In lesson2 why since there is UefiLessonsPkg why we have include MdePkg/MdePkg.dec in SimplestApp.inf?

[joyoseller20100802]

[Kostr]

Generally if we use a library or PCD from another package, we would need to include declaration (DEC) file for that package.

Why is that? Let's comment the Packages section in our UefiLessonsPkg/SimplestApp/SimplestApp.inf file:

#[Packages]
#  MdePkg/MdePkg.dec

In this case the build would fail with this error:

edk2/Build/UefiLessonsPkg/RELEASE_GCC5/X64/UefiLessonsPkg/SimplestApp/SimplestApp/DEBUG/AutoGen.h:16:10: fatal error: Uefi.h: No such file or directory
   16 | #include <Uefi.h>
      |          ^~~~~~~~

Here the build system complains that it can't find the header file Uefi.h. If you'll search for this file in the edk2 repo, you will find it in the MdePkg/Include folder (https://github.com/tianocore/edk2/blob/master/MdePkg/Include/Uefi.h)

But as you see the build system couldn't find it in our case.

How does including MdePkg/MdePkg.dec fixes the issue? If you'll look at the https://github.com/tianocore/edk2/blob/master/MdePkg/MdePkg.dec you'll see that it has these statements:

...
[Includes]
  Include
...

This adds another folder (MdePkg/Include) folder to search for include files. And this is what we need to make our build system happy.

Just in case here is a documentation for the INF file [Packages] section: https://edk2-docs.gitbook.io/edk-ii-inf-specification/3_edk_ii_inf_file_format/37_-packages-_sections

[joyoseller20100802]

My thought about it is that the uefi add package(dsc dec) and class(inf) Based on c (.c file and .h file) ,a class (a inf) have many c file and their reference information of c file of other package and corresponding class can get from its dsc file.but it has it own MdePkg/MdePkg.dec to reflect the reference information,I think it's more intuitive.

[joyoseller20100802]

In lesson 20 i thought it again ,inf can not use the information in dsc because dsc `s component include inf w if inf w use the information in its dsc can lead to cycle use ,so inf can not use the information in dsc in theory.

[joyoseller20100802]

Of cause this is only on case ,other cases such as lesson 2 inf can use the information of its dsc .