Thanks for the detailed explanation.  Makes sense now.

On Thu, Jan 28, 2021 at 5:43 PM Nate DeSimone <
nathaniel.l.desimone@intel.com> wrote:

> Hi There,
>
> First of all, welcome!
>
> So, “NULL” library classes are conceptually an "anonymous library". It
> enables one to statically link code into a module even if the module
> doesn't directly call functions in that library. All libraries, both
> regular libraries with a declared LibraryClass as well as these anonymous
> libraries, can publish both a constructor and a destructor. The EDK II
> build system will automatically generate a small amount of C code that
> invokes all library constructors before the entry point for the module is
> invoked, and all destructors after the entry point returns. This is useful
> for building statically linked plug-ins.
>
> I think my favorite example of this in-action is the UEFI Shell, here is
> what a typical DSC declaration for the UEFI Shell looks like:
>
> ShellPkg/Application/Shell/Shell.inf {
>   <PcdsFixedAtBuild>
>     gEfiShellPkgTokenSpaceGuid.PcdShellLibAutoInitialize|FALSE
>   <LibraryClasses>
>
> NULL|ShellPkg/Library/UefiShellLevel1CommandsLib/UefiShellLevel1CommandsLib.inf
>
> NULL|ShellPkg/Library/UefiShellLevel2CommandsLib/UefiShellLevel2CommandsLib.inf
>
> NULL|ShellPkg/Library/UefiShellLevel3CommandsLib/UefiShellLevel3CommandsLib.inf
>
> NULL|ShellPkg/Library/UefiShellDriver1CommandsLib/UefiShellDriver1CommandsLib.inf
>
> NULL|ShellPkg/Library/UefiShellInstall1CommandsLib/UefiShellInstall1CommandsLib.inf
>
> NULL|ShellPkg/Library/UefiShellDebug1CommandsLib/UefiShellDebug1CommandsLib.inf
>
> NULL|ShellPkg/Library/UefiShellNetwork1CommandsLib/UefiShellNetwork1CommandsLib.inf
>
> NULL|ShellPkg/Library/UefiShellNetwork2CommandsLib/UefiShellNetwork2CommandsLib.inf
>
> ShellCommandLib|ShellPkg/Library/UefiShellCommandLib/UefiShellCommandLib.inf
>
> HandleParsingLib|ShellPkg/Library/UefiHandleParsingLib/UefiHandleParsingLib.inf
>
> BcfgCommandLib|ShellPkg/Library/UefiShellBcfgCommandLib/UefiShellBcfgCommandLib.inf
>
> ShellCEntryLib|ShellPkg/Library/UefiShellCEntryLib/UefiShellCEntryLib.inf
>     ShellLib|ShellPkg/Library/UefiShellLib/UefiShellLib.inf
> }
>
> If you take a look at
> ShellPkg/Library/UefiShellLevel1CommandsLib/UefiShellLevel1CommandsLib.inf,
> you will see that the constructor for that anonymous library is named
> ShellLevel1CommandsLibConstructor(). Now, let's go and look at the
> definition for ShellLevel1CommandsLibConstructor() in
> ShellPkg/Library/UefiShellLevel1CommandsLib/UefiShellLevel1CommandsLib.c
> and note the following code snippet:
>
> ShellCommandRegisterCommandName(L"stall",  ShellCommandRunStall   ...
> ShellCommandRegisterCommandName(L"for",    ShellCommandRunFor     ...
> ShellCommandRegisterCommandName(L"goto",   ShellCommandRunGoto    ...
> ShellCommandRegisterCommandName(L"if",     ShellCommandRunIf      ...
> ShellCommandRegisterCommandName(L"shift",  ShellCommandRunShift   ...
> ShellCommandRegisterCommandName(L"exit",   ShellCommandRunExit    ...
> ShellCommandRegisterCommandName(L"else",   ShellCommandRunElse    ...
> ShellCommandRegisterCommandName(L"endif",  ShellCommandRunEndIf   ...
> ShellCommandRegisterCommandName(L"endfor", ShellCommandRunEndFor  ...
>
> This library is installing new commands into the UEFI shell during its
> initialization procedure. This allows one to add custom commands to the
> shell as statically linked built-ins. A typical use case would be
> implementing platform specific diagnostic/recovery utilities.
>
> Hope that helps!
> Nate
>
> > From: devel@edk2.groups.io <devel@edk2.groups.io> On Behalf Of
> wonderfly@waymo.com
> > Sent: Thursday, January 28, 2021 10:22 AM
> > To: devel@edk2.groups.io
> > Subject: [edk2-devel] What is a "NULL" library class?
> >
> > Hi,
> >
> > I am super new to EDK2 development so excuse my ignorance.  While
> browsing the code base I find a lot places where the keyword "NULL" is used
> in place of a LibraryClass, specifically in DSC files.  For example,
> > in edk2/UefiPayloadPkg/UefiPayloadPkgIa32.dsc:
>
> >   MdeModulePkg/Core/Dxe/DxeMain.inf {
> >     <LibraryClasses>
> >
>  NULL|MdeModulePkg/Library/LzmaCustomDecompressLib/LzmaCustomDecompressLib.inf
> >   }
> >
> > What does this mean?  Does it mean that the LzmaCustomDecompressLib.inf
> library instance is unconditionally linked for this package?
> >
>
>
> 
>
>
>

-- 
Best,
Daniel