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<concept id="GUID-50BE8F1A-513A-433C-B045-66658FC226D7" xml:lang="en"><title>Client/server communication</title><prolog><metadata><keywords/></metadata></prolog><conbody>
<p><xref href="GUID-E530CBB9-C20D-5426-89AA-DC0281C05381.dita">Inter
Process Communication</xref> (IPC) is the basis for client/server
architecture. Clients and servers run in different threads, although
not necessarily in different processes, and communicate through message
passing protocol. There are no direct pointers between the parties,
leaving the integrity of the server and its resources untouched by
clients. For more information, see <xref href="GUID-D4C7A714-F7BB-5A8D-A875-4796395B9B6C.dita">Introduction to the
client/server architecture</xref>.</p>
<p>Communication between the client and the server is managed by <xref href="GUID-E651C7A7-D6EB-533E-A97A-360D089DE7A5.dita">Inter-thread data
transfer</xref>. Only the server and its clients can decode the parameters
of the message. This is also a security feature, because process boundaries
are separators of memory space, direct pointers to the client's data
structures cannot be delivered. All data not fitting in the provided
integers must be represented as a <xref href="GUID-261ADCEC-C8C8-46E3-A7DC-804AC868C233.dita">descriptor</xref>, and an address to the descriptor (or actually a <codeph><xref href="jar:GUID-35228542-8C95-4849-A73F-2B4F082F0C44.jar!/sdk/doc_source/reference/reference-cpp/Kernel_Architecture_2/TPckgClass.html" format="application/java-archive">TPckg</xref></codeph> object) is delivered within the message.
The server then uses safe inter-thread read and write functions to
access the provided descriptor.</p>
<p>The following figure shows the relationship of the <codeph><xref href="jar:GUID-35228542-8C95-4849-A73F-2B4F082F0C44.jar!/sdk/doc_source/reference/reference-cpp/Kernel_Architecture_2/e32cmn.hGlobals.html#%3a%3aTDesC" format="application/java-archive">TDesC</xref></codeph> and <codeph><xref href="jar:GUID-35228542-8C95-4849-A73F-2B4F082F0C44.jar!/sdk/doc_source/reference/reference-cpp/Kernel_Architecture_2/TPckgClass.html" format="application/java-archive">TPckg</xref></codeph> classes.</p>
<fig id="GUID-715F1B96-AFC4-4FB2-AF51-6E720077F83A"><title>Relationship of a message package and a descriptor class</title><image href="GUID-D99BA4CA-9823-4968-953D-36AA7F266B39_d0e12157_href.png"/></fig>
<p>Consider the following issues when implementing the client-side
API:</p>
<ul>
<li><p>Determine if the server is already running with the <codeph><xref href="jar:GUID-35228542-8C95-4849-A73F-2B4F082F0C44.jar!/sdk/doc_source/reference/reference-cpp/Kernel_Architecture_2/TFindServerClass.html" format="application/java-archive">TFindServer</xref></codeph> class. If not, then it should be
launched.</p></li>
<li><p>Once the server is running, create the connection
with <codeph><xref href="jar:GUID-35228542-8C95-4849-A73F-2B4F082F0C44.jar!/sdk/doc_source/reference/reference-cpp/Kernel_Architecture_2/RSessionBaseClass.html" format="application/java-archive">RSessionBase::CreateSession</xref></codeph> and make sure there
are free message slots available to avoid lost messages.</p>
</li>
<li><p>When packaging message arguments, make sure they
are in a format that the server understands. For more information,
see <codeph><xref href="jar:GUID-35228542-8C95-4849-A73F-2B4F082F0C44.jar!/sdk/doc_source/reference/reference-cpp/Kernel_Architecture_2/TIpcArgsClass.html" format="application/java-archive">TIpcArgs</xref></codeph>.</p></li>
</ul>
</conbody></concept>