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    10 <!DOCTYPE concept

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

    12 <concept id="GUID-3631971E-424B-5B10-B44C-98FB7C265843" xml:lang="en"><title>Kernel

    13 services</title><shortdesc>Describes asynchronous services provided by the kernel.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

    14 <p>Many asynchronous services are provided to user threads by the kernel.</p>

    15 <p>The <codeph>RTimer</codeph> class is an example. It includes functions

    16 such as:</p>

    17 <codeblock id="GUID-9BF774FC-9DA2-5426-A4C5-F6A9DCB7E889" xml:space="preserve">After(TRequestStatus &amp;aStatus,TTimeIntervalMicroSeconds32 anInterval);</codeblock>

    18 <p>This makes a timer request that completes after a given number of microseconds.</p>

    19 <p>The <codeph>RTimer</codeph> class is a client-side handle to the kernel’s

    20 timer service. When an <codeph>RTimer</codeph> is opened, using its <codeph>CreateLocal()</codeph> function,

    21 a Kernel object is created which holds the state of the timer and the ID of

    22 the client’s thread. When a request function is issued, the client-side handle:</p>

    23 <ul>

    24 <li id="GUID-D7D4F444-C1CB-5FEA-B885-FDC6764CE093"><p>sets the passed <codeph>TRequestStatus</codeph> to <codeph>KRequestPending</codeph> </p> </li>

    25 <li id="GUID-0CB9A142-ED99-5455-B503-B9AB78F43993"><p>sends a message to the

    26 Kernel identifying the <codeph>RTimer</codeph> client-side handle, the <codeph>TRequestStatus</codeph> object

    27 and the time interval in microseconds.</p> </li>

    28 <li id="GUID-04280A19-96A4-526A-9E5A-7295112A16F0"><p>returns to the caller.</p> </li>

    29 </ul>

    30 <p>When the Kernel side timer completes, it posts the <codeph>TRequestStatus</codeph> with

    31 a completion code, normally <codeph>KErrNone</codeph>, and uses the thread

    32 id stored with the Kernel side timer object to signal the thread’s request

    33 semaphore.</p>

    34 <p>Provision of asynchronous services by the Kernel involves sending messages

    35 between the client thread and the Kernel.</p>

    36 <p>The Kernel provides the most basic services available. Its main services

    37 are defined in <filepath>e32std.h</filepath>, including timing, notification

    38 and the classes used in the client-server framework.</p>

    39 </conbody></concept>