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From: mheaney@ni.net (Matthew Heaney)
Subject: Re: Environment variables
Date: 1996/11/03
Message-ID: <mheaney-ya023180000311961747550001@news.ni.net>#1/1
X-Deja-AN: 194231915
references: <55819q$mql@newslink.runet.edu>
 <EACHUS.96Oct30201754@spectre.mitre.org>
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organization: Estormza Software
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newsgroups: comp.lang.ada
Date: 1996-11-03T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <EACHUS.96Oct30201754@spectre.mitre.org>,
eachus@spectre.mitre.org (Robert I. Eachus) wrote:


>with System; use System;
>function Get_Environment(Variable : String) return String is
>-- Return the value of the given environment variable.
>-- If there's no such environment variable, return an empty string.
>  type String_Pointer is access String(1..Integer'LAST);
>  function getenv(Variable : System.Address) return String_Pointer;
>  pragma Interface(C, getenv);
>  -- getenv is a standard C library function; see K&R 2, 1988, page 253.
>  -- it returns a pointer to the first character; do NOT free its results.
>  Variable_In_C_Format : constant String := Variable&Ascii.Nul;
>  Result_Ptr : String_Pointer := getenv(Variable_In_C_Format(1)'ADDRESS);
>  function strlen(Pointer: in String_Pointer) return Integer;
>  pragma Interface(C,strlen);
>begin
>  if Result_Ptr = null
>  then return "";
>  else return Result_Ptr(1..strlen(Result_Ptr));
>  end if;
>end Get_Environment;

This isn't a very good way to interface Ada string pointers to C, because
this example assumes the string access object designates the first
character of the string.  This is a very dangerous assumption, because a
string access object could be pointing to a string descriptor, not the
string itself.  In fact, this is how access types are implemented in DEC
Ada.

The issue is with pointers to unconstrained arrays.  A more portable way is
to make the string access type designate a constrained array; thay way, no
dope vector is required.

function Get_Environment (Variable : String) return String is

   type Character_Array is array (Positive) of Character;
   type String_Access is access Character_Array;    -- more portable
   pragma String_Access'Storage_Size = 0;

   Variable_In_C_Format : constant String := Variable & ASCII.NUL;

   function Getenv (S : System.Address) return String_Access;
   pragma Interface (C, Getenv);

   The_Environment_Value : constant String_Access :=
      Getenv (Variable_In_C_Format (1)'Address);
...

Generally, if the accessed type is constrained, no dope vector is required,
and access objects designate the array (in this case), not a descriptor, so
that the Ada string pointer matches the C string pointer.

And remember to set the storage_size of the pool to 0, to explicitly
indicate that no memory is being allocated by the Ada runtime environment.

--------------------------------------------------------------------
Matthew Heaney
Software Development Consultant
mheaney@ni.net
(818) 985-1271