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<!DOCTYPE concept
  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="GUID-983F0ABD-470C-51C3-B6AE-1B1AA55AB4A2" xml:lang="en"><title>File
Server Extensions</title><shortdesc>This document provides an overview of the file server extension
architecture, and of the APIs that allows file server extensions to be developed
and used. </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<p>A file server extension is a library that allows the functionality of a
file server logical drive and its corresponding file system to be extended
or modified. </p>
<p>The Symbian platform uses a file server extension to implement its Flash
Transition Layer (FTL) that allows a FAT file system to run on a NAND flash
device. Because this layer is not needed when using FAT on a RAM disk or a
MultiMediaCard, it is not necessary to add this functionality to the FAT file
system itself. So the FTL is implemented as a file server extension and is
only used on NAND local drives. </p>
<p>The file server uses libraries called file systems, which implement support
for particular file system formats, such as FAT and LFFS. These file system
libraries in turn make calls to the media sub-system, which provides device
drivers that handles interaction with the actual storage hardware. </p>
<p>A file server extension is an additional layer that intercepts all calls
from a file system to the media sub-system, as shown below: </p>
<fig id="GUID-5034F500-1CC7-5A8A-B1C0-DB99FAF8FE93">
<title>           File server extension architecture          </title>
<image href="GUID-E36B0B87-7948-5288-9F4D-9A1F1EBDF972_d0e373227_href.png" placement="inline"/>
</fig>
<p>All calls to the media sub-system from a mounted file system go through
a "proxy" drive interface provided by the loaded extension (or extensions),
before the call is passed on to the media sub-system interface. </p>
<p>An extension has the same lifetime as its corresponding file system object.
This is usually a <xref href="GUID-FADDE053-CC1C-39BC-A52D-27093041BE20.dita"><apiname>CMountCB</apiname></xref> object but can also be a subsession
object (for example <xref href="GUID-63DF657C-434D-353D-A536-8AF9D97C8260.dita"><apiname>CFileCB</apiname></xref>). On removable drives, if a
mount with an extension is destroyed when the media is removed, then when
the same media is reinserted, the mount object and extension should be created
at the same time. </p>
<p>Extensions can be referred to as being <i>primary</i> or <i>secondary</i>: </p>
<ul>
<li id="GUID-58C4DAEE-22E3-506E-81C2-4D5C9B69C7F8"><p>An extension is called
primary if it is required for the file system to be used. Such extensions
must be mounted before or at the same time as a file system is mounted on
a drive. This can be done by modifying the initial mounting of the local file
system in the <xref href="GUID-80698E62-E310-59CA-A524-5CF44C437BE4.dita">Base
Starter</xref> component. The file system mounted on the drive must be dismounted
before a primary extension can be dismounted. </p> <p>Only one primary extension
is permitted to be mounted on a drive at a time. </p> </li>
<li id="GUID-DB65DE20-5F80-5FC6-A847-EAE74F544A8C"><p>Secondary extensions
can be added after a file system is mounted. The maximum number of extensions
allowed is two. </p> <p>When two extensions are loaded, calls are passed through
both extensions in turn. This is referred to as <i>chaining</i>. The order
of chaining is determined by the order that extensions are mounted on the
drive. The extension mounted first is closest to the media sub-system interface.
Therefore the primary extension is always closest to the media sub-system
interface. </p> </li>
</ul>
</conbody></concept>