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------------------------------------------------------------------------------
-- --
-- GNAT RUN-TIME COMPONENTS --
-- --
-- G N A T . B Y T E _ S W A P P I N G --
-- --
-- S p e c --
-- --
-- Copyright (C) 2006-2007, AdaCore --
-- --
-- GNAT is free software; you can redistribute it and/or modify it under --
-- terms of the GNU General Public License as published by the Free Soft- --
-- ware Foundation; either version 2, or (at your option) any later ver- --
-- sion. GNAT is distributed in the hope that it will be useful, but WITH- --
-- OUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY --
-- or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License --
-- for more details. You should have received a copy of the GNU General --
-- Public License distributed with GNAT; see file COPYING. If not, write --
-- to the Free Software Foundation, 51 Franklin Street, Fifth Floor, --
-- Boston, MA 02110-1301, USA. --
-- --
-- As a special exception, if other files instantiate generics from this --
-- unit, or you link this unit with other files to produce an executable, --
-- this unit does not by itself cause the resulting executable to be --
-- covered by the GNU General Public License. This exception does not --
-- however invalidate any other reasons why the executable file might be --
-- covered by the GNU Public License. --
-- --
-- GNAT was originally developed by the GNAT team at New York University. --
-- Extensive contributions were provided by Ada Core Technologies Inc. --
-- --
------------------------------------------------------------------------------
-- Simple routines for swapping the bytes of 16-, 32-, and 64-bit objects
-- The generic functions should be instantiated with types that are of a size
-- in bytes corresponding to the name of the generic. For example, a 2-byte
-- integer type would be compatible with Swapped2, 4-byte integer with
-- Swapped4, and so on. Failure to do so will result in a warning when
-- compiling the instantiation; this warning should be heeded. Ignoring this
-- warning can result in unexpected results.
-- An example of proper usage follows:
-- declare
-- type Short_Integer is range -32768 .. 32767;
-- for Short_Integer'Size use 16; -- for confirmation
-- X : Short_Integer := 16#7FFF#;
-- function Swapped is new Byte_Swapping.Swapped2 (Short_Integer);
-- begin
-- Put_Line (X'Img);
-- X := Swapped (X);
-- Put_Line (X'Img);
-- end;
-- Note that the generic actual types need not be scalars, but must be
-- 'definite' types. They can, for example, be constrained subtypes of
-- unconstrained array types as long as the size is correct. For instance,
-- a subtype of String with length of 4 would be compatible with the
-- Swapped4 generic:
-- declare
-- subtype String4 is String (1 .. 4);
-- function Swapped is new Byte_Swapping.Swapped4 (String4);
-- S : String4 := "ABCD";
-- begin
-- Put_Line (S);
-- S := Swapped (S);
-- Put_Line (S);
-- end;
-- Similarly, a constrained array type is also acceptable:
-- declare
-- type Mask is array (0 .. 15) of Boolean;
-- for Mask'Component_Size use Boolean'Size;
-- X : Mask := (0 .. 7 => True, others => False);
-- function Swapped is new Byte_Swapping.Swapped2 (Mask);
-- begin
-- ...
-- X := Swapped (X);
-- ...
-- end;
-- A properly-sized record type will also be acceptable, and so forth
-- However, as described, a size mismatch must be avoided. In the following we
-- instantiate one of the generics with a type that is too large. The result
-- of the function call is undefined, such that assignment to an object can
-- result in garbage values.
-- Wrong: declare
-- subtype String16 is String (1 .. 16);
-- function Swapped is new Byte_Swapping.Swapped8 (String16);
-- -- Instantiation generates a compiler warning about
-- -- mismatched sizes
-- S : String16;
-- begin
-- S := "ABCDEFGHDEADBEEF";
--
-- Put_Line (S);
--
-- -- the following assignment results in garbage in S after the
-- -- first 8 bytes
--
-- S := Swapped (S);
--
-- Put_Line (S);
-- end Wrong;
-- When the size of the type is larger than 8 bytes, the use of the non-
-- generic procedures is an alternative because no function result is
-- involved; manipulation of the object is direct.
-- The procedures are passed the address of an object to manipulate. They will
-- swap the first N bytes of that object corresponding to the name of the
-- procedure. For example:
-- declare
-- S2 : String := "AB";
-- for S2'Alignment use 2;
-- S4 : String := "ABCD";
-- for S4'Alignment use 4;
-- S8 : String := "ABCDEFGH";
-- for S8'Alignment use 8;
-- begin
-- Swap2 (S2'Address);
-- Put_Line (S2);
-- Swap4 (S4'Address);
-- Put_Line (S4);
-- Swap8 (S8'Address);
-- Put_Line (S8);
-- end;
-- If an object of a type larger than N is passed, the remaining bytes of the
-- object are undisturbed. For example:
-- declare
-- subtype String16 is String (1 .. 16);
-- S : String16;
-- for S'Alignment use 8;
-- begin
-- S := "ABCDEFGHDEADBEEF";
-- Put_Line (S);
-- Swap8 (S'Address);
-- Put_Line (S);
-- end;
with System;
package GNAT.Byte_Swapping is
pragma Pure;
-- NB: all the routines in this package treat the application objects as
-- unsigned (modular) types of a size in bytes corresponding to the routine
-- name. For example, the generic function Swapped2 manipulates the object
-- passed to the formal parameter Input as a value of an unsigned type that
-- is 2 bytes long. Therefore clients are responsible for the compatibility
-- of application types manipulated by these routines and these modular
-- types, in terms of both size and alignment. This requirement applies to
-- the generic actual type passed to the generic formal type Item in the
-- generic functions, as well as to the type of the object implicitly
-- designated by the address passed to the non-generic procedures. Use of
-- incompatible types can result in implementation- defined effects.
generic
type Item is limited private;
function Swapped2 (Input : Item) return Item;
-- Return the 2-byte value of Input with the bytes swapped
generic
type Item is limited private;
function Swapped4 (Input : Item) return Item;
-- Return the 4-byte value of Input with the bytes swapped
generic
type Item is limited private;
function Swapped8 (Input : Item) return Item;
-- Return the 8-byte value of Input with the bytes swapped
procedure Swap2 (Location : System.Address);
-- Swap the first 2 bytes of the object starting at the address specified
-- by Location.
procedure Swap4 (Location : System.Address);
-- Swap the first 4 bytes of the object starting at the address specified
-- by Location.
procedure Swap8 (Location : System.Address);
-- Swap the first 8 bytes of the object starting at the address specified
-- by Location.
pragma Inline (Swap2, Swap4, Swap8, Swapped2, Swapped4, Swapped8);
end GNAT.Byte_Swapping;
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