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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Checking for OS in gnatstudio project file(.gpr)
Date: Fri, 19 Jun 2020 17:43:43 +0100
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ldries46 <bertus.dries@planet.nl> writes:

> I em in neede for a possibility to check within a .gpr file which
> operating system is runnung. Is there a way to do that? So I can use
> the same project file without changing  the .gpr file when compiling a
> program on different OS'ses.

Alire settled on this[1] in an aggregate project after some
heartsearching:

   Host_OS := external ("OS", "default");
   --  On Windows an OS environment variable is defined, we can use it to
   --  determine if we are compiling on Windows.
   --
   --  On macOS, the nearest equivalent is OSTYPE; however this is
   --  e.g. "darwin18", so not useful here. Set "macOS" by hand.

   --  ALIRE_OS is used in alire_common.gpr.
   --  GNATCOLL_OS is used in gnatcoll.gpr.
   case Host_OS is
      when "Windows_NT" =>
         for External ("ALIRE_OS") use "windows";
         for External ("GNATCOLL_OS") use "windows";
      when "macOS"      =>
         for External ("ALIRE_OS") use "osx";
         for External ("GNATCOLL_OS") use "osx";
      when others       =>
         for External ("ALIRE_OS") use "unix";
         for External ("GNATCOLL_OS") use "unix";
   end case;

but I've had a lot of trouble with aggregate projects (I don't think
I've grasped what the use cases are). As I remember, it was package
Builder that was the issue.

[1] https://github.com/alire-project/alire/blob/master/alr_env.gpr#L22