This page gives an overview of EDK II libraries.

Table of Contents Classes vs. Implementation Classes Using a library class Defining a class Implementations Using a library implementation Library Constructors How library constructors work

Libraries in edk2 are defined in two distinct ways. Each library has a 'class' and one or more 'implementations'.

A library class defines the C language based interface to the library. Generally this involves prototypes of C based functions.

A library implementation is an instance of a library that implements the library class. It is possible to have multiple implementations for the same library class.

Code which uses a library in edk2 should generally only focus on the library class interface. The details of which library implementation are used can be decided at a later point, and even altered easily within the build process.

If a driver or another library wants to make use of a library, then only a few changes must be made.

1. Include the package .dec file in the driver or library .inf file. The syntax in the .inf will look like this:

[Packages]

  MdePkg/MdePkg.dec

This will enable the code built in the driver or library to have the package include directories added to its include path. So, when the .inf change above is made, now the MdePkg/Include directory will be added to the include path.

2. Include the library class .h file. Within a .c or .h file within the driver or library code, add the #include. For example, to use the MdePkg/Include/Library/CpuLib.h library class:

include "Library/CpuLib.h"

3. Add the library class name to the driver or library's .inf file. For example:

[LibraryClasses]

  CpuLib

A library class is defined in a C language .h file. A simple example is MdePkg/Include/Library/CpuLib.h. This library class declares a C prototype for a function:

VOID
EFIAPI
CpuSleep (

  VOID
  );

In addition to defining the class .h file, you will need to ensure that the package .dec file contains the include path necessary to include the library class .h file. For example, in MdePkg/MdePkg.dec:

[Includes]

  Include

Now, when a .inf file uses MdePkg/MdePkg.dec:

[Packages]

  MdePkg/MdePkg.dec

The MdePkg/Include path will be added to the include paths for building that module.

The .dsc file used in your build process defines which library implementation is actually used by a driver.

For example, in MdeModulePkg/MdeModulePkg.dsc, BaseMemoryLib is mapped as:

[LibraryClasses]

  BaseMemoryLib|MdePkg/Library/BaseMemoryLib/BaseMemoryLib.inf

But, note that this mapping only applies when MdeModulePkg/MdeModulePkg.dsc is built. If another .dsc is used, it must specify the library mappings itself, and the MdeModulePkg/MdeModulePkg.dsc will have no impact on the mapping when the other .dsc file is built.

If a library requires some initialization code to run, then it can define a constructor function. For example, in MdePkg/Library/UefiBootServicesTableLib/UefiBootServicesTableLib.inf:

  CONSTRUCTOR                    = UefiBootServicesTableLibConstructor

This function in MdePkg/Library/UefiBootServicesTableLib/UefiBootServicesTableLib.c is then called before the driver entry point.

During the build process, the library constructors are auto-generated to be called from a function named ProcessLibraryConstructorList.

To see an example of this, after building look for a driver's AutoGen.c file underneath the build directory.

The ProcessLibraryConstructorList will then be called by the entry point library code. For an example of this, see MdePkg/Library/UefiDriverEntryPoint/DriverEntryPoint.c.