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    10 <!DOCTYPE concept

    11   PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">

    12 <concept xml:lang="en" id="GUID-FE3825C5-BDEE-5F18-9FFD-2E794E618FEC"><title>Data Access Example</title><shortdesc>This example describes typical processing of the data in an RMBufChain. </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody><section><title>Downwards flow</title> <fig id="GUID-43219AE7-A0E4-570A-98ED-07D44D849979"><title>

    13              Example : flow going downwards 

    14           </title> <image href="GUID-30987768-8B0A-5A35-820C-DBB410D2053B_d0e117545_href.png" placement="inline"/></fig> <p>The following function is a simplified example of how a component in the middle of the stack could process data sent to the driver. As shown in the above diagram, the upper layer provides the data in the <codeph>RMBufChain</codeph> parameter. </p> <codeblock id="GUID-4C87CEB3-DCFD-5765-BF89-6EF9275EB584" xml:space="preserve">

    15 void Send (RMBufChain &aChain) 

    16 {

    17    TBuf8<KHeaderLength> aHeader;

    18    

    19    // [...] fill the header

    20 

    21    // request space in front of the data 

    22    aChain.Prepend(KHeaderLength);

    23 

    24    // add the protocol header

    25    aChain.CopyIn(aHeader);

    26 

    27    // send the updated chain to the next layer

    28    downwardComponent->Send(aChain);

    29 }

    30    </codeblock> <p>Note that depending on the available space remaining in front of the data in the MBuf, the <xref href="GUID-F024208C-ED19-3301-85C1-53F397C9910F.dita#GUID-F024208C-ED19-3301-85C1-53F397C9910F/GUID-A7CEF452-AB4E-378C-A262-B1C5FCBA68BB"><apiname>RMBufChain::Prepend()</apiname></xref> method may request the pond to add a new MBuf to the front of the chain. For more information, see <xref href="GUID-B4F15CA3-CAD4-5A87-9610-A656CA337B72.dita#GUID-B4F15CA3-CAD4-5A87-9610-A656CA337B72/GUID-5EBCD46C-5E07-5DB1-90D9-6A5CA3E36C0B">Buffer layout</xref>. </p> </section> <section><title>Upwards flow</title> <fig id="GUID-402331EB-6DE8-5DC5-9FF7-7D5DD8D816D5"><title>

    31              Example : flow going upwards 

    32           </title> <image href="GUID-11CBC0BA-3683-584A-9DC9-8BD3C9573F01_d0e117575_href.png" placement="inline"/></fig> <p>The following function is a simplified example of how a component in the middle of the stack could process data received by the driver. As shown in the above diagram, the underlying layer provides the data in the <codeph>RMBufChain</codeph> parameter. </p> <codeblock id="GUID-C36656D9-AEB0-5DBF-A469-0183F80AE057" xml:space="preserve">

    33 void Process (RMBufChain &aChain)

    34 {

    35    TBuf8<KHeaderLength> aHeader;

    36    

    37    // get a copy of the header

    38    aChain.CopyOut(KHeaderLength, &aHeader);

    39    

    40    // discard the header in the chain

    41    aChain.TrimStart(KHeaderLength);

    42 

    43    // [...] process the header

    44    

    45    // send the updated chain to the next layer

    46    appropriateUpwardComponent->Process(aChain);

    47 }

    48 </codeblock> <p>Note that the header might not be stored in a contiguous memory area: it can be splitted accross several chained MBufs. For a better performance, you should define a buffer size slightly bigger than your MTU: for more information, see <xref href="GUID-6FEE5676-8BFB-53E6-A652-7178295C342E.dita#GUID-6FEE5676-8BFB-53E6-A652-7178295C342E/GUID-8F0E62BA-455A-5C58-9AB1-70C79F5BA573">Pond Configuration : Recommendations</xref>. </p> </section> </conbody><related-links><link href="GUID-55E4D84B-1B90-5BA4-9CE0-6D26EA208F13.dita"><linktext>Overview</linktext> </link> <link href="GUID-B4F15CA3-CAD4-5A87-9610-A656CA337B72.dita"><linktext>Comms Buffers (MBuf)

    49                 and Comms Chains</linktext> </link> </related-links></concept>