Add lesson 36

Signed-off-by: Konstantin Aladyshev <aladyshev22@gmail.com>

1 files changed, 157 insertions, 0 deletions

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+In this lesson we will try to create the most simple library.
+
+Usually libraries are present in these directories:
+```
+<Pkg Name>/Library/<Library Name>/               <---- inf and source files for the library (=library implementation)
+<Pkg Name>/Include/Library/                      <---- library headers (=library interface)
+```
+
+Create folders for our `SimpleLibrary`:
+```
+$ mkdir -p UefiLessonsPkg/Library/SimpleLibrary/
+$ mkdir -p UefiLessonsPkg/Include/Library/
+```
+
+First let's implement the header file, the interface for our library. Our `SimpleLibrary` would contain the only function `Plus2` that would receive a `number` and return a `number+2`.
+Therefore the content in the header file (`UefiLessonsPkg/Include/Library/SimpleLibrary.h`) would look like this:
+```
+UINTN Plus2(UINTN number);
+```
+
+No harder the library implementation file `UefiLessonsPkg/Library/SimpleLibrary/SimpleLibrary.c`:
+```
+#include <Library/SimpleLibrary.h>
+
+UINTN Plus2(UINTN number) {
+  return number+2;
+}
+```
+
+This is really a simple library, it stands to its name!
+
+Now we need to create an INF file for the library `UefiLessonsPkg/Library/SimpleLibrary/SimpleLibrary.inf`:
+```
+[Defines]
+  INF_VERSION                    = 1.25
+  BASE_NAME                      = SimpleLibrary
+  FILE_GUID                      = 826c8951-5bd2-4d72-a9d9-f7ab48684117
+  MODULE_TYPE                    = UEFI_APPLICATION
+  VERSION_STRING                 = 1.0
+  LIBRARY_CLASS                  = SimpleLibrary | UEFI_APPLICATION
+
+[Sources]
+  SimpleLibrary.c
+
+[Packages]
+  MdePkg/MdePkg.dec
+```
+
+The interesting string here is the:
+```
+LIBRARY_CLASS = SimpleLibrary | UEFI_APPLICATION
+```
+It says that this library can only be used in modules with a type `UEFI_APPLICATION`. If you would say here `DXE_DRIVER` and try to link it to some of you UEFI applications, build process would fail. The error message would look like this:
+```
+build.py...
+/home/kostr/tiano/edk2/UefiLessonsPkg/UefiLessonsPkg.dsc(...): error 1001: Module type [UEFI_APPLICATION] is not supported by library instance [/home/kostr/tiano/edk2/UefiLessonsPkg/Library/SimpleLibrary/SimpleLibrary.inf]
+        consumed by [/home/kostr/tiano/edk2/UefiLessonsPkg/<path to your app inf file>]
+```
+
+Now we need to include our library to our package DSC file `UefiLessonsPkg/UefiLessonsPkg.dsc`, so it would get build on a package build:
+```
+[Components]
+  ...
+  UefiLessonsPkg/Library/SimpleLibrary/SimpleLibrary.inf
+```
+
+But if you try to build our package now build would fail with a message:
+```
+/home/kostr/tiano/edk2/UefiLessonsPkg/Library/SimpleLibrary/SimpleLibrary.c:1:10: fatal error: Library/SimpleLibrary.h: No such file or directory
+    1 | #include <Library/SimpleLibrary.h>
+      |          ^~~~~~~~~~~~~~~~~~~~~~~~~
+compilation terminated.
+```
+The reason of that is a fact that our `UefiLessonsPkg/Include/Library/` folder is not recognized by a build system as a place where headers might be.
+
+To fix it we need to add to our `UefiLessonsPkg/UefiLessonsPkg.dec` file `[Includes]` section:
+```
+[Includes]
+  Include
+```
+
+And include this `UefiLessonsPkg/UefiLessonsPkg.dec` file to the library module INF file section `[Packages]`:
+```
+[Packages]
+  MdePkg/MdePkg.dec
+  UefiLessonsPkg/UefiLessonsPkg.dec
+```
+
+Now we are good, build would succeed.
+
+# SimpleLibraryUser
+
+Now let's create an application that would use our library.
+
+UefiLessonsPkg/SimpleLibraryUser/SimpleLibraryUser.c:
+```
+#include <Library/UefiBootServicesTableLib.h>
+#include <Library/UefiLib.h>
+
+#include <Library/SimpleLibrary.h>
+
+EFI_STATUS
+EFIAPI
+UefiMain (
+  IN EFI_HANDLE        ImageHandle,
+  IN EFI_SYSTEM_TABLE  *SystemTable
+  )
+{
+  Print(L"%d\n", Plus2(3));
+
+  return EFI_SUCCESS;
+}
+```
+
+UefiLessonsPkg/SimpleLibraryUser/SimpleLibraryUser.inf
+```
+[Defines]
+  INF_VERSION                    = 1.25
+  BASE_NAME                      = SimpleLibraryUser
+  FILE_GUID                      = 22a1f57c-21ca-4011-9133-e3df0d01dace
+  MODULE_TYPE                    = UEFI_APPLICATION
+  VERSION_STRING                 = 1.0
+  ENTRY_POINT                    = UefiMain
+
+[Sources]
+  SimpleLibraryUser.c
+
+[Packages]
+  MdePkg/MdePkg.dec
+  UefiLessonsPkg/UefiLessonsPkg.dec                  <--- we need to include this for the same reason as in library INF file (for the header search)
+
+[LibraryClasses]
+  UefiApplicationEntryPoint
+  UefiLib
+  SimpleLibrary                                      <--- library is included as usual
+```
+
+Now add modifications to the `UefiLessonsPkg/UefiLessonsPkg.dsc`:
+```
+[LibraryClasses]
+  ...
+  SimpleLibrary|UefiLessonsPkg/Library/SimpleLibrary/SimpleLibrary.inf
+
+[Components]
+  ...
+  UefiLessonsPkg/SimpleLibraryUser/SimpleLibraryUser.inf
+```
+Here we've added implementation for our library class and added our new module to the package components.
+
+If you build everything now and execute it under OVMF, you would get:
+```
+FS0:\> SimpleLibraryUser.efi
+5
+```
+
+`3+2` is indeed `5`, so our library works correctly!
+