Message -Queue Overview

diff -r 51a74ef9ed63 -r ae94777fff8f Symbian3/SDK/Source/GUID-000ACB11-EDD0-5160-BC5E-4593F1BAF293.dita --- a/Symbian3/SDK/Source/GUID-000ACB11-EDD0-5160-BC5E-4593F1BAF293.dita Wed Mar 31 11:11:55 2010 +0100 +++ b/Symbian3/SDK/Source/GUID-000ACB11-EDD0-5160-BC5E-4593F1BAF293.dita Fri Jun 11 12:39:03 2010 +0100 @@ -1,64 +1,64 @@ - - - - - -Message -Queue OverviewThis topic describes the message queue overview. -

A message queue is a mechanism for passing data:

between threads within -a process
between threads that -run in separate processes.

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.

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.

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.

A message queue is represented by a DMsgQueue kernel side -object, to which the reader and the writer can open a handle, a RMsgQueue object. -A message queue is a reference counted object, being derived from CObject, -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 DMsgQueue object.

- - - -

Handle to a -message queue

A message queue is created, opened, written to and -read from through a message queue handle, an RMsgQueue object. -This is a templated class, where the template parameter defines the message -type.

RMsgQueue is derived from RMsgQueueBase, -which together form a thin template class/base class pair. RMsgQueueBase provides -the implementation, while RMsgQueue provides type safety. -An RMsgQueueBase object is a valid message queue handle, -but does not offer the type safety that RMsgQueue does.

- -

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.

Visibility -of a message queue

A message queue can be:

named and be visible -to all processes - a global queue
local to the current -process, i.e. not visible to any other process - a local queue

+ + + + + +Message +Queue OverviewThis topic describes the message queue overview. +

A message queue is a mechanism for passing data:

between threads within +a process
between threads that +run in separate processes.

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.

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.

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.

A message queue is represented by a DMsgQueue kernel side +object, to which the reader and the writer can open a handle, a RMsgQueue object. +A message queue is a reference counted object, being derived from CObject, +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 DMsgQueue object.

+ + + +

Handle to a +message queue

A message queue is created, opened, written to and +read from through a message queue handle, an RMsgQueue object. +This is a templated class, where the template parameter defines the message +type.

RMsgQueue is derived from RMsgQueueBase, +which together form a thin template class/base class pair. RMsgQueueBase provides +the implementation, while RMsgQueue provides type safety. +An RMsgQueueBase object is a valid message queue handle, +but does not offer the type safety that RMsgQueue does.

+ +

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.

Visibility +of a message queue

A message queue can be:

named and be visible +to all processes - a global queue
local to the current +process, i.e. not visible to any other process - a local queue

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