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<reference id="GUID-C1612638-4C5A-5C37-AF1A-16F9EBEF09FD" xml:lang="en"><title>transient:
transient server example</title><shortdesc>This example demonstrates the use of transient servers that are
started on demand when a client needs to use the server, and exit after the
last client has disconnected, sometimes after a short delay. It demonstrates
how to handle startup and shutdown situations correctly. </shortdesc><prolog><metadata><keywords/></metadata></prolog><refbody>
<section id="GUID-BF4C4CB4-D495-49D6-8035-F8BA724851B7"><title>Class summary</title><ul>
<li><p><xref href="GUID-6D8A458C-9A39-3000-A3BC-060A2A3663E6.dita"><apiname>RSessionBase</apiname></xref> - Client-side handle to a session with
a server.</p></li>
<li><p><xref href="GUID-5733DF34-5B54-3044-BBED-3BFECB7ACE7A.dita"><apiname>CTimer</apiname></xref> - Base class for a timer active object.</p></li>
<li><p><xref href="GUID-8E316AC4-4676-301A-9A23-659E83AA1D1C.dita"><apiname>CServer2</apiname></xref> - Abstract base class for servers (version
2).</p></li>
<li><p><xref href="GUID-D5A30C75-E22C-34E8-913B-7D2CA6AD5C51.dita"><apiname>CSession2</apiname></xref> - Represents a session (version 2) for
a client thread on the server-side.</p></li>
</ul></section>
<section id="GUID-E688F8C0-0A4A-4D25-B7B4-779CB6A08157"><title>Download</title> <p>Click on the following link to download
the example: <xref href="guid-6013a680-57f9-415b-8851-c4fa63356636/zips/guid-cbf7e18e-3788-4ec1-a225-268ce1a22c51.zip" scope="external">transient.zip</xref></p><p>click: <xref href="guid-6013a680-57f9-415b-8851-c4fa63356636/guid-cbf7e18e-3788-4ec1-a225-268ce1a22c51.html" scope="peer">browse</xref> to view the example code.</p> </section>
<section id="GUID-87B18963-ACE8-464B-9863-A37ADA57379C"><title>Background</title> <p>The client-server framework is an important
part of Symbian platform. There is a level of complexity in implementing a
server for an application, but this example provides client interface code
and server code that can be re-used in your own implementations. </p> <p>Symbian
platform servers are of three basic types: </p> <ul>
<li id="GUID-6D0F8C69-CFEA-5BB1-A0CB-C4D178DFAE25"><p> <b>Transient servers:</b> these
are started on demand when a client needs the server, and exit after the last
client has disconnected - sometimes after a short delay, for example, the
socket server. </p> </li>
<li id="GUID-5B824584-C8E6-57F7-A83C-564DA256F622"><p> <b>System servers:</b> these
are started as part of the system boot process, for example, the kernel, the
file server, the window server etc. </p> </li>
<li id="GUID-9BE0FD09-B112-5AF6-A5D1-FBDD6A4EC00B"><p> <b>Local servers:</b> these
are "local" to a process to provide multi-threaded access to thread-specific
resources, for example, the posix server (in the C Standard Library). </p> </li>
</ul> </section>
<section id="GUID-EEA0A38B-F8A6-4308-A50D-B3E74B12420A"><title>Description</title> <p>The transient server supplied here
runs in a thread in its own process. This gives the server independence from
its clients. This is important because if the client were to create the server
as a separate thread within its own process, then the server would exit, i.e.
terminate, if its owning process terminated. This would be true even if clients
in other processes were still connected to it. </p> <p>Equally as important: </p> <ul>
<li id="GUID-432FCF7B-2393-5E59-BDE5-0A5B4BAFD155"><p>a client does not explicitly
need to start the server prior to connecting; this is done by the client interface. </p> </li>
<li id="GUID-D89C8F48-2CA2-5769-8DB9-42FD8FFA5879"><p>the server terminates
when not in use shortly after the last client disconnects. </p> </li>
<li id="GUID-968AB243-908B-5709-BB82-4213860CBC8A"><p>the code handles concurrency
issues, the most common of which is handling the case where the server is
closing down or exiting at the same time that another client is trying to
connect. </p> </li>
</ul> <p>The example consists of four main parts: </p> <ul>
<li id="GUID-BC6A15E0-07CB-562C-99E8-0AAF39B0ED26"><p>Client interface: this
is a DLL (<filepath>t-client.dll</filepath>) that is used by clients. </p> <p>The
client interface effectively forms the gateway to the server. All requests
to the server, including requests to connect to it are routed through this. </p> <p>The
client interface defines just one class: <codeph>RMySession</codeph>, derived
from <xref href="GUID-6D8A458C-9A39-3000-A3BC-060A2A3663E6.dita"><apiname>RSessionBase</apiname></xref>. An <codeph>RSessionBase</codeph> (derived)
object represents the client side session and forms a channel of communication
between the client and the server. Requests to the server, and information
received from the server are channelled through this object. Note that the
client side session object is mirrored by a server side session object; an
instance of a class derived from <xref href="GUID-D5A30C75-E22C-34E8-913B-7D2CA6AD5C51.dita"><apiname>CSession2</apiname></xref>, which in this
server is called <codeph>CMySession</codeph>. </p> <p>The client calls <codeph>Connect()</codeph> to
set up a connection to the server. This function attempts to start the server
if necessary and then issues a request to create a session with the server.
If the server fails to start, or a session cannot be created then this is
reported back.Simultaneous launching of two server processes is also detected,
with the second attempt failing with <xref href="GUID-D1D25122-F2B8-3C78-8599-84905BFD47B8.dita"><apiname>KErrAlreadyExists</apiname></xref>.
On successful completion the connection is established, and a session is created
with the server. </p> <p>The client uses the <codeph>Send()</codeph> method
to send messages to the server and the <codeph>Receive()</codeph> method to
request information from the server. <codeph>Send()</codeph> is implemented
by calling down to the synchronous variant of <xref href="GUID-6D8A458C-9A39-3000-A3BC-060A2A3663E6.dita#GUID-6D8A458C-9A39-3000-A3BC-060A2A3663E6/GUID-479FE087-DD9F-31D7-98F7-9F96535F0584"><apiname>RSessionBase::SendReceive()</apiname></xref> in
the base class. Synchronous means that it does not return until the server
has dealt with the request. </p> <p> <codeph>Send()</codeph> passes an operation
code (or function number) identifying the request and a descriptor containing
information to the server. The descriptor is wrapped into a <xref href="GUID-4AD02F14-1142-372F-9D11-224595932034.dita"><apiname>TIpcArgs</apiname></xref> object.
In practice, your client interface would probably be richer, having variants
of the <codeph>Send()</codeph> function that take more than one parameter.
You might also choose to have asynchronous variants of <codeph>Send()</codeph>;
your client code (not the client interface) would then need to use active
objects to deal with this. </p> <p> <codeph>Receive()</codeph> is implemented
by calling down to the asynchronous variant of <xref href="GUID-6D8A458C-9A39-3000-A3BC-060A2A3663E6.dita#GUID-6D8A458C-9A39-3000-A3BC-060A2A3663E6/GUID-479FE087-DD9F-31D7-98F7-9F96535F0584"><apiname>RSessionBase::SendReceive()</apiname></xref> in
the base class. Asynchronous means that the call returns immediately, but
the request itself may not complete until some time later. </p> <p> <codeph>Receive()</codeph> passes
an operation code (or function number) identifying the request, a descriptor
into which the server will place the information, and a <xref href="GUID-E0B34F3E-D4C4-3232-B8B1-7DB35B454646.dita"><apiname>TRequestStatus</apiname></xref> object,
which the server will use to report how the request completes. The descriptor
is wrapped into a <xref href="GUID-4AD02F14-1142-372F-9D11-224595932034.dita"><apiname>TIpcArgs</apiname></xref> object. Again, you might want
a richer client interface; perhaps different function names to represent different
request types. </p> <p> <codeph>CancelReceive()</codeph> allows the client
to cancel a pending <codeph>Receive()</codeph> request. It is usual to provide
some mechanism to cancel asynchronous requests. This is implemented by calling
down into the asynchronous variant of <xref href="GUID-6D8A458C-9A39-3000-A3BC-060A2A3663E6.dita#GUID-6D8A458C-9A39-3000-A3BC-060A2A3663E6/GUID-479FE087-DD9F-31D7-98F7-9F96535F0584"><apiname>RSessionBase::SendReceive()</apiname></xref> in
the base class. Note: the <codeph>Receive()</codeph> method simply passes
an operation code (or function number) and no data. </p> <p>The operation
codes (or function numbers) are defined by an enum in the header file <filepath>clientserver.h</filepath>.
This file is included in both the client interface code and server code as
both need access to these common symbols. </p> </li>
<li id="GUID-A76A5A09-CB86-5D58-8771-3DE4131E51B9"><p>Server: this is an EXE
(<filepath>t-server.exe</filepath>) that runs in its own process, and E32Main()
is the entry point. After setting up heap checking (in debug builds only)
the implementation creates a cleanup stack and then executes the main body
of the code inside a trap harness. </p> <p>The server code implements several
classes: </p> <p> <codeph>CMyServer</codeph>: this class derives from the
class <xref href="GUID-8E316AC4-4676-301A-9A23-659E83AA1D1C.dita"><apiname>CServer2</apiname></xref>. The main role of this class is to handle
messages sent by client threads; this includes requests from clients for connection
to the server. The connection is established by starting a session with the
server. The session is represented by a session object, in this case the <xref href="GUID-8B50E9C7-CA33-3E11-A881-16E230A082CB.dita"><apiname>CMySession</apiname></xref> object. </p> <p>Once
the <codeph>CMyServer</codeph> object has been created, the active scheduler
can be started by the call to <xref href="GUID-B4C76104-EA1B-3FC3-A31E-86A976598171.dita#GUID-B4C76104-EA1B-3FC3-A31E-86A976598171/GUID-3281C85C-1A45-3049-B711-64CA8BFBC452"><apiname>CActiveScheduler::Start()</apiname></xref>.
This call will not return until the active scheduler is stopped by the <codeph>CShutdown</codeph> timer,
which is primed after the last client has disconnected. In simple terms, the
active scheduler is a wait loop. It sits inactive until it receives a message
from (potential) clients (or until the timer completes and stops the active
scheduler). Messages that represent requests for connection result in the
creation of a session. Other messages are passed to the relevant <codeph>CMySession</codeph> active
object. </p> <p> <codeph> CMySession</codeph>: one of these objects is created
for each session, i.e. as a result of a request by a client for connection
to the server. Once sessions have been created, requests received by <codeph>CMyServer</codeph> are
then forwarded to the corresponding session, and specifically to the virtual
function <codeph>ServiceL()</codeph>, which handles <codeph>Send()</codeph>, <codeph>Receive()</codeph> and <codeph>CancelReceive()</codeph> requests. </p> <p> <codeph>CShutdown</codeph>: this class represents a timer that is started
before the first client has connected, and after the last client has disconnected.
The timer is stopped once the server has at least one client attached to it.
If the server has no clients, then there is 2 second interval before the server
terminates. This is implemented as an active object. </p> </li>
<li id="GUID-07B2ACEC-C457-547D-B1FE-78FCF980AEEF"><p>Test code:this corresponds
to the client, and uses the client interface to communicate with the server
- application developers should make use of the principles applied here and
include them into their own client code. </p> </li>
<li id="GUID-91B89143-B380-524D-89CF-918FAE5CE055"><p>bld.inf: which is a
component description file. </p> </li>
</ul><p><b>Related APIs</b></p><ul>
<li><p><xref href="GUID-B4C76104-EA1B-3FC3-A31E-86A976598171.dita#GUID-B4C76104-EA1B-3FC3-A31E-86A976598171/GUID-3281C85C-1A45-3049-B711-64CA8BFBC452"><apiname>CActiveScheduler::Start()</apiname></xref></p></li>
<li><p><xref href="GUID-8B50E9C7-CA33-3E11-A881-16E230A082CB.dita"><apiname>CMySession</apiname></xref></p></li>
<li><p><xref href="GUID-8E316AC4-4676-301A-9A23-659E83AA1D1C.dita"><apiname>CServer2</apiname></xref> - Abstract base class for servers (version
2).</p></li>
<li><p><xref href="GUID-D5A30C75-E22C-34E8-913B-7D2CA6AD5C51.dita"><apiname>CSession2</apiname></xref> - Represents a session (version 2) for
a client thread on the server-side.</p></li>
<li><p><xref href="GUID-D1D25122-F2B8-3C78-8599-84905BFD47B8.dita"><apiname>KErrAlreadyExists</apiname></xref> - System wide error code -11 :
an object already exists.</p></li>
<li><p><xref href="GUID-6D8A458C-9A39-3000-A3BC-060A2A3663E6.dita"><apiname>RSessionBase</apiname></xref> - Client-side handle to a session with
a server.</p></li>
<li><p><xref href="GUID-6D8A458C-9A39-3000-A3BC-060A2A3663E6.dita#GUID-6D8A458C-9A39-3000-A3BC-060A2A3663E6/GUID-479FE087-DD9F-31D7-98F7-9F96535F0584"><apiname>RSessionBase::SendReceive()</apiname></xref></p></li>
<li><p><xref href="GUID-4AD02F14-1142-372F-9D11-224595932034.dita"><apiname>TIpcArgs</apiname></xref> - A Version 2 client/server class that
clients use to package the arguments to be sent to a server.</p></li>
<li><p><xref href="GUID-E0B34F3E-D4C4-3232-B8B1-7DB35B454646.dita"><apiname>TRequestStatus</apiname></xref> - Indicates the completion status
of a request made to a service provider.</p></li>
</ul> </section>
<section id="GUID-86D3479E-216B-4D5D-AF57-C4C9963CD9F0"><title>Building and configuring</title> <p>To build the example: </p> <ul>
<li id="GUID-FDC690A6-D68B-58E5-A10D-89220DB9B89E"><p>You can build the example
from your IDE or the command line. </p> <p>If you use an IDE, import the <filepath>bld.inf</filepath> file
of the example into your IDE, and use the build command of the IDE. </p> <p>If
you use the command line, open a command prompt, and set the current directory
to the source code directory of the example. You can then build the example
with the SBSv1 build tools with the following commands: </p> <p><userinput>bldmake
bldfiles</userinput> </p> <p><userinput>abld build</userinput> </p> <p><xref href="GUID-793A5EF9-CC16-5EEB-9011-6431EA76EB15.dita">How to use bldmake</xref> and <xref href="GUID-B6B54E07-3B34-5D5C-8815-93383FA8FB4B.dita">How to use abld</xref> describe
how to use the SBSv1 build tools. </p> </li>
<li id="GUID-DD18F4AF-6AB1-5E0F-95C3-9EB899BA99C9"><p>For the emulator, the
example builds the following executables : </p> <p> <filepath>t-client.dll</filepath> </p> <p> <filepath>t-server.exe</filepath>  </p> <p> <filepath>t-test.exe</filepath> (test executable) </p> <p> <filepath>t-testc.dll</filepath> (test
client) </p> <p>in the <filepath>epoc32\release\winscw\&lt;udeb or urel&gt;\</filepath> folder. </p> </li>
</ul> </section>
</refbody><related-links>
<link href="GUID-4BFEDD79-9502-526A-BA7B-97550A6F0601.dita"><linktext>Platform
security</linktext></link>
<link href="GUID-B21141D4-3BFE-59C9-8D5F-147A93BE1C95.dita"><linktext>Build tools
guide</linktext></link>
<link href="GUID-594488FF-02C9-5066-85DB-5F88A754AE18.dita"><linktext>How     
           to build DLLs</linktext></link>
<link href="GUID-BBBB5000-90A2-503A-9521-2FC6DEC69DC8.dita"><linktext>MMP file
syntax</linktext></link>
</related-links></reference>