Re: Linker_Alias

[jhb.games@gmail.com]

On Wednesday, January 3, 2018 at 4:59:11 PM UTC-5, Simon Wright wrote:
> Jere writes:
> 
> > I know this is more of a GNAT question than an Ada question
> > but was hoping someone else had some insight to this pragma.
> >
> > I am trying to use pragma Linker_Alias to set up a default
> > operation for an operation defined with pragma Weak_External.  
> [...]
> > The intent is that GNAT links in a default operation unless
> > the client exports a procedure with the intended link name.
> >
> > The use case is for ZFP runtimes where I don't have access
> > to protected procedures.  I want to supply a dummy ISR
> > unless the client decides to override it.  
> >
> > I know GCC itself supports this (I tested it in another
> > language just to make sure), but I cannot figure out how
> > to get GNAT to support it.
> >
> > Am I setting up the pragma's wrong?  I've tried a few different
> > combinations, but with no luck.
> >
> > I'm on Windows 10 using GNAT GPL 2017 (same problem on 2015 and 2016).
> 
> On macOS with GCC8 it failed to link (symbol "target" missing).
> 
> I never came across pragma Linker_Alias (and the description isn't easy
> to get my head round). I had a similar problem in Cortex GNAT RTS: the
> default was
> 
>    procedure HardFault_Handler
>    with Export, Convention => Ada, External_Name => "HardFault_Handler";
>    pragma Weak_External (HardFault_Handler);
>    procedure HardFault_Handler is
>    begin
>       loop
>          null;
>       end loop;
>    end HardFault_Handler;
> 
> and then, if required, override by providing a package containing
> 
>    procedure Handler
>    with
>      Export,
>      Convention => Asm,
>      External_Name => "HardFault_Handler";
>    pragma Machine_Attribute (Handler, "naked");
> 
> whose body works out which stack is in use and sets up for debugging.


Thanks!

I tried that but ran into the same problem.  I added:

   procedure Target_Op_2
      with
         Convention => Ada,
         Export,
         External_Name => "t2",
         Link_Name => "t2";
   pragma Weak_External(Target_Op_2);

to Operations.ads, and

   procedure Target_Op_2 is
   begin
      Put_Line("t2");
   end Target_Op_2;

to Operations.adb but still got the same exception when calling
from the entry procedure:

with Ada.Text_IO; use Ada.Text_IO;
with Operations;

procedure Main is
begin
   Put_Line("Hello");
   Operations.Target_Op_2;
end Main;  

********************
Hello

raised PROGRAM_ERROR : EXCEPTION_ACCESS_VIOLATION
********************

So it looks like it still isn't defaulting the procedure.

Note
that I am testing this in the standard windows GNAT GPL
version as opposed to the cross compiler.  I'll get to
that, but I had to redo my environment for it.  I just
wanted to test out the functionality.

Any thoughts?