From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00
	autolearn=unavailable autolearn_force=no version=3.4.4
Path: 
 eternal-september.org!reader01.eternal-september.org!reader02.eternal-september.org!.POSTED!not-for-mail
From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Linker_Alias
Date: Wed, 03 Jan 2018 21:59:08 +0000
Organization: A noiseless patient Spider
Message-ID: <lypo6qd2oj.fsf@pushface.org>
References: <6042baa6-7743-48aa-8237-87181ab41a02@googlegroups.com>
Mime-Version: 1.0
Content-Type: text/plain
Injection-Info: reader02.eternal-september.org;
 posting-host="374867fda94719e6d8d0cf912deeb372";
	logging-data="27099"; mail-complaints-to="abuse@eternal-september.org";
	posting-account="U2FsdGVkX18sqiYupxEBnVkpXHQatyjJ3AMHURZE6ak="
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/25.3 (darwin)
Cancel-Lock: sha1:TmkOqGzGElzM2Uz8gTXFlvuo0b0=
	sha1:s8tdvNN0Yvq14jkMz2wjjZEtd1k=
Xref: reader02.eternal-september.org comp.lang.ada:49740
Date: 2018-01-03T21:59:08+00:00
List-Id: <comp.lang.ada>

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).