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Newsgroups: comp.lang.ada
Date: Mon, 22 Jun 2020 10:52:20 -0700 (PDT)
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Subject: Re: How to supply O/S linker arguments with gprbuild?
From: Warren <ve3wwg@gmail.com>
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Xref: reader01.eternal-september.org comp.lang.ada:59164
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On Monday, 22 June 2020 12:39:05 UTC-4, Dmitry A. Kazakov  wrote:
> On 22/06/2020 17:51, Warren wrote:
> > I have a GNAT project that has these pragma statements embedded to get =
the sucker to link. However, the directory is only valid for an instance on=
 my Mac, so I need a better solution to replace:
> >=20
> >     pragma Linker_Options("-L/usr/local/Cellar/readline/8.0.4/lib");
> >     pragma Linker_Options("-lreadline");
> >=20
> > I've tried to find the solution in the gnat documents but they seem to =
fall short of what I need to do (could not find an example nor direction in=
 this -- perhaps I missed it).
> >=20
> > My project file looks something like this:
> >=20
> > project MyProject is
> >      for Languages use ("ada", "c");
> >      for Source_Dirs use ("src");
> >      for Object_Dir use "obj";
> >      for Main use ("main.adb");
> > end MyProject;
> >=20
> > I've tried a variety of gpr things, but unsuccessfully.
>=20
> There are many ways to do this.
>=20
> Here I give one rarely used but most flexible and consistent with the=20
> idea of projects.
>=20
> If you have an alien library like readline, you can pack it in a=20
> separate library project like this:
>=20
> project Readline is
>     for Externally_Built use "true"; -- Do not build it
>     for Source_Files use ();         -- No sources
>     for Library_Dir use "/usr/local/Cellar/readline/8.0.4/lib";
>     for Library_Name use "readline"; -- OS naming conventions applied
>     for Library_Kind use "dynamic";  -- Or static,
> end Readline;                       -- depending on what you have
>=20
> In your project or projects you would simply do
>=20
> with "readline.gpr";
>=20
> and that is all.
>=20
> Adjust the above using a scenario variable to select the target OS, e.g.
>=20
>     type OS_Type is
>          (  "Windows", "Windows_NT",
>             "Linux",
>             "UNIX",
>             "OSX",
>             "FreeBSD",
>             ...
>          );
>     Target_OS : OS_Type :=3D external ("Target_OS", "MS-DOS");
>=20
>     case Target_OS is
>        when "ms-dos" =3D>
>           for Library_Dir use "...";
>        when "osx" =3D>
>           for Library_Dir use "...";
>           ...

The above might be useful for some projects, but I didn't want to define a =
package for each and every library I link with. I have a simple Makefile th=
at drives this project, and I simply wanted a central place to configure th=
e pathnames.=20

So I managed to get this to work:

project MyProject is  =20
  for Languages use ("ada", "c");
  for Source_Dirs use ("src");
  for Object_Dir use "obj";
  for Main use ("main.adb");
  package Linker is
    for Required_Switches use ( "-L/usr/local/Cellar/readline/8.0.4/lib", "=
-lreadline" );
  end Linker;
end MyProject;  =20

In the final version of the .gpr file, I'll put some sentinel text like <re=
adline_dir> and substitute it as required using sed from the Makefile.  Cur=
rently there is only one path I have to deal with but it might grow to seve=
ral. I didn't want to define a package for each and every library reference=
d.

I find it amazing that there isn't a single instance of an example like thi=
s that I could goggle. The gprbuild doc seems to expend a great deal explai=
ning every other use case, including building of library packages. All I wa=
nted to do was to link an executable. That said, it is possible that my fai=
ling eyesight may have overlooked it.

Thanks everyone,
Warren