<concept id="GUID-A188606F-4A8E-5A85-A241-E15219960EFA" xml:lang="en"><title>Loader Overview</title><shortdesc>This topic describes the overview, purpose, key concepts

and architecture of loader.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<section id="GUID-E36370EC-CD5E-54AE-AD62-2BA3B70F5379"><title>Purpose</title> <p>The loader makes EXEs and DLLs ready to run. </p> <p>The build

tools compile the source code into an executable file. An executable

file is called a binary. The build tools arrange the compiled code

into EXEs and DLLs. The source code, header files and other information

that defines an executable file is defined in an mmp file. The loader

applies relocation information to the code and data sections of EXEs

and DLLs. The loader also loads DLLs that are referred to by other

EXEs and DLLs. EXEs and DLLs can then run in the process into which

they are loaded. </p> </section>

<section id="GUID-89A8CABD-6B31-5D10-87F6-922CCFB83186"><title>Key

concepts</title> <p><b>The loader server</b> </p> <p>The Symbian platform loader is a

server. The loader server has no public client interface. The client

interface is internal to Symbian platform. Requests to load an EXE

or a DLL are passed to the loader when you do the following: </p> <ul>

<li id="GUID-5DFBB7F5-2F30-50C7-9144-3D6D85C0ACA1"><p>start a new

process - a call to <xref href="GUID-9DD1EA2B-DC59-315C-8E9C-CE6D9461B695.dita#GUID-9DD1EA2B-DC59-315C-8E9C-CE6D9461B695/GUID-DD214BA3-907E-3C7F-93C6-924A9A115A02"><apiname>RProcess::Create()</apiname></xref>  </p> </li>

<li id="GUID-B88FC041-7643-5CA0-9796-A51E64CAA1DF"><p>load a DLL -

a call to <xref href="GUID-25327159-83D6-3507-B187-09EA4BB3727F.dita#GUID-25327159-83D6-3507-B187-09EA4BB3727F/GUID-4F1B2717-D34F-32A4-B6E6-03D0533186A3"><apiname>RLibrary::Load()</apiname></xref>  </p> </li>

<li id="GUID-A7578989-6BDB-5CEA-B3C3-0B5A08E57166"><p>load a logical

device driver - a call to <xref href="GUID-C197C9A7-EA05-3F24-9854-542E984C612D.dita#GUID-C197C9A7-EA05-3F24-9854-542E984C612D/GUID-AE0D51B7-7ADC-3C9F-ACAA-8F6D9EA0AEFA"><apiname>User::LoadLogicalDevice()</apiname></xref>. A logical device driver is a type of DLL. </p> </li>

<li id="GUID-34DC54FE-FCEF-5F83-B205-9BA5B7140680"><p>load a physical

device driver - a call to <xref href="GUID-C197C9A7-EA05-3F24-9854-542E984C612D.dita#GUID-C197C9A7-EA05-3F24-9854-542E984C612D/GUID-A0F4BF4A-9C58-3E5E-88E1-6D98597DDA18"><apiname>User::LoadPhysicalDevice()</apiname></xref>. A physical device driver is a type of DLL. </p> </li>

<li id="GUID-51318E5E-4534-589B-B4BD-92D73F832A6F"><p>load a file

system - a call to <xref href="GUID-E263C747-946F-35AA-9F1D-41833BD350FC.dita#GUID-E263C747-946F-35AA-9F1D-41833BD350FC/GUID-51353BE2-9133-39F5-83F3-A5DB88D858CA"><apiname>RFs::AddFileSystem()</apiname></xref>. A file

system is a type of DLL. A call to <codeph>RFs::AddFileSystem()</codeph> requires DiskAdmin capability. See <xref href="GUID-99D8A4E8-BC4F-39FF-A3DF-832CF0411629.dita"><apiname>ECapabilityDiskAdmin</apiname></xref>. </p> </li>

<li id="GUID-8394C231-341B-5EA6-B9C2-01F75685813D"><p>load a file

server extension - call to <xref href="GUID-E263C747-946F-35AA-9F1D-41833BD350FC.dita#GUID-E263C747-946F-35AA-9F1D-41833BD350FC/GUID-D45FD363-EEF9-3F77-8FA1-49270D8E89A0"><apiname>RFs::AddExtension()</apiname></xref>.

A file server extension is a type of DLL. This function requires DiskAdmin

<li id="GUID-F1889916-80EB-55B6-BEAF-F66164F3838C"><p>load a locale

- a call to <xref href="GUID-454ACAB5-1573-323C-820E-AF542B149D2A.dita#GUID-454ACAB5-1573-323C-820E-AF542B149D2A/GUID-9F04584E-DD0B-39CE-8666-88F67F3B83BF"><apiname>TExtendedLocale::LoadLocale()</apiname></xref>. </p> </li>

</ul> <p><b>Version numbers</b> </p> <p><b>The reason for version numbers</b> </p> <p>More than one version

of an EXE can exist on a device at the same time. More than one version

of a DLL can exist on a device at the same time. A device can store

versions of an EXE or DLL that are not compatible with an older version

of that EXE or DLL. </p> <p>A version number is a set of two numbers.

The loader uses these numbers to identify copies of the executable

file. The two numbers are called the major version number and the

minor version number. The build tools encode the two numbers into

the header of the executable file. When a DLL is built, the version

numbers are encoded into the import library. </p> <p><b>The structure of version information</b> </p> <p>DLLs are the

most common users of versions. You assign new values to the version

numbers of a DLL when you change the published API of the DLL. Symbian

recommends the following procedure: </p> <ul>

<li id="GUID-A3D6B7C6-E48E-5374-8EFE-29EB129FEE04"><p>change the minor

version number if you make a change to a DLL that is backward compatible.

An example is a new function in the DLL. </p> </li>

<li id="GUID-F6813400-1E6A-58B9-8360-7FD364F2153A"><p>change the major

version number if you make a change to a DLL that breaks an interface.

An example is the removal of a function or a change to the signature

of a function. An interface break is disruptive and must be prevented. </p> </li>

</ul> <p>The major and minor version numbers are defined in the <filepath>.mmp</filepath> file that defines the source code, header files and

other options that apply to the executable file. </p> <p>If you do

not specify version numbers, the build tools apply the following default

values: </p> <ul>

<li id="GUID-CBF4D098-3A83-53E6-9E54-0A9B487D6E22"><p>Major version

number : 10 </p> </li>

<li id="GUID-46E7516E-988B-5F92-9877-FEE17D3F26A7"><p>Minor version

number : 0 </p> </li>

</ul> <p>The major version number and the minor version number are

stored in the header of an EXE and DLL. The two numbers are stored

in a single 32 bit value. The high 16 bits hold the major version

number. The low 16 bits hold the minor version number. </p> <p>When

you build a DLL, the build tools encode the version numbers into the

filename of the import library (the <filepath>.lib</filepath> file).

The build tools also create an import library that does not have the

version numbers encoded as part of the filename.The following example

shows how the version numbers are encoded in the filename. The major

version number is (decimal) 11 and the minor version number is (decimal)

2. The version numbers in the filename are in hexadecimal. </p> <p> <filepath>abcdsrv{000B0002}.lib</filepath>  </p> <p>Notes: </p> <ul>

<li id="GUID-7DC90016-5A54-5939-84B1-DDB0B2D4D9F5"><p>The left bracket

and the right bracket are part of the name. </p> </li>

<li id="GUID-BC291976-EB6D-59D2-8941-64F6ED3AC191"><p>The first four

digits after the open bracket are the hexadecimal representation of

the major version number. </p> </li>

<li id="GUID-51590778-985F-5DCC-9191-E937B1058F51"><p>The second four

digits are the hexadecimal representation of the minor version number. </p> </li>

</ul> <p>The build tools do not encode the version numbers in the

filename of the executable file. </p> <p><b>Loader search rules</b> </p> <p>EXEs and DLLs can exist on more

than one drive. There can be more than one version of an EXE and more

than one version of a DLL. The loader uses search rules to select

the EXE and DLL to load. </p> <p>See <xref href="GUID-D8837969-74D0-5E17-AD42-3F10DD1FD5BF.dita">Loader search rules</xref>  </p> <p><b>Emulator</b> </p> <p>Version numbers are stored in import files

for DLLs that run on devices and the emulator. Version numbers change

how the loader functions on devices, but not in the emulator. You

cannot encode the version numbers in the import library file name

on the emulator. </p> <p><b>Tools support</b> </p> <p>Use the build tools to apply version

numbers to EXEs and DLLs. You specify version numbers in the <filepath>.mmp</filepath> file that describes the EXE or DLL. </p> <p>See also <uicontrol>MMP file syntax</uicontrol> in <uicontrol>Symbian Tools Guide &gt; Building

 &gt; Symbian Build System (SBSv1) &gt; Build tools reference</uicontrol>.</p> </section>

relationships</title> <p>The loader is a server that runs in a separate

thread in the file server process. The loader is built as part of

the file server. </p> <p>The loader server starts when the file server

starts. The file server starts after the kernel is loaded at system

start. </p> <p>The loader runs on the user side, but calls the kernel

for services that are not available to general applications. Examples

of services are process creation and code segment creation. A code

segment is an object that represents a piece of code loaded into RAM.

A code segment also represents a piece of code that runs directly

from ROM. The kernel services that the loader uses are protected.

The file server process only is allowed to access these services. </p> <p>The following picture is an example of program flow. In this

example, a client calls <xref href="GUID-25327159-83D6-3507-B187-09EA4BB3727F.dita#GUID-25327159-83D6-3507-B187-09EA4BB3727F/GUID-4F1B2717-D34F-32A4-B6E6-03D0533186A3"><apiname>RLibrary::Load()</apiname></xref>, which

exists in <filepath>euser.dll</filepath>. Internal functions make

client-server calls to the loader server. The loader server code exists

in <filepath>efile.exe</filepath>. The loader server uses kernel services. </p> <fig id="GUID-5FCAE81A-19BC-5BE7-89D2-096A87735381">

<image href="GUID-2735BDAF-295F-5F1E-8925-8F4212D95CD1_d0e269008_href.png" placement="inline"/>