. This event can be detected using the MServerSocketObserverclass. Other alternatives include de ning a special command that allows the PC client to tell the server to close down, and terminating when all connections are dropped by the PC. Which one you choose depends on the protocol that you design and how you expect to use the server.
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