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.
{shift}
{rotate}
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 non-overloadable
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. {
IEEE floating point arithmetic}
However, if IEEE arithmetic is supported, then the
names should be IEEE_Float_32 and IEEE_Float_64 for single and double
precision, respectively.
{
AI95-00204-01}
Support for interfacing to any foreign language is optional. However,
an implementation shall not provide any attribute, library unit, or pragma
having the same name as an attribute, library unit, or pragma (respectively)
specified in the following clauses of this Annex unless the provided
construct is either as specified in those clauses or is more limited
in capability than that required by those clauses. A program that attempts
to use an unsupported capability of this Annex shall either be identified
by the implementation before run time or shall raise an exception at
run time.
Discussion: The intent is that the same
rules apply for language interfacing as apply for Specialized Needs Annexes.
See
1.1.3 for a discussion of the purpose
of these rules.
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}
A child package of package Interfaces with the name of a convention may
be provided independently of whether the convention is supported by the
pragma Convention 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.
An implementation supporting an interface to C, COBOL,
or Fortran should provide the corresponding package or packages described
in the following clauses.
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.
Sponsored by Ada-Europe