Including edk2-libc in efi shell application

Question

How would one approach adding support for https://github.com/tianocore/edk2-libc, say I want to include stdio and use printf in my edk2 application? I followed StdLib/Readme.txt, and am able to successfully build examples in the AppPkg, however, when I try to add StdLib to my project I get errors like these:

LibString.lib(Searching.obj) : error LNK2005: strspn already defined in LibString.lib(Searching.obj)
LibCtype.lib(CClass.obj) : error LNK2005: isspace already defined in LibCtype.lib(CClass.obj)
(...)
LibC.lib(Main.obj) : error LNK2001: unresolved external symbol main

I do have the boilerplate (!include StdLib/StdLib.inc) added to my dsc file and in inf, I have StdLib.dec added to Packages and LibC and LibStdio added to LibraryClasses. I am using VS2017 toolchain for compilation and am using edk2-stable202108 release.

Answer 1

I was able to achieve this using below configuration for Hello Application of AppPkg.

Hello.inf

[Defines]
  INF_VERSION                    = 0x00010006
  BASE_NAME                      = Hello
  FILE_GUID                      = a912f198-7f0e-4803-b908-b757b806ec83
  MODULE_TYPE                    = UEFI_APPLICATION
  VERSION_STRING                 = 0.1
  ENTRY_POINT                    = ShellCEntryLib

#
#  VALID_ARCHITECTURES           = IA32 X64
#

[Sources]
  Hello.c

[Packages]
  MdePkg/MdePkg.dec
  ShellPkg/ShellPkg.dec
  StdLib/StdLib.dec

[LibraryClasses]
  UefiLib
  ShellCEntryLib
  BaseLib
  BaseMemoryLib
  MemoryAllocationLib
  LibStdLib
  LibStdio
  LibString
  DevConsole

Hello.c

#include  <Uefi.h>
#include  <Library/UefiLib.h>
#include  <Library/ShellCEntryLib.h>
#include  <stdio.h>

int
main (
  IN int Argc,
  IN char **Argv
  )
{
    printf("Hello, world!\n");
    
  return 0;
}

What I have understood is that LibC has ShellAppMain() defined in it which internally calls extern main(). So You need to provide definition of main() in your source just like I did in Hello.c