source: bootcd/isolinux/syslinux-6.03/gnu-efi/gnu-efi-3.0/apps/t.c

#include <efi.h>
#include <efilib.h>

static CHAR16 *
a2u (char *str)
{
        static CHAR16 mem[2048];
        int i;

        for (i = 0; str[i]; ++i)
                mem[i] = (CHAR16) str[i];
        mem[i] = 0;
        return mem;
}

EFI_STATUS
efi_main (EFI_HANDLE image_handle, EFI_SYSTEM_TABLE *systab)
{
        SIMPLE_TEXT_OUTPUT_INTERFACE *conout;

        InitializeLib(image_handle, systab);
        conout = systab->ConOut;
        uefi_call_wrapper(conout->OutputString, 2, conout, (CHAR16 *)L"Hello World!\n\r");
        uefi_call_wrapper(conout->OutputString, 2, conout, a2u("Hello World!\n\r"));

        return EFI_SUCCESS;
}