Loader Overview
This topic describes the overview, purpose, key concepts and architecture of loader.
Purpose
The loader makes EXEs and DLLs ready to run.
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.
Key concepts
The loader server
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:
start a new process - a call to RProcess::Create()
load a DLL - a call to RLibrary::Load()
load a logical device driver - a call to User::LoadLogicalDevice(). A logical device driver is a type of DLL.
load a physical device driver - a call to User::LoadPhysicalDevice(). A physical device driver is a type of DLL.
load a file system - a call to RFs::AddFileSystem(). A file system is a type of DLL. A call to RFs::AddFileSystem() requires DiskAdmin capability. See ECapabilityDiskAdmin.
load a file server extension - call to RFs::AddExtension(). A file server extension is a type of DLL. This function requires DiskAdmin capability. See ECapabilityDiskAdmin.
load a locale - a call to TExtendedLocale::LoadLocale().
Version numbers
The reason for version numbers
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.
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.
The structure of version information
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:
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.
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.
The major and minor version numbers are defined in the .mmp file that defines the source code, header files and other options that apply to the executable file.
If you do not specify version numbers, the build tools apply the following default values:
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.
When you build a DLL, the build tools encode the version numbers into the filename of the import library (the .lib 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.
abcdsrv{000B0002}.lib
Notes:
The left bracket and the right bracket are part of the name.
The first four digits after the open bracket are the hexadecimal representation of the major version number.
The second four digits are the hexadecimal representation of the minor version number.
The build tools do not encode the version numbers in the filename of the executable file.
Loader search rules
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.
Emulator
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.
Tools support
Use the build tools to apply version numbers to EXEs and DLLs. You specify version numbers in the .mmp file that describes the EXE or DLL.
See also mmp file syntax and Build tools guide.
Architectural relationships
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.
The loader server starts when the file server starts. The file server starts after the kernel is loaded at system start.
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.
The following picture is an example of program flow. In this example, a client calls RLibrary::Load(), which exists in euser.dll. Internal functions make client-server calls to the loader server. The loader server code exists in efile.exe. The loader server uses kernel services.
