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<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_d0e357270_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>