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

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
...