How can I access a symbol from the linker script in my Ada code?

Question

I am building my Ada/SPARK project using GNAT and I am using a linker script. Here is an excerpt:

SECTIONS
{
    .code :
    {
        . = ALIGN(0x4);
        *(.text.section1)
        _end_of_section1 = .;
        *(.text.section2)
        ...
    }
}

The symbol _end_of_section1 is the address between the two sections. I'd like to be able to access this in my Ada code. I know it's possible in C using extern char _end_of_section1[];. Is it possible to do something like this in Ada? If not, is there some other way to get this address in the code?

Answer 1

You can import a linker symbol by using the Importand Link_Name aspects (see also RM B.1):

main.adb (updated on 25-jan)

with System.Storage_Elements;
with System.Address_Image;
with Ada.Text_IO; use Ada.Text_IO;

procedure Main is   
      
   package SSE renames System.Storage_Elements;

   package Storage_Element_IO is
     new Ada.Text_IO.Modular_IO (SSE.Storage_Element);
   use Storage_Element_IO;
       
   Start_Symbol : aliased SSE.Storage_Element
     with Import, Link_Name => "_start";
   
   Start_Symbol_Addr : constant System.Address := Start_Symbol'Address;

begin
   Put ("Address : ");
   Put (System.Address_Image (Start_Symbol_Addr));   
   New_Line;
   
   Put ("Value   : ");
   Put (Start_Symbol, Base => 16);
   New_Line;
   
end Main;

output

$ ./obj/main 
Address : 0000000000403300
Value   : 16#F3#

output (objdump)

$ objdump -d -M intel ./obj/main | grep -A5 "<_start>"
0000000000403300 <_start>:
  403300:   f3 0f 1e fa             endbr64 
  403304:   31 ed                   xor    ebp,ebp
  403306:   49 89 d1                mov    r9,rdx
  403309:   5e                      pop    rsi
  40330a:   48 89 e2                mov    rdx,rsp
  ...