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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: How to supply O/S linker arguments with gprbuild?
Date: Mon, 22 Jun 2020 18:39:02 +0200
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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:
> 
>     pragma Linker_Options("-L/usr/local/Cellar/readline/8.0.4/lib");
>     pragma Linker_Options("-lreadline");
> 
> 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).
> 
> My project file looks something like this:
> 
> 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;
> 
> I've tried a variety of gpr things, but unsuccessfully.

There are many ways to do this.

Here I give one rarely used but most flexible and consistent with the 
idea of projects.

If you have an alien library like readline, you can pack it in a 
separate library project like this:

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

In your project or projects you would simply do

with "readline.gpr";

and that is all.

Adjust the above using a scenario variable to select the target OS, e.g.

    type OS_Type is
         (  "Windows", "Windows_NT",
            "Linux",
            "UNIX",
            "OSX",
            "FreeBSD",
            ...
         );
    Target_OS : OS_Type := external ("Target_OS", "MS-DOS");

    case Target_OS is
       when "ms-dos" =>
          for Library_Dir use "...";
       when "osx" =>
          for Library_Dir use "...";
          ...

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de