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<!DOCTYPE concept
  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="GUID-132349A6-9A5F-4866-A54D-F01B6F58ABDD" xml:lang="en"><title>Shared
Chunks</title><shortdesc>This document describes the use of shared chunks to share data.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<section id="GUID-DBFFE006-4E6C-4B4C-9E50-0A9FC9CB51FF">       <title> Shared
chunks</title>       <p>Device drivers often need to share data between user
space and kernel space threads. Though there are APIs for doing this, such
as <xref href="GUID-C6946ECB-775F-3EC2-A56F-78F25B9FBE3D.dita#GUID-C6946ECB-775F-3EC2-A56F-78F25B9FBE3D/GUID-904A42A8-8077-3FC6-BEF2-29619F079842"><apiname>Kern::ThreadRawRead()</apiname></xref> and <xref href="GUID-C6946ECB-775F-3EC2-A56F-78F25B9FBE3D.dita#GUID-C6946ECB-775F-3EC2-A56F-78F25B9FBE3D/GUID-182C88F4-326C-376E-9FBE-889E3CB9B68A"><apiname>Kern::ThreadRawWrite()</apiname></xref>,
these involve an overhead of a memory copy, which can be a problem when there
is a large amount of data. </p> <p>To avoid unnecessary data transfer, Symbian
platform provides shared chunks, which are similar to shared memory in other
operating systems. Shared chunks can be created and used by both a driver
and user-side code directly. An example application for shared chunks is for
a camera driver. Without shared chunks, the image data would have to be copied
from the camera driver to the user process, which would then copy it to the
display driver. This would have a high memory copy overhead. Using shared
chunks instead would improve the performance of the camera view finder. </p> <p>A
shared chunk is created and controlled by the kernel side code, for example,
the device driver, rather than the user code. This memory is safe to be used
by ISRs and DMA. A shared chunk can be mapped into multiple user processes
in sequence or at the same time. The memory object can be transferred between
another process and another device driver. Multiple drivers and multiple user
processes can have access to the same shared chunk. </p>     </section>
<section id="GUID-8C27DA12-5396-4229-8723-480D5846604C"><title>Comparison
between shared memory and shared I/O buffers</title><p>The EKA1 versions of
Symbian platform had a mechanism called shared I/O buffers. These are now
deprecated, and shared chunks should be used instead. The disadvantage of
a shared I/O buffer is that only one user process can access the chunk at
a time, and that it does not have a user handle: the user process is only
supplied with the address and size of the buffer. Shared chunks solve these
issues, and give more flexibility for users to do safe operations. </p></section>
</conbody></concept>