<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. -->
<!-- This component and the accompanying materials are made available under the terms of the License 
"Eclipse Public License v1.0" which accompanies this distribution, 
and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". -->
<!-- Initial Contributors:
    Nokia Corporation - initial contribution.
Contributors: 
-->
<!DOCTYPE concept
  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="GUID-7AB51180-7A1A-40C7-B28F-EA46314A6E5B-GENID-1-2-1-9-1-5-1-4-1" xml:lang="en"><title>Synchronous
Requests</title><shortdesc>This document describes the use of synchronous requests by device
drivers.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<section id="GUID-6C508A3B-B2E6-4D16-9421-03EE1A0B595F-GENID-1-2-1-9-1-5-1-4-1-3-1">       <title><b>Synchronous
requests</b></title>       <p>Synchronous requests are typically used to set
or retrieve some information for the device driver. These requests almost
never need to access the hardware itself, and usually complete relatively
quickly. They return only after the completion of the request and the user
side thread is blocked till completion. Synchronous requests are usuallly
initiated by a call to <xref href="GUID-6FBFA078-8253-3E24-B1F8-5F75E86C3066.dita#GUID-6FBFA078-8253-3E24-B1F8-5F75E86C3066/GUID-D774DE92-6431-374A-A1F6-1C7045BD4FE5"><apiname>RBusLogicalChannel::DoControl()</apiname></xref>.
The request is most likely to be executed in the context of the client user-side
thread.</p> <p>A driver lists its available synchronous requests
in an enumeration, for example: </p> <codeblock id="GUID-09E42EC9-CF71-59C0-876E-0651F27E591F-GENID-1-2-1-9-1-5-1-4-1-3-1-4" xml:space="preserve">// Synchronous control messages used with DoControl()
enum TControl 
    {
    EControlConfigure, // Configure the device (UART)
    EControlTransmitData, // Transmit data over the device (UART)    
    EControlReceiveData,    // Receive the data from the device 
    ENumRequests, // Number of synchronous control requests
    AllRequests = (1&lt;&lt;ENumRequests)-1
    };</codeblock>     </section>
<section id="GUID-EC33A9C8-CE41-4937-8008-35502C290581-GENID-1-2-1-9-1-5-1-4-1-3-2"><title>Implementation</title><p>Drivers
generally implement the <xref href="GUID-06C73075-6095-3D8F-AFC9-FD832958A49C.dita"><apiname>DoControl()</apiname></xref> function in the LDD
to handle the received synchronous messages. The implementation reads the
request type and other passed arguments, and handles the request appropriately.
For example, the following handles <codeph>RExDriver::EControlConfigure</codeph> requests: </p> <codeblock id="GUID-1996BA3A-A895-58C8-B394-4219510DF6D3-GENID-1-2-1-9-1-5-1-4-1-3-2-3" xml:space="preserve">TInt DExDriverLogicalChannel::DoControl(TInt aFunction, TAny* a1, TAny* /*a2*/)
    {
    switch (aFunction)
        {
        case RExDriver::EControlConfigure:
            memclr(&amp;c, sizeof(c)));
            TPtr8 cfg((TUint8*)&amp;c, 0, sizeof(c)));
            r = Kern::ThreadDesRead(iClient,a1,cfg,0,0);
            if (r==KErrNone)
                Pdd()-&gt;Configure(&amp;c);
            break;
            ...
        default:
                r = KErrNotSupported;
        }
    return r;
    }</codeblock></section>
</conbody></concept>