B.2 The Package Interfaces
Package Interfaces is the parent of several library
packages that declare types and other entities useful for interfacing
to foreign languages. It also contains some implementation-defined types
that are useful across more than one language (in particular for interfacing
to assembly language).
Implementation defined: The contents
of the visible part of package Interfaces and its language-defined descendants.
Static Semantics
The library package
Interfaces has the following skeletal declaration:
package Interfaces
is
pragma Pure(Interfaces);
type Integer_n is range -2**(n-1) .. 2**(n-1) - 1; --2's complement
type Unsigned_n is mod 2**n;
function Shift_Left (Value : Unsigned_
n; Amount : Natural)
return Unsigned_
n;
function Shift_Right (Value : Unsigned_
n; Amount : Natural)
return Unsigned_
n;
function Shift_Right_Arithmetic (Value : Unsigned_
n; Amount : Natural)
return Unsigned_
n;
function Rotate_Left (Value : Unsigned_
n; Amount : Natural)
return Unsigned_
n;
function Rotate_Right (Value : Unsigned_
n; Amount : Natural)
return Unsigned_
n;
...
end Interfaces;
Implementation Requirements
An implementation shall
provide the following declarations in the visible part of package Interfaces:
Signed and modular integer types of
n bits,
if supported by the target architecture, for each
n that is at
least the size of a storage element and that is a factor of the word
size. The names of these types are of the form Integer_
n for the
signed types, and Unsigned_
n for the modular types;
Ramification: For example, for a typical
32-bit machine the corresponding types might be Integer_8, Unsigned_8,
Integer_16, Unsigned_16, Integer_32, and Unsigned_32.
The wording above implies, for example, that
Integer_16'Size = Unsigned_16'Size = 16. Unchecked conversions between
same-Sized types will work as expected.
For each
such modular type in Interfaces, shifting and rotating subprograms as
specified in the declaration of Interfaces above. These subprograms are
Intrinsic. They operate on a bit-by-bit basis, using the binary representation
of the value of the operands to yield a binary representation for the
result. The Amount parameter gives the number of bits by which to shift
or rotate. For shifting, zero bits are shifted in, except in the case
of Shift_Right_Arithmetic, where one bits are shifted in if Value is
at least half the modulus.
Reason: We considered making shifting
and rotating be primitive operations of all modular types. However, it
is a design principle of Ada that all predefined operations should be
operators (not functions named by identifiers). (Note that an early version
of Ada had "abs" as an identifier, but it was changed
to a reserved word operator before standardization of Ada 83.) This is
important because the implicit declarations would hide nonoverloadable
declarations with the same name, whereas operators are always overloadable.
Therefore, we would have had to make shift and rotate into reserved words,
which would have been upward incompatible, or else invent new operator
symbols, which seemed like too much mechanism.
Floating point types corresponding to each floating
point format fully supported by the hardware.
Implementation Note: The names for these
floating point types are not specified.
However,
if IEEE arithmetic is supported, then the names should be IEEE_Float_32
and IEEE_Float_64 for single and double precision, respectively.
Implementation Permissions
An implementation may provide implementation-defined
library units that are children of Interfaces, and may add declarations
to the visible part of Interfaces in addition to the ones defined above.
Implementation defined: Implementation-defined
children of package Interfaces.
{
AI95-00204-01}
{
AI05-0229-1}
A child package of package Interfaces with the name of a convention may
be provided independently of whether the convention is supported by the
Convention aspect and vice versa. Such a child package should contain
any declarations that would be useful for interfacing to the language
(implementation) represented by the convention. Any declarations useful
for interfacing to any language on the given hardware architecture should
be provided directly in Interfaces.
Ramification: For example, package Interfaces.XYZ_Pascal
might contain declarations of types that match the data types provided
by the XYZ implementation of Pascal, so that it will be more convenient
to pass parameters to a subprogram whose convention is XYZ_Pascal.
Implementation Advice
This paragraph
was deleted.
{
AI05-0299-1}
An implementation supporting an interface to C, COBOL, or Fortran should
provide the corresponding package or packages described in the following
subclauses.
Implementation Advice: If an interface
to C, COBOL, or Fortran is provided, the corresponding package or packages
described in
Annex B, “
Interface
to Other Languages” should also be provided.
Implementation
Note: The intention is that an implementation might support several
implementations of the foreign language: Interfaces.This_Fortran and
Interfaces.That_Fortran might both exist. The “default” implementation,
overridable by the user, should be declared as a renaming:
package Interfaces.Fortran renames Interfaces.This_Fortran;
Wording Changes from Ada 95
{
AI95-00204-01}
Clarified that interfacing to foreign languages is optional and has the
same restrictions as a Specialized Needs Annex.
Wording Changes from Ada 2005
{
AI05-0262-1}
Move the restrictions on implementations of optional features to the
start of this Annex.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
