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B.1 Interfacing Pragmas

1
   A pragma Import is used to import an entity defined in a foreign language into an Ada program, thus allowing a foreign-language subprogram to be called from Ada, or a foreign-language variable to be accessed from Ada. In contrast, a pragma Export is used to export an Ada entity to a foreign language, thus allowing an Ada subprogram to be called from a foreign language, or an Ada object to be accessed from a foreign language. The pragmas Import and Export are intended primarily for objects and subprograms, although implementations are allowed to support other entities.
2
   A pragma Convention is used to specify that an Ada entity should use the conventions of another language. It is intended primarily for types and ``callback'' subprograms. For example, ``pragma Convention(Fortran, Matrix);'' implies that Matrix should be represented according to the conventions of the supported Fortran implementation, namely column-major order.
3
   A pragma Linker_Options is used to specify the system linker parameters needed when a given compilation unit is included in a partition.

Syntax

4
{interfacing pragma [distributed]} {interfacing pragma (Import) [partial]} {pragma, interfacing (Import) [partial]} {interfacing pragma (Export) [partial]} {pragma, interfacing (Export) [partial]} {interfacing pragma (Convention) [partial]} {pragma, interfacing (Convention) [partial]} {pragma, interfacing (Linker_Options) [partial]} An interfacing pragma is a representation pragma that is one of the pragmas Import, Export, or Convention. Their forms, together with that of the related pragma Linker_Options, are as follows:
5
  pragma Import(
     [Convention =>] convention_identifier, [Entity =>] local_name
  [, [External_Name =>] string_expression] [, [Link_Name =>] string_expression]);
6
  pragma Export(
     [Convention =>] convention_identifier, [Entity =>] local_name
  [, [External_Name =>] string_expression] [, [Link_Name =>] string_expression]);
7
  pragma Convention([Convention =>] convention_identifier,[Entity =>] local_name);
8
  pragma Linker_Options(string_expression);
9
A pragma Linker_Options is allowed only at the place of a declarative_item.
9.1/1
{8652/0058} For pragmas Import and Export, the argument for Link_Name shall not be given without the pragma_argument_identifier unless the argument for External_Name is given.

Name Resolution Rules

10
    {expected type (link name) [partial]} The expected type for a string_expression in an interfacing pragma or in pragma Linker_Options is String.
10.a
Ramification: There is no language-defined support for external or link names of type Wide_String, or of other string types. Implementations may, of course, have additional pragmas for that purpose. Note that allowing both String and Wide_String in the same pragma would cause ambiguities.

Legality Rules

11
    {convention} The convention_identifier of an interfacing pragma shall be the name of a convention. The convention names are implementation defined, except for certain language-defined ones, such as Ada and Intrinsic, as explained in 6.3.1, ``Conformance Rules''. [Additional convention names generally represent the calling conventions of foreign languages, language implementations, or specific run-time models.] {calling convention} The convention of a callable entity is its calling convention.
11.a
Implementation defined: Implementation-defined convention names.
11.b
Discussion: We considered representing the convention names using an enumeration type declared in System. Then, convention_identifier would be changed to convention_name, and we would make its expected type be the enumeration type. We didn't do this because it seems to introduce extra complexity, and because the list of available languages is better represented as the list of children of package Interfaces -- a more open-ended sort of list.
12
    {compatible (a type, with a convention)} If L is a convention_identifier for a language, then a type T is said to be compatible with convention L, (alternatively, is said to be an L-compatible type) if any of the following conditions are met:
13
14
15
16
17
18
19
20
20.a
Discussion: For example, an implementation might permit Integer as a C-compatible type, though the C type to which it corresponds might be different in different environments.
21
    If pragma Convention applies to a type, then the type shall either be compatible with or eligible for the convention specified in the pragma.
21.a
Ramification: If a type is derived from an L-compatible type, the derived type is by default L-compatible, but it is also permitted to specify pragma Convention for the derived type.
21.b
It is permitted to specify pragma Convention for an incomplete type, but in the complete declaration each component must be L-compatible.
21.c
If each component of a record type is L-compatible, then the record type itself is only L-compatible if it has a pragma Convention.
22
    A pragma Import shall be the completion of a declaration. {notwithstanding} Notwithstanding any rule to the contrary, a pragma Import may serve as the completion of any kind of (explicit) declaration if supported by an implementation for that kind of declaration. If a completion is a pragma Import, then it shall appear in the same declarative_part, package_specification, task_definition or protected_definition as the declaration. For a library unit, it shall appear in the same compilation, before any subsequent compilation_units other than pragmas. If the local_name denotes more than one entity, then the pragma Import is the completion of all of them.
22.a
Discussion: For declarations of deferred constants and subprograms, we mention pragma Import explicitly as a possible completion. For other declarations that require completions, we ignore the possibility of pragma Import. Nevertheless, if an implementation chooses to allow a pragma Import to complete the declaration of a task, protected type, incomplete type, private type, etc., it may do so, and the normal completion is then not allowed for that declaration.
23
    {imported entity} {exported entity} An entity specified as the Entity argument to a pragma Import (or pragma Export) is said to be imported (respectively, exported).
24
    The declaration of an imported object shall not include an explicit initialization expression. [Default initializations are not performed.]
24.a
Proof: This follows from the ``Notwithstanding ...'' wording in the Dynamics Semantics paragraphs below.
25
    The type of an imported or exported object shall be compatible with the convention specified in the corresponding pragma.
25.a
Ramification: This implies, for example, that importing an Integer object might be illegal, whereas importing an object of type Interfaces.C.int would be permitted.
26
    For an imported or exported subprogram, the result and parameter types shall each be compatible with the convention specified in the corresponding pragma.
27
    The external name and link name string_expressions of a pragma Import or Export, and the string_expression of a pragma Linker_Options, shall be static.

Static Semantics

28
    {representation pragma (Import) [partial]} {pragma, representation (Import) [partial]} {representation pragma (Export) [partial]} {pragma, representation (Export) [partial]} {representation pragma (Convention) [partial]} {pragma, representation (Convention) [partial]} {aspect of representation (convention, calling convention) [partial]} {convention (aspect of representation)} Import, Export, and Convention pragmas are representation pragmas that specify the convention aspect of representation. {aspect of representation (imported) [partial]} {imported (aspect of representation)} {aspect of representation (exported) [partial]} {exported (aspect of representation)} In addition, Import and Export pragmas specify the imported and exported aspects of representation, respectively.
29
    {program unit pragma (Import) [partial]} {pragma, program unit (Import) [partial]} {program unit pragma (Export) [partial]} {pragma, program unit (Export) [partial]} {program unit pragma (Convention) [partial]} {pragma, program unit (Convention) [partial]} An interfacing pragma is a program unit pragma when applied to a program unit (see 10.1.5).
30
    An interfacing pragma defines the convention of the entity denoted by the local_name. The convention represents the calling convention or representation convention of the entity. For an access-to-subprogram type, it represents the calling convention of designated subprograms. In addition:
31
32
33
34
    {external name} An external name is a string value for the name used by a foreign language program either for an entity that an Ada program imports, or for referring to an entity that an Ada program exports.
35
    {link name} A link name is a string value for the name of an exported or imported entity, based on the conventions of the foreign language's compiler in interfacing with the system's linker tool.
36
    The meaning of link names is implementation defined. If neither a link name nor the Address attribute of an imported or exported entity is specified, then a link name is chosen in an implementation-defined manner, based on the external name if one is specified.
36.a
Implementation defined: The meaning of link names.
36.b
Ramification: For example, an implementation might always prepend "_", and then pass it to the system linker.
36.c
Implementation defined: The manner of choosing link names when neither the link name nor the address of an imported or exported entity is specified.
36.d
Ramification: Normally, this will be the entity's defining name, or some simple transformation thereof.
37
    Pragma Linker_Options has the effect of passing its string argument as a parameter to the system linker (if one exists), if the immediately enclosing compilation unit is included in the partition being linked. The interpretation of the string argument, and the way in which the string arguments from multiple Linker_Options pragmas are combined, is implementation defined.
37.a
Implementation defined: The effect of pragma Linker_Options.

Dynamic Semantics

38
    {elaboration (declaration named by a pragma Import) [partial]} {notwithstanding} Notwithstanding what this International Standard says elsewhere, the elaboration of a declaration denoted by the local_name of a pragma Import does not create the entity. Such an elaboration has no other effect than to allow the defining name to denote the external entity.
38.a
Ramification: This implies that default initializations are skipped. (Explicit initializations are illegal.) For example, an imported access object is not initialized to null.
38.b
Note that the local_name in a pragma Import might denote more than one declaration; in that case, the entity of all of those declarations will be the external entity.
38.c
Discussion: This ``notwithstanding'' wording is better than saying ``unless named by a pragma Import'' on every definition of elaboration. It says we recognize the contradiction, and this rule takes precedence.

Implementation Advice

39
    If an implementation supports pragma Export to a given language, then it should also allow the main subprogram to be written in that language. It should support some mechanism for invoking the elaboration of the Ada library units included in the system, and for invoking the finalization of the environment task. On typical systems, the recommended mechanism is to provide two subprograms whose link names are "adainit" and "adafinal". Adainit should contain the elaboration code for library units. Adafinal should contain the finalization code. These subprograms should have no effect the second and subsequent time they are called. {adainit} {adafinal} {Elaboration (of library units for a foreign language main subprogram)} {Finalization (of environment task for a foreign language main subprogram)}
39.a
Ramification: For example, if the main subprogram is written in C, it can call adainit before the first call to an Ada subprogram, and adafinal after the last.
40
    Automatic elaboration of preelaborated packages should be provided when pragma Export is supported.
41
    For each supported convention L other than Intrinsic, an implementation should support Import and Export pragmas for objects of L-compatible types and for subprograms, and pragma Convention for L-eligible types and for subprograms, presuming the other language has corresponding features. Pragma Convention need not be supported for scalar types.
41.a
Reason: Pragma Convention is not necessary for scalar types, since the language interface packages declare scalar types corresponding to those provided by the respective foreign languages.
41.b
Implementation Note: If an implementation supports interfacing to C++, it should do so via the convention identifier C_Plus_Plus (in additional to any C++-implementation-specific ones).
41.c
Reason: The reason for giving the advice about C++ is to encourage uniformity among implementations, given that the name of the language is not syntactically legal as an identifier. We place this advice in the AARM, rather than the RM95 proper, because (as of this writing) C++ is not an international standard, and we don't want to refer to a such a language from an international standard.
NOTES
42
1  Implementations may place restrictions on interfacing pragmas; for example, requiring each exported entity to be declared at the library level.
42.a
Proof: Arbitrary restrictions are allowed by 13.1.
42.b
Ramification: Such a restriction might be to disallow them altogether. Alternatively, the implementation might allow them only for certain kinds of entities, or only for certain conventions.
43
2  A pragma Import specifies the conventions for accessing external entities. It is possible that the actual entity is written in assembly language, but reflects the conventions of a particular language. For example, pragma Import(Ada, ...) can be used to interface to an assembly language routine that obeys the Ada compiler's calling conventions.
44
3  To obtain ``call-back'' to an Ada subprogram from a foreign language environment, pragma Convention should be specified both for the access-to-subprogram type and the specific subprogram(s) to which 'Access is applied.
45
4  It is illegal to specify more than one of Import, Export, or Convention for a given entity.
46
5  The local_name in an interfacing pragma can denote more than one entity in the case of overloading. Such a pragma applies to all of the denoted entities.
47
6  See also 13.8, ``Machine Code Insertions''.
47.a
Ramification: The Intrinsic convention (see 6.3.1) implies that the entity is somehow ``built in'' to the implementation. Thus, it generally does not make sense for users to specify Intrinsic in a pragma Import. The intention is that only implementations will specify Intrinsic in a pragma Import. The language also defines certain subprograms to be Intrinsic.
47.b
Discussion: There are many imaginable interfacing pragmas that don't make any sense. For example, setting the Convention of a protected procedure to Ada is probably wrong. Rather than enumerating all such cases, however, we leave it up to implementations to decide what is sensible.
48
7  If both External_Name and Link_Name are specified for an Import or Export pragma, then the External_Name is ignored.
49
8  An interfacing pragma might result in an effect that violates Ada semantics.

Examples

50
    Example of interfacing pragmas:
51
package Fortran_Library is
  function Sqrt (X : Float) return Float;
  function Exp  (X : Float) return Float;
private
  pragma Import(Fortran, Sqrt);
  pragma Import(Fortran, Exp);
end Fortran_Library;

Extensions to Ada 83

51.a
{extensions to Ada 83} Interfacing pragmas are new to Ada 95. Pragma Import replaces Ada 83's pragma Interface. Existing implementations can continue to support pragma Interface for upward compatibility.

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