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From: "Rego, P." <pvrego@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: Read Windows login username in Ada 95
Date: Wed, 9 Nov 2011 12:47:57 -0800 (PST)
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Date: 2011-11-09T12:47:57-08:00
List-Id: <comp.lang.ada>

Well, answering myself, we could use

function GetUsername return String is
   function GetEnv (Variable : String) return Interfaces.C.Strings.chars_ptr;
   pragma Import (C, GetEnv, "getenv");

   Command : constant String := "USERNAME";
   Answer_Ptr : constant Interfaces.C.Strings.chars_ptr := GetEnv (Command);
   Answer : constant String := Interfaces.C.Strings.Value (Answer_Ptr);
begin
   return Answer;
end GetUsername;

Not pure Ada, but fits very well.
Thank to all suggestions.