------------------------------------------------------------------------------
--                                                                          --
--                GNU ADA RUN-TIME LIBRARY (GNARL) COMPONENTS               --
--                                                                          --
--                  S Y S T E M . O S _ P R I M I T I V E S                 --
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--                                  S p e c                                 --
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--                             $Revision: 1.2 $
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--          Copyright (C) 1998-2001 Free Software Foundation, Inc.          --
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------------------------------------------------------------------------------

--  This package provides low level primitives used to implement clock and
--  delays in non tasking applications on Alpha/VMS

--  The choice of the real clock/delay implementation (depending on whether
--  tasking is involved or not) is done via soft links (see s-tasoli.ads)

--  NEVER add any dependency to tasking packages here

package System.OS_Primitives is

   subtype OS_Time is Long_Integer;
   --  System time on VMS is used for performance reasons.
   --  Note that OS_Time is *not* the same as Ada.Calendar.Time, the
   --  difference being that relative OS_Time is negative, but relative
   --  Calendar.Time is positive.
   --  See Ada.Calendar.Delays for more information on VMS Time.

   Max_Sensible_Delay : constant Duration := 183 * 24 * 60 * 60.0;
   --  Max of half a year delay, needed to prevent exceptions for large
   --  delay values. It seems unlikely that any test will notice this
   --  restriction, except in the case of applications setting the clock at
   --  at run time (see s-tastim.adb). Also note that a larger value might
   --  cause problems (e.g overflow, or more likely OS limitation in the
   --  primitives used).

   function OS_Clock return OS_Time;
   --  Returns "absolute" time, represented as an offset
   --  relative to "the Epoch", which is Nov 17, 1858 on VMS.

   function Clock return Duration;
   pragma Inline (Clock);
   --  Returns "absolute" time, represented as an offset
   --  relative to "the Epoch", which is Jan 1, 1970 on unixes.
   --  This implementation is affected by system's clock changes.

   function Monotonic_Clock return Duration;
   pragma Inline (Monotonic_Clock);
   --  Returns "absolute" time, represented as an offset
   --  relative to "the Epoch", which is Jan 1, 1970.
   --  This clock implementation is immune to the system's clock changes.

   Relative          : constant := 0;
   Absolute_Calendar : constant := 1;
   Absolute_RT       : constant := 2;
   --  Values for Mode call below. Note that the compiler (exp_ch9.adb)
   --  relies on these values. So any change here must be reflected in
   --  corresponding changes in the compiler.

   procedure Timed_Delay (Time : Duration; Mode : Integer);
   --  Implements the semantics of the delay statement when no tasking is
   --  used in the application.
   --
   --    Mode is one of the three values above
   --
   --    Time is a relative or absolute duration value, depending on Mode.
   --
   --  Note that currently Ada.Real_Time always uses the tasking run time, so
   --  this procedure should never be called with Mode set to Absolute_RT.
   --  This may change in future or bare board implementations.

   function To_Duration (T : OS_Time; Mode : Integer) return Duration;
   --  Convert VMS system time to Duration
   --  Mode is one of the three values above

   function To_OS_Time (D : Duration; Mode : Integer) return OS_Time;
   --  Convert Duration to VMS system time
   --  Mode is one of the three values above

end System.OS_Primitives;