Re: Linker_Alias

[Simon Wright <simon@pushface.org>]

Jere <jhb.chat@gmail.com> 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.

One thing that confused me for a while was that you have to ensure that
the overriding package actually gets linked; my comment says

   --  Normally, we'd mark this unit as preelaborable, but that would
   --  mean that the binder-generated code wouldn't reference it, so
   --  it wouldn't get referenced at all (because the weak symbol in
   --  startup.o would satisfy the link).

https://github.com/simonjwright/cortex-gnat-rts/blob/master/common/hardfault_handling.ads
(and .adb).