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B.5 Interfacing with Fortran

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{AI05-0229-1} The facilities relevant to interfacing with the Fortran language are the package Interfaces.Fortran and support for specifying the Convention aspect with convention_identifier Fortran.
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The package Interfaces.Fortran defines Ada types whose representations are identical to the default representations of the Fortran intrinsic types Integer, Real, Double Precision, Complex, Logical, and Character in a supported Fortran implementation. These Ada types can therefore be used to pass objects between Ada and Fortran programs.

Static Semantics

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The library package Interfaces.Fortran has the following declaration: 
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with Ada.Numerics.Generic_Complex_Types;  -- see G.1.1
pragma Elaborate_All(Ada.Numerics.Generic_Complex_Types);
package Interfaces.Fortran is
   pragma Pure(Fortran);
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   type Fortran_Integer is range implementation-defined;
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   type Real             is digits implementation-defined;
   type Double_Precision is digits implementation-defined;
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   type Logical is new Boolean;
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   package Single_Precision_Complex_Types is
      new Ada.Numerics.Generic_Complex_Types (Real);
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   type Complex is new Single_Precision_Complex_Types.Complex;
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   subtype Imaginary is Single_Precision_Complex_Types.Imaginary;
   i : Imaginary renames Single_Precision_Complex_Types.i;
   j : Imaginary renames Single_Precision_Complex_Types.j;
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   type Character_Set is implementation-defined character type;
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{AI05-0229-1}    type Fortran_Character is array (Positive range <>) of Character_Set
      with Pack;
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   function To_Fortran (Item : in Character) return Character_Set;
   function To_Ada (Item : in Character_Set) return Character;
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   function To_Fortran (Item : in String) return Fortran_Character;
   function To_Ada     (Item : in Fortran_Character) return String;
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   procedure To_Fortran (Item       : in String;
                         Target     : out Fortran_Character;
                         Last       : out Natural);
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   procedure To_Ada (Item     : in Fortran_Character;
                     Target   : out String;
                     Last     : out Natural);
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end Interfaces.Fortran;
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Implementation defined: The types Fortran_Integer, Real, Double_Precision, and Character_Set in Interfaces.Fortran.
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Ramification: The means by which the Complex type is provided in Interfaces.Fortran creates a dependence of Interfaces.Fortran on Numerics.Generic_Complex_Types (see G.1.1). This dependence is intentional and unavoidable, if the Fortran-compatible Complex type is to be useful in Ada code without duplicating facilities defined elsewhere. 
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The types Fortran_Integer, Real, Double_Precision, Logical, Complex, and Fortran_Character are Fortran-compatible.
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The To_Fortran and To_Ada functions map between the Ada type Character and the Fortran type Character_Set, and also between the Ada type String and the Fortran type Fortran_Character. The To_Fortran and To_Ada procedures have analogous effects to the string conversion subprograms found in Interfaces.COBOL. 

Implementation Requirements

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 {AI05-0229-1} An implementation shall support specifying aspect Convention with a Fortran convention_identifier for a Fortran-eligible type (see B.1). 

Implementation Permissions

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An implementation may add additional declarations to the Fortran interface packages. For example, the Fortran interface package for an implementation of Fortran 77 (ANSI X3.9-1978) that defines types like Integer*n, Real*n, Logical*n, and Complex*n may contain the declarations of types named Integer_Star_n, Real_Star_n, Logical_Star_n, and Complex_Star_n. (This convention should not apply to Character*n, for which the Ada analog is the constrained array subtype Fortran_Character (1..n).) Similarly, the Fortran interface package for an implementation of Fortran 90 that provides multiple kinds of intrinsic types, e.g. Integer (Kind=n), Real (Kind=n), Logical (Kind=n), Complex (Kind=n), and Character (Kind=n), may contain the declarations of types with the recommended names Integer_Kind_n, Real_Kind_n, Logical_Kind_n, Complex_Kind_n, and Character_Kind_n
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Discussion: Implementations may add auxiliary declarations as needed to assist in the declarations of additional Fortran-compatible types. For example, if a double precision complex type is defined, then Numerics.Generic_Complex_Types may be instantiated for the double precision type. Similarly, if a wide character type is defined to match a Fortran 90 wide character type (accessible in Fortran 90 with the Kind modifier), then an auxiliary character set may be declared to serve as its component type. 

Implementation Advice

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An Ada implementation should support the following interface correspondences between Ada and Fortran: 
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An Ada procedure corresponds to a Fortran subroutine.
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An Ada function corresponds to a Fortran function.
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An Ada parameter of an elementary, array, or record type T is passed as a TF argument to a Fortran procedure, where TF is the Fortran type corresponding to the Ada type T, and where the INTENT attribute of the corresponding dummy argument matches the Ada formal parameter mode; the Fortran implementation's parameter passing conventions are used. For elementary types, a local copy is used if necessary to ensure by-copy semantics.
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An Ada parameter of an access-to-subprogram type is passed as a reference to a Fortran procedure whose interface corresponds to the designated subprogram's specification. 
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Implementation Advice: If Fortran interfacing is supported, the interface correspondences between Ada and Fortran should be supported.
NOTES
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14  An object of a Fortran-compatible record type, declared in a library package or subprogram, can correspond to a Fortran common block; the type also corresponds to a Fortran “derived type”. 

Examples

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Example of Interfaces.Fortran:
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with Interfaces.Fortran;
use Interfaces.Fortran;
procedure Ada_Application is
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{AI05-0229-1}    type Fortran_Matrix is array (Integer range <>,
                                 Integer range <>) of Double_Precision
      with Convention => Fortran;                  -- stored in Fortran's
                                                   -- column-major order
   procedure Invert (Rank : in Fortran_Integer; X : in out Fortran_Matrix)
      with Import => True, Convention => Fortran; -- a Fortran subroutine
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   Rank      : constant Fortran_Integer := 100;
   My_Matrix : Fortran_Matrix (1 .. Rank, 1 .. Rank);
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begin
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   ...
   My_Matrix := ...;
   ...
   Invert (Rank, My_Matrix);
   ...
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end Ada_Application;

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