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<concept id="GUID-000ACB11-EDD0-5160-BC5E-4593F1BAF293" xml:lang="en"><title>Message

Queue Overview</title><shortdesc>This topic describes the message queue overview.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<p>A message queue is a mechanism for passing data: </p>

<ul>

<li id="GUID-B4740898-D06D-5B19-A3AD-E8FE3B72C397"><p>between threads within

a process </p> </li>

<li id="GUID-F9F8E8FF-02F7-5E11-BC57-61FFE1122DB1"><p>between threads that

run in separate processes. </p> </li>

</ul>

<p>The mechanism provides a way to send data (messages) to an interested party

without needing to know whether anyone is listening nor needing to know the

identity of a recipient. </p>

<p>A message is an object, usually an instance of a class, that is placed

into a queue for delivery to recipients. A queue is normally created to deal

with messages of a given type. This means that a queue is created to deal

with messages of a defined (fixed) length. The size of a queue, i.e. the maximum

number of messages, or slots, it can contain is defined and fixed when the

queue is created. The size of message for which a queue is created, and the

size of the queue is arbitrary, being limited only by system resources. </p>

<p>A single queue can be shared by many readers and writers. Several threads

may be reading from and writing to the same message queue, but only one thread

can access the queue at a time. If multiple clients attempt to read from the

message queue a panic will be raised. </p>

<p>A message queue is represented by a <codeph>DMsgQueue</codeph> kernel side

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.

A message queue is a reference counted object, being derived from <codeph>CObject</codeph>,

which means that it is not persistent; it is deleted when the last handle

to it is closed. The queue itself is simply a block of memory divided into

slots, managed by the <codeph>DMsgQueue</codeph> object. </p>

<fig id="GUID-BDA4A786-57B2-529D-BDFB-72D2B2B30670">

<image href="GUID-7C061090-83A8-5E0F-B42B-2D722BE054EE_d0e356014_href.png" placement="inline"/>

</fig>

<section id="GUID-1D2EF848-8068-56B9-850B-56AE88E192DE"><title>Handle to a

message queue</title> <p>A message queue is created, opened, written to and

read from through a message queue handle, an <xref href="GUID-5195B8D1-851E-3BEE-A72D-1841C0937300.dita"><apiname>RMsgQueue</apiname></xref> object.

This is a templated class, where the template parameter defines the message

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>,

which together form a thin template class/base class pair. <codeph>RMsgQueueBase</codeph> provides

the implementation, while <codeph>RMsgQueue</codeph> provides type safety.

An <codeph>RMsgQueueBase</codeph> object is a valid message queue handle,

but does not offer the type safety that <codeph>RMsgQueue</codeph> does. </p> <fig id="GUID-B9685695-0E07-5CC1-AFED-0ABF4DC93ACB">

<image href="GUID-2F6EF388-CA87-5AAD-A4C5-37FC3CE15331_d0e356053_href.png" placement="inline"/>

</fig> <p>Message queues are used for one-way, one to one communications only.

The message queue is therefore unicast. If there are multiple readers of the

message queue only one will receive the message. </p> </section>

<section id="GUID-910216A9-EE2D-5C45-8AB3-E1CBEF3B7514"><title>Visibility

of a message queue</title> <p>A message queue can be: </p> <ul>

<li id="GUID-A773AF82-EACC-57DB-9A6F-31F74A9EDF94"><p>named and be visible

to all processes - a global queue </p> </li>

<li id="GUID-9EFB0165-798F-5B5F-8537-199C9C16CE33"><p>local to the current

process, i.e. not visible to any other process - a local queue </p> </li>

</ul> </section>