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1 <?xml version="1.0" encoding="utf-8"?> |
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2 <!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. --> |
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3 <!-- This component and the accompanying materials are made available under the terms of the License |
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4 "Eclipse Public License v1.0" which accompanies this distribution, |
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5 and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". --> |
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6 <!-- Initial Contributors: |
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7 Nokia Corporation - initial contribution. |
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8 Contributors: |
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9 --> |
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10 <!DOCTYPE concept |
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11 PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd"> |
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12 <concept id="GUID-000ACB11-EDD0-5160-BC5E-4593F1BAF293" xml:lang="en"><title>Message |
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13 Queue Overview</title><shortdesc>This topic describes the message queue overview.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody> |
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14 <p>A message queue is a mechanism for passing data: </p> |
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15 <ul> |
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16 <li id="GUID-B4740898-D06D-5B19-A3AD-E8FE3B72C397"><p>between threads within |
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17 a process </p> </li> |
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18 <li id="GUID-F9F8E8FF-02F7-5E11-BC57-61FFE1122DB1"><p>between threads that |
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19 run in separate processes. </p> </li> |
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20 </ul> |
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21 <p>The mechanism provides a way to send data (messages) to an interested party |
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22 without needing to know whether anyone is listening nor needing to know the |
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23 identity of a recipient. </p> |
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24 <p>A message is an object, usually an instance of a class, that is placed |
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25 into a queue for delivery to recipients. A queue is normally created to deal |
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26 with messages of a given type. This means that a queue is created to deal |
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27 with messages of a defined (fixed) length. The size of a queue, i.e. the maximum |
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28 number of messages, or slots, it can contain is defined and fixed when the |
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29 queue is created. The size of message for which a queue is created, and the |
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30 size of the queue is arbitrary, being limited only by system resources. </p> |
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31 <p>A single queue can be shared by many readers and writers. Several threads |
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32 may be reading from and writing to the same message queue, but only one thread |
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33 can access the queue at a time. If multiple clients attempt to read from the |
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34 message queue a panic will be raised. </p> |
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35 <p>A message queue is represented by a <codeph>DMsgQueue</codeph> kernel side |
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36 object, to which the reader and the writer can open a handle, a <xref href="GUID-5195B8D1-851E-3BEE-A72D-1841C0937300.dita"><apiname>RMsgQueue</apiname></xref> object. |
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37 A message queue is a reference counted object, being derived from <codeph>CObject</codeph>, |
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38 which means that it is not persistent; it is deleted when the last handle |
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39 to it is closed. The queue itself is simply a block of memory divided into |
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40 slots, managed by the <codeph>DMsgQueue</codeph> object. </p> |
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41 <fig id="GUID-BDA4A786-57B2-529D-BDFB-72D2B2B30670"> |
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42 <image href="GUID-7C061090-83A8-5E0F-B42B-2D722BE054EE_d0e246744_href.png" placement="inline"/> |
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43 </fig> |
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44 <section id="GUID-1D2EF848-8068-56B9-850B-56AE88E192DE"><title>Handle to a |
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45 message queue</title> <p>A message queue is created, opened, written to and |
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46 read from through a message queue handle, an <xref href="GUID-5195B8D1-851E-3BEE-A72D-1841C0937300.dita"><apiname>RMsgQueue</apiname></xref> object. |
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47 This is a templated class, where the template parameter defines the message |
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48 type. </p> <p> <xref href="GUID-5195B8D1-851E-3BEE-A72D-1841C0937300.dita"><apiname>RMsgQueue</apiname></xref> is derived from <xref href="GUID-2DCEE7F5-9EA3-3546-8779-7299318176E2.dita"><apiname>RMsgQueueBase</apiname></xref>, |
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49 which together form a thin template class/base class pair. <codeph>RMsgQueueBase</codeph> provides |
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50 the implementation, while <codeph>RMsgQueue</codeph> provides type safety. |
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51 An <codeph>RMsgQueueBase</codeph> object is a valid message queue handle, |
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52 but does not offer the type safety that <codeph>RMsgQueue</codeph> does. </p> <fig id="GUID-B9685695-0E07-5CC1-AFED-0ABF4DC93ACB"> |
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53 <image href="GUID-2F6EF388-CA87-5AAD-A4C5-37FC3CE15331_d0e246783_href.png" placement="inline"/> |
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54 </fig> <p>Message queues are used for one-way, one to one communications only. |
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55 The message queue is therefore unicast. If there are multiple readers of the |
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56 message queue only one will receive the message. </p> </section> |
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57 <section id="GUID-910216A9-EE2D-5C45-8AB3-E1CBEF3B7514"><title>Visibility |
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58 of a message queue</title> <p>A message queue can be: </p> <ul> |
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59 <li id="GUID-A773AF82-EACC-57DB-9A6F-31F74A9EDF94"><p>named and be visible |
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60 to all processes - a global queue </p> </li> |
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61 <li id="GUID-9EFB0165-798F-5B5F-8537-199C9C16CE33"><p>local to the current |
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62 process, i.e. not visible to any other process - a local queue </p> </li> |
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63 </ul> </section> |
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64 </conbody></concept> |