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From: jerry@jvdsys.stuyts.nl (Jerry van Dijk)
Subject: Re: "system" Call in GNAT?
Date: 1999/04/05
Message-ID: <F9qqJM.37@jvdsys.stuyts.nl>#1/1
X-Deja-AN: 463065777
References: <3708E4C0.F13C2023@alphasoft-inc.com>
Organization: * JerryWare *, Leiden, Holland
Newsgroups: comp.lang.ada
Date: 1999-04-05T00:00:00+00:00
List-Id: <comp.lang.ada>

M. Smith (work@alphasoft-inc.com) wrote:

:     I was looking for a call in GNAT that would make invoke a command to the OS
: for me and  I found the function "system" and "execl".

Calling the OS is, of course, OS specific, so not addressed by Ada or an
Ada compiler (although a compiler may provided a vendor specific package
for it).

The system() call is the easiest to use, since you simply give it the command
to execute in a string, and of it goes.

However, the system() call is part of the C runtime library, not of the Win32
API, so you are making things more difficult than they need to be.

Here is example:

--  using the system call on Win32 to list a directory

with System;   --  to use the Address type

procedure List is

   --------------------------------------------------------------------
   procedure System_Call (S : in String) is

      --  The system() call is not part of the Win32 API, but of the C
      --  runtime library, which GNAT links in by default.

      --  Note that the system() call will go away and not return until
      --  its job is done.

      --  The return value of the system() call in the Win32 environment
      --  indicates the result of the system call, not the result of the
      --  command it tried to execute. Since system() is unlikely to
      --  fail I discard the return value here.

      procedure System_Call (Command : in System.Address);
      pragma Import (C, System_Call, "system");
      --  The system() call expects a C string pointer, which is the
      --  address of the first character in a character array terminated
      --  by a zero.

      --  Note the distingishing between the Ada name (System_Call) and
      --  the C name (system) which are 'linked' by the import pragma.

      Command : constant String := S & ASCII.NUL;
      --  Add a terminating zero to the Ada string to make it look like
      --  a C string.

   begin
      System_Call (Command'Address);
      --  Command'Address returns the address of the first character
   end System_Call;

-----------------------------------------------------------------------
begin
   System_Call ("dir");
end List;

--
-- Jerry van Dijk | Leiden, Holland
-- Team Ada       | jdijk@acm.org
-- see http://stad.dsl.nl/~jvandyk