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    10 <!DOCTYPE concept

    11   PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">

    12 <concept id="GUID-983F0ABD-470C-51C3-B6AE-1B1AA55AB4A2" xml:lang="en"><title>File

    13 Server Extensions</title><shortdesc>This document provides an overview of the file server extension

    14 architecture, and of the APIs that allows file server extensions to be developed

    15 and used. </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

    16 <p>A file server extension is a library that allows the functionality of a

    17 file server logical drive and its corresponding file system to be extended

    18 or modified. </p>

    19 <p>The Symbian platform uses a file server extension to implement its Flash

    20 Transition Layer (FTL) that allows a FAT file system to run on a NAND flash

    21 device. Because this layer is not needed when using FAT on a RAM disk or a

    22 MultiMediaCard, it is not necessary to add this functionality to the FAT file

    23 system itself. So the FTL is implemented as a file server extension and is

    24 only used on NAND local drives. </p>

    25 <p>The file server uses libraries called file systems, which implement support

    26 for particular file system formats, such as FAT and LFFS. These file system

    27 libraries in turn make calls to the media sub-system, which provides device

    28 drivers that handles interaction with the actual storage hardware. </p>

    29 <p>A file server extension is an additional layer that intercepts all calls

    30 from a file system to the media sub-system, as shown below: </p>

    31 <fig id="GUID-5034F500-1CC7-5A8A-B1C0-DB99FAF8FE93">

    32 <title>           File server extension architecture          </title>

    33 <image href="GUID-E36B0B87-7948-5288-9F4D-9A1F1EBDF972_d0e357270_href.png" placement="inline"/>

    34 </fig>

    35 <p>All calls to the media sub-system from a mounted file system go through

    36 a "proxy" drive interface provided by the loaded extension (or extensions),

    37 before the call is passed on to the media sub-system interface. </p>

    38 <p>An extension has the same lifetime as its corresponding file system object.

    39 This is usually a <xref href="GUID-FADDE053-CC1C-39BC-A52D-27093041BE20.dita"><apiname>CMountCB</apiname></xref> object but can also be a subsession

    40 object (for example <xref href="GUID-63DF657C-434D-353D-A536-8AF9D97C8260.dita"><apiname>CFileCB</apiname></xref>). On removable drives, if a

    41 mount with an extension is destroyed when the media is removed, then when

    42 the same media is reinserted, the mount object and extension should be created

    43 at the same time. </p>

    44 <p>Extensions can be referred to as being <i>primary</i> or <i>secondary</i>: </p>

    45 <ul>

    46 <li id="GUID-58C4DAEE-22E3-506E-81C2-4D5C9B69C7F8"><p>An extension is called

    47 primary if it is required for the file system to be used. Such extensions

    48 must be mounted before or at the same time as a file system is mounted on

    49 a drive. This can be done by modifying the initial mounting of the local file

    50 system in the <xref href="GUID-80698E62-E310-59CA-A524-5CF44C437BE4.dita">Base

    51 Starter</xref> component. The file system mounted on the drive must be dismounted

    52 before a primary extension can be dismounted. </p> <p>Only one primary extension

    53 is permitted to be mounted on a drive at a time. </p> </li>

    54 <li id="GUID-DB65DE20-5F80-5FC6-A847-EAE74F544A8C"><p>Secondary extensions

    55 can be added after a file system is mounted. The maximum number of extensions

    56 allowed is two. </p> <p>When two extensions are loaded, calls are passed through

    57 both extensions in turn. This is referred to as <i>chaining</i>. The order

    58 of chaining is determined by the order that extensions are mounted on the

    59 drive. The extension mounted first is closest to the media sub-system interface.

    60 Therefore the primary extension is always closest to the media sub-system

    61 interface. </p> </li>

    62 </ul>

    63 </conbody></concept>