GNAT Reference Manual

GNAT, The GNU Ada 95 Compiler

Version 3.12p

Document revision level $Revision: 1.90 $

Date: $Date: 1999/06/28 22:42:46 $

(C) Copyright 1995-1999, Ada Core Technologies - All Rights Reserved

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About This Guide

This manual contains useful information in writing programs using the GNAT compiler. It includes information on implementation dependent characteristics of GNAT, including all the information required by Annex M of the standard.

Ada 95 is designed to be highly portable,and guarantees that, for most programs, Ada 95 compilers behave in exactly the same manner on different machines. However, since Ada 95 is designed to be used in a wide variety of applications, it also contains a number of system dependent features to be used in interfacing to the external world.

Note: Any program that makes use of implementation-dependent features may be non-portable. You should follow good programming practice and isolate and clearly document any sections of your program that make use of these features in a non-portable manner.

• What This Reference Manual Contains
• Conventions
• Related Information

What This Reference Manual Contains

This reference manual contains the following chapters:

• section Implementation Defined Pragmas lists GNAT implementation-dependent pragmas, which can be used to extend and enhance the functionality of the compiler.
• section Implementation Defined Attributes lists GNAT implementation-dependent attributes which can be used to extend and enhance the functionality of the compiler.
• section Implementation Advice provides information on generally desirable behavior which are not requirements that all compilers must follow since it cannot be provided on all systems, or which may be undesirable on some systems.
• section Implementation Defined Characteristics provides a guide to minimizing implementation dependent features.
• section Standard Library Routines provides a listing of packages and a brief description of the functionality that is provided by Ada's extensive set of standard library routines as implemented by GNAT.
• section The Implementation of Standard I/O details how the GNAT implementation of the input-output facilities.
• section Interfacing to Other Languages describes how programs written in Ada using GNAT can be interfaced to other programming languages.
• section Specialized Needs Annexes describes the GNAT implementation of all of the special needs annexes.
• section Compatibility Guide includes sections on compatibility of GNAT with other Ada 83 and Ada 95 compilation systems, to assist in porting code from other environments.

This reference manual assumes that you are familiar with Ada 95 language, as described in the International Standard ANSI/ISO/IEC-8652:1995, Jan 1995.

Conventions

Following are examples of the typographical and graphic conventions used in this guide:

• Functions, utility program names, standard names, and classes.
• `Option flags'
• `File Names', `button names', and `field names'.
• Variables.
• Emphasis.
• [optional information or parameters]
• Examples are described by text

  and then shown this way.
  

Commands that are entered by the user are preceded in this manual by the characters "$ " (dollar sign followed by space). If your system uses this sequence as a prompt, then the commands will appear exactly as you see them in the manual. If your system uses some other prompt, then the command will appear with the $ replaced by whatever prompt character you are using.

Related Information

See the following documents for further information on GNAT

• GNAT User's Guide, which provides information on how to use the GNAT compiler system.
• Ada 95 Reference Manual, which contains all reference material for the Ada 95 programming language.
• Ada 95 Annotated Reference Manual, which is an annotated version of the standard reference manual cited above. The annotations describe detailed aspects of the design decision, and in particular contain useful sections on Ada 83 compatibility.
• DEC Ada, Technical Overview and Comparison on DIGITAL Platforms, which contains specific information on compatibility between GNAT and DEC Ada 83 systems.
• DEC Ada, Language Reference Manual, part number AA-PYZAB-TK which describes in detail the pragmas and attributes provided by the DEC Ada 83 compiler system.

Implementation Defined Pragmas

Ada 95 defines a set of pragmas that can be used to supply additional information to the compiler. These language defined pragmas are implemented in GNAT and work as described in the Ada 95 Reference Manual.

In addition, Ada 95 allows implementations to define additional pragmas whose meaning is defined by the implementation. GNAT provides a number of these implementation-dependent pragmas which can be used to extend and enhance the functionality of the compiler. This section of the GNAT Reference Manual describes these additional pragmas.

Note that any program using these pragmas may not be portable to other compilers (although GNAT implements this set of pragmas on all platforms). Therefore if portability to other compilers is an important consideration, the use of these pragmas should be minimized.

pragma Abort_Defer

Syntax:

pragma Abort_Defer;

This pragma must appear at the start of the statement sequence of a handled sequence of statements (right after the begin). It has the effect of deferring aborts for the sequence of statements (but not for the declarations or handlers, if any, associated with this statement sequence).

pragma Ada_83

Syntax:

pragma Ada_83;

A configuration pragma that establishes Ada 83 mode for the unit to which it applies, regardless of the mode set by the command line switches. In Ada 83 mode, GNAT attempts to be as compatible with the syntax and semantics of Ada 83, as defined in the original Ada 83 Reference Manual as possible. In particular, the new Ada 95 keywords are not recognized, optional package bodies are allowed, and generics may name types with unknown discriminants without using the (<>) notation. In addition, some but not all of the additional restrictions of Ada 83 are enforced. Ada 83 mode is intended for two purposes. Firstly, it allows existing legacy Ada 83 code to be compiled and adapted to GNAT with less effort. Secondly, it aids in keeping code backwards compatible with Ada 83. However, there is no guarantee that code that is processed correctly by GNAT in Ada 83 mode will in fact compile and execute with an Ada 83 compiler, since GNAT does not enforce all the additional checks required by Ada 83.

pragma Ada_95

Syntax:

pragma Ada_95;

A configuration pragma that establishes Ada 95 mode for the unit to which it applies, regardless of the mode set by the command line switches. This mode is set automatically for the Ada and System packages and their children, so you need not specify it in these contexts. This pragma is useful when writing a reusable component that itself uses Ada 95 features, but which is intended to be usable from either Ada 83 or Ada 95 programs.

pragma Annotate

Syntax:

pragma Annotate (IDENTIFIER {, ARG});

ARG ::= NAME | EXPRESSION

This pragma is used to annotate programs. identifier identifies the type of annotation. GNAT verifies this is an identifier, but does not otherwise analyze it. The arg argument can be either a string literal or an expression. String literals are assumed to be of type Standard.String. Names of entities are simply analyzed as entity names. All other expressions are analyzed as expressions, and must be unambiguous. The analyzed pragma is retained in the tree, but not otherwise processed by any part of the GNAT compiler. This pragma is intended for use by external tools, including ASIS.

pragma Assert

Syntax:

pragma Assert (
  boolean_EXPRESSION
  [, static_string_EXPRESSION])

The effect of this pragma depends on whether the corresponding command line switch is set to activate assertions. If assertions are inactive, the pragma has no effect. If assertions are enabled, then the semantics of the pragma is exactly equivalent to:

if not boolean_EXPRESSION then
   System.Assertions.Raise_Assert_Failure (string_EXPRESSION); 
end if;

The effect of the call is to raise System.Assertions.Assert_Failure. The string argument, if given, is the message associated with the exception occurrence. If no second argument is given, the default message is `file:nnn', where file is the name of the source file containing the assert, and nnn is the line number of the assert. A pragma is not a statement, so if a statement sequence contains nothing but a pragma assert, then a null statement is required in addition, as in: Note that, as with the if statement to which it is equivalent, the type of the expression is either Standard.Boolean, or any type derived from this standard type.

...
if J > 3 then
   pragma (Assert (K > 3, "Bad value for K"));
   null;
end if;

If the boolean expression has side effects, these side effects will turn on and off with the setting of the assertions mode, resulting in assertions that have an effect on the program. You should generally avoid side effects in the expression arguments of this pragma. However, the expressions are analyzed for semantic correctness whether or not assertions are enabled, so turning assertions on and off cannot affect the legality of a program.

pragma Ast_Entry

Syntax:

pragma AST_Entry (entry_IDENTIFIER);

This pragma is implemented only in the OpenVMS implementation of GNAT. The argument is the simple name of a single entry; at most one AST_Entry pragma is allowed for any given entry. This pragma must be used in conjunction with the AST_Entry attribute, and is only allowed after the entry declaration and in the same task type specification or single task as the entry to which it applies. This pragma specifies that the given entry may be used to handle an OpenVMS asynchronous system trap (AST) resulting from an OpenVMS system service call. The pragma does not affect normal use of the entry. For further details on this pragma, see the DEC Ada Language Reference Manual, section 9.12a.

pragma C_Pass_By_Copy

Syntax:

pragma C_Pass_By_Copy
  ([Max_Size =>] static_integer_EXPRESSION);

Normally the default mechanism for passing C convention records to C convention subprograms is to pass them by reference, as suggested by RM B.3(69). Use the configuration pragma C_Pass_By_Copy to change this default, by requiring that record formal parameters be passed by copy if all of the following conditions are met:

• The size of the record type does not exceed static_integer_expression.
• The record type has Convention C.
• The formal parameter has this record type, and the subprogram has a foreign (non-Ada) convention.

If these conditions are met the argument is passed by copy, i.e. in a manner consistent with what C expects if the corresponding formal in the C prototype is a struct (rather than a pointer to a struct). You can also pass records by copy by specifying the convention C_Pass_By_Copy for the record type, or by using the extended Import and Export pragmas, which allow specification of passing mechanisms on a parameter by parameter basis.

pragma Comment

Syntax:

pragma Comment (static_string_EXPRESSION);

Synonymous for pragma Ident.

pragma Common_Object

Syntax:

pragma Common_Object
     [Internal =>] LOCAL_NAME,
  [, [External =>] EXTERNAL_SYMBOL,
  [, [Size     =>] EXTERNAL_SYMBOL]

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION

This pragma enables the shared use of variables stored in overlaid linker areas corresponding to the use of COMMON in Fortran. The single object local_name is assigned to the area designated by the External argument. You may define a record to correspond to a series of fields. The size argument is syntax checked in GNAT, but otherwise ignored.

pragma Complex_Representation

Syntax:

pragma Complex_Representation ([Entity =>] LOCAL_NAME);

The Entity argument must be the name of a record type which has two fields of the same floating-point type. The effect of this pragma is to force gcc to use the special internal complex representation form for this record, which may be more efficient. Note that this may result in the code for this type not conforming to standard ABI (application binary interface) requirements for the handling of record types. For example, in some environments, there is a requirement for passing records by pointer, and the use of this pragma may result in passing this type in floating-point registers.

pragma Component_Alignment

Syntax:

pragma Component_Alignment (
     [Form =>] ALIGNMENT_CHOICE
  [, [Name =>] type_LOCAL_NAME]);

ALIGNMENT_CHOICE ::=
  Component_Size
| Component_Size_4
| Storage_Unit
| Default

Specifies the alignment of components in array or record types. The meaning of the Form argument is as follows:

Component_Size

Aligns scalar components and subcomponents of the array or record type on boundaries appropriate to their inherent size (naturally aligned). For example, 1-byte components are aligned on byte boundaries, 2-byte integer components are aligned on 2-byte boundaries, 4-byte integer components are aligned on 4-byte boundaries and so on. These alignment rules correspond to the normal rules for C compilers on all machines except the VAX.

Component_Size_4

Naturally aligns components with a size of four or fewer bytes. Components that are larger than 4 bytes are placed on the next 4-byte boundary.

Storage_Unit

Specifies that array or record components are byte aligned, i.e. aligned on boundaries determined by the value of the constant System.Storage_Unit.

Default

Specifies that array or record components are aligned on default boundaries, appropriate to the underlying hardware or operating system or both. For OpenVMS VAX systems, the Default choice is the same as the Storage_Unit choice (byte alignment). For all other systems, the Default choice is the same as Component_Size (natural alignment).

If the Name parameter is present, type_local_name must refer to a local record or array type, and the specified alignment choice applies to the specified type. The use of Component_Alignment together with a pragma Pack causes the Component_Alignment pragma to be ignored. The use of Component_Alignment together with a record representation clause is only effective for fields not specified by the representation clause. If the Name parameter is absent, the pragma can be used as either a configuration pragma, in which case it applies to one or more units in accordance with the normal rules for configuration pragmas, or it can be used within a declarative part, in which case it applies to types that are declared within this declarative part, or within any nested scope within this declarative part. In either case it specifies the alignment to be applied to any record or array type which has otherwise standard representation. If the alignment for a record or array type is not specified (using pragma Pack, pragma Component_Alignment, or a record rep clause), the GNAT uses the default alignment as described previously.

pragma CPP_Class

Syntax:

pragma CPP_Class ([Entity =>] LOCAL_NAME);

The argument denotes an entity in the current declarative region that is declared as a tagged or untagged record type. It indicates that the type corresponds to an externally declared C++ class type, and is to be laid out the same way that C++ would lay out the type. If (and only if) the type is tagged, at least one component in the record must be of type Interfaces.CPP.Vtable_Ptr, corresponding to the C++ Vtable (or Vtables in the case of multiple inheritance) used for dispatching. Types for which CPP_Class is specified do not have assignment or equality operators defined (such operations can be imported or declared as subprograms as required). Initialization is allowed only by constructor functions (see pragma CPP_Constructor). Pragma CPP_Class is intended primarily for automatic generation using an automatic binding generator tool. Ada Core Technologies does not currently supply such a tool; See section Interfacing to C++ for more details.

pragma CPP_Constructor

Syntax:

pragma CPP_Constructor ([Entity =>] LOCAL_NAME);

This pragma identifies an imported function (imported in the usual way with pragma Import) as corresponding to a C++ constructor. The argument is a name that must have been previously mentioned in a pragma Import with Convention CPP, and must be of one of the following forms:

• function Fname return T'Class
• function Fname (...) return T'Class

where T is a tagged type to which the pragma CPP_Class applies. The first form is the default constructor, used when an object of type T is created on the Ada side with no explicit constructor. Other constructors (including the copy constructor, which is simply a special case of the second form in which the one and only argument is of type T), can only appear in two contexts:

• On the right side of an initialization of an object of type T.
• In an extension aggregate for an object of a type derived from T.

Although the constructor is described as a function that returns a value on the Ada side, it is typically a procedure with an extra implicit argument (the object being initialized) at the implementation level. GNAT issues the appropriate call, whatever it is, to get the object properly initialized. In the case of derived objects, you may use one of two possible forms for declaring and creating an object:

• New_Object : Derived_T
• New_Object : Derived_T := (constructor-function-call with ...)

In the first case the default constructor is called and extension fields if any are initialized according to the default initialization expressions in the Ada declaration. In the second case, the given constructor is called and the extension aggregate indicates the explicit values of the extension fields. If no constructors are imported, it is impossible to create any objects on the Ada side. If no default constructor is imported, only the initialization forms using an explicit call to a constructor are permitted. Pragma CPP_Constructor is intended primarily for automatic generation using an automatic binding generator tool. Ada Core Technologies does not currently supply such a tool; See section Interfacing to C++ for more details.

pragma CPP_Destructor ([Entity =>] LOCAL_NAME);

This pragma identifies an imported procedure (imported in the usual way with pragma Import) as corresponding to a C++ destructor. LOCAL_NAME must be previously mentioned in a pragma Import with Convention CPP, and be of the following form:

procedure Fname (obj : in out T'Class);

where T is a tagged type to which pragma CPP_Class applies. This procedure will be called automatically on scope exit if any objects of T are created on the Ada side. Pragma CPP_Destructor is intended primarily for automatic generation using an automatic binding generator tool. Ada Core Technologies does not currently supply such a tool; See section Interfacing to C++ for more details.

pragma CPP_Virtual

Syntax:

pragma CPP_Virtual
     [Entity     =>] ENTITY,
  [, [Vtable_Ptr =>] vtable_ENTITY,]
  [, [Position   =>] static_integer_EXPRESSION])

This pragma serves the same function as pragma Import in that case of a virtual function imported from C++. The Entity argument must be a primitive subprogram of a tagged type to which pragma CPP_Class applies. The Vtable_Ptr argument specifies the Vtable_Ptr component which contains the entry for this virtual function. The Position argument is the sequential number counting virtual functions for this Vtable starting at 1. The Vtable_Ptr and Position arguments may be omitted if there is one Vtable_Ptr present (single inheritance case) and all virtual functions are imported. In that case the compiler can deduce both these values. No External_Name or Link_Name arguments are required for a virtual function, since it is always accessed indirectly via the appropriate Vtable entry. Pragma CPP_Virtual is intended primarily for automatic generation using an automatic binding generator tool. Ada Core Technologies does not currently supply such a tool; See section Interfacing to C++ for more details.

pragma CPP_Vtable

Syntax:

pragma CPP_Vtable (
  [Entity      =>] ENTITY,
  [Vtable_Ptr  =>] vtable_ENTITY,
  [Entry_Count =>] static_integer_EXPRESSION);

Given a record to which the pragma CPP_Class applies, this pragma can be specified for each component of type CPP.Interfaces.Vtable_Ptr. Entity is the tagged type, Vtable_Ptr is the record field of type Vtable_Ptr, and Entry_Count is the number of virtual functions on the C++ side. Not all of these functions need to be imported on the Ada side. You may omit the CPP_Vtable pragma if there is only one Vtable_Ptr component in the record and all virtual functions are imported on the Ada side (the default value for the entry count in this case is simply the total number of virtual functions). Pragma CPP_Vtable is intended primarily for automatic generation using an automatic binding generator tool. Ada Core Technologies does not currently supply such a tool; See section Interfacing to C++ for more details.

pragma Debug

Syntax:

pragma Debug (PROCEDURE_CALL_STATEMENT);

If assertions are not enabled on the command line, this pragma has no effect. If asserts are enabled, the semantics of the pragma is exactly equivalent to the procedure call. Pragmas are permitted in sequences of declarations, so you can use pragma Debug to intersperse calls to debug procedures in the middle of declarations.

pragma Eliminate

Syntax:

pragma Eliminate (
    [Unit_Name       =>] IDENTIFIER |
                         SELECTED_COMPONENT
  [,[Entity          =>] IDENTIFIER |
                         SELECTED_COMPONENT |
                         STRING_LITERAL]
  [,[Parameter_Types =>] PARAMETER_TYPES]
  [,[Result_Type     =>] result_SUBTYPE_MARK]);

PARAMETER_TYPES ::=
  null
| SUBTYPE_MARK {, SUBTYPE_MARK}

This pragma indicates that the given entity is unused in a program. The entity may be either a subprogram or a variable. If the entity to be eliminated is a library level subprogram, then only the first argument, specifying the corresponding unit name, is required. If the item is an entity of a library package, then the first argument specifies the unit name, and the second argument specifies the particular entity. If the second argument is in string form, it must correspond to the internal manner in which GNAT stores entity names (see compilation unit Namet in the compiler sources for details). The third and fourth parameters are optionally used to distinguish between overloaded subprograms, in the same manner as is used for pragma Import_Procedure. The effect of the pragma is to allow the compiler to optionally eliminate the code or data associated with the named entity. If the declaration of the entity would have resulted in side effects, these side effects may or may not occur in the resulting program. Any reference to an eliminated entity may cause a compile time error, link time error, or incorrect results at runtime. The intention of pragma Eliminate is to allow a program to be compiled in a system independent manner, with unused entities eliminated, without the requirement of modifying the source text. Normally the required set of Eliminate pragmas is constructed automatically using the gnatelim tool.

pragma Export_Exception

Syntax:

pragma Export_Exception (
     [Internal =>] LOCAL_NAME,
  [, [External =>] EXTERNAL_SYMBOL,]
  [, [Form     =>] Ada | VMS]
  [, [Code     =>] static_integer_EXPRESSION]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION

This pragma is implemented only in the OpenVMS implementation of GNAT. It causes the specified exception to be propagated outside of the Ada program, so that it can be handled by programs written in other OpenVMS languages. This pragma establishes an external name for an Ada exception and makes the name available to the OpenVMS Linker as a global symbol. For further details on this pragma, see the DEC Ada Language Reference Manual, section 13.9a3.2.

pragma Export_Function ...

Syntax:

pragma Export_Function (
     [Internal         =>] LOCAL_NAME,      
  [, [External         =>] EXTERNAL_SYMBOL]
  [, [Parameter_Types  =>] PARAMETER_TYPES]
  [, [Result_Type      =>] result_SUBTYPE_MARK]
  [, [Mechanism        =>] MECHANISM]
  [, [Result_Mechanism =>] MECHANISM_NAME]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION

PARAMETER_TYPES ::=
  null
| SUBTYPE_MARK {, SUBTYPE_MARK}

MECHANISM ::=
  MECHANISM_NAME
| (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})

MECHANISM_ASSOCIATION ::=
  [formal_parameter_NAME =>] MECHANISM_NAME

MECHANISM_NAME ::=
  Value
| Reference
| Descriptor [([Class =>] CLASS_NAME)]

CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca

Use this pragma to make a function externally callable and optionally provide information on mechanisms to be used for passing parameter and result values. We recommend, for the purposes of improving portability, this pragma always be used in conjunction with a separate pragma Export, which must precede the pragma Export_Function. GNAT does not require a separate pragma Export, but if none is present, it assumes Convention C. Pragma Export_Function (and Export, if present) must appear in the same declarative region as the function to which they apply. internal_name must uniquely designate the function to which the pragma applies. If more than one function name exists of this name in the declarative part you must use the Parameter_Types and Result_Type parameters is mandatory to achieve the required unique designation. subtype_ marks in these parameters must exactly match the subtypes in the corresponding function specification, using positional notation to match parameters with subtype marks. Passing by descriptor is supported only on the OpenVMS ports of GNAT.

pragma Export_Object ...

Syntax:

pragma Export_Object
      [Internal =>] LOCAL_NAME,
   [, [External =>] EXTERNAL_SYMBOL]
   [, [Size     =>] EXTERNAL_SYMBOL]

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION

This pragma designates an object as exported, and apart from the extended rules for external symbols, is identical in effect to the use of the normal Export pragma applied to an object. You may use a separate Export pragma (and you probably should from the point of view of portability), but it is not required. Size is syntax checked, but otherwise ignored by GNAT.

pragma Export_Procedure ...

Syntax:

pragma Export_Procedure (
     [Internal        =>] LOCAL_NAME
  [, [External        =>] EXTERNAL_SYMBOL]
  [, [Parameter_Types =>] PARAMETER_TYPES]
  [, [Mechanism       =>] MECHANISM]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION

PARAMETER_TYPES ::=
  null
| SUBTYPE_MARK {, SUBTYPE_MARK}

MECHANISM ::=
  MECHANISM_NAME
| (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})

MECHANISM_ASSOCIATION ::=
  [formal_parameter_NAME =>] MECHANISM_NAME

MECHANISM_NAME ::=
  Value
| Reference
| Descriptor [([Class =>] CLASS_NAME)]

CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca

This pragma is identical to Export_Function except that it applies to a procedure rather than a function and the parameters Result_Type and Result_Mechanism are not permitted.

pragma Export_Valued_Procedure

Syntax:

pragma Export_Valued_Procedure (
     [Internal        =>] LOCAL_NAME
  [, [External        =>] EXTERNAL_SYMBOL]
  [, [Parameter_Types =>] PARAMETER_TYPES]
  [, [Mechanism       =>] MECHANISM]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION

PARAMETER_TYPES ::=
  null
| SUBTYPE_MARK {, SUBTYPE_MARK}

MECHANISM ::=
  MECHANISM_NAME
| (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})

MECHANISM_ASSOCIATION ::=
  [formal_parameter_NAME =>] MECHANISM_NAME

MECHANISM_NAME ::=
  Value
| Reference
| Descriptor [([Class =>] CLASS_NAME)]

CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca

This pragma is identical to Export_Procedure except that the first parameter of local_name, which must be present, must be of mode OUT, and externally the subprogram is treated as a function with this parameter as the result of the function. GNAT provides for this capability to allow the use of OUT and IN OUT parameters in interfacing to external functions (which are not permitted in Ada functions).

pragma Extend_System

Syntax:

pragma Extend_System ([Name =>] IDENTIFIER);

This pragma is used to provide backwards compatibility with other implementations that extend the facilities of package System. In GNAT, System contains only the definitions that are present in the Ada 95 RM. However, other implementations, notably the DEC Ada 83 implementation, provide many extensions to package System. For each such implementation accommodated by this pragma, GNAT provides a package Aux_xxx, e.g. Aux_DEC for the DEC Ada 83 implementation, which provides the required additional definitions. You can use this package in two ways. You can with it in the normal way and access entities either by selection or using a use clause. In this case no special processing is required. However, if existing code contains references such as System.xxx where xxx is an entity in the extended definitions provided in package System, you may use this pragma to extend visibility in System in a non-standard way that provides greater compatibility with the existing code. Pragma Extend_System is a configuration pragma whose single argument is the name of the package containing the extended definition (e.g. Aux_DEC for the DEC Ada case). A unit compiled under control of this pragma will be processed using special visibility processing that looks in package System.Aux_xxx where Aux_xxx is the pragma argument for any entity referenced in package System, but not found in package System.

pragma Finalize_Storage_Only

Syntax:

pragma Finalize_Storage_Only (first_subtype_LOCAL_NAME);

This pragma allows the compiler not to emit a Finalize call for objects defined at the library level. This is mostly useful for types where finalization is only used to deal with storage reclamation since in most environments it is not necessary to reclaim memory just before terminating execution, hence the name.

pragma Float_Representation

Syntax:

pragma Float_Representation (FLOAT_REP);

FLOAT_REP ::= VAX_Float | IEEE_Float

This pragma is implemented only in the OpenVMS implementation of GNAT. It allows control over the internal representation chosen for the predefined floating point types declared in the packages Standard and System. For further details on this pragma, see the DEC Ada Language Reference Manual, section 3.5.7a. Note that to use this pragma, the standard runtime libraries must be recompiled. See the description of the GNAT LIBRARY command in the OpenVMS version of the GNAT Users Guide for details on the use of this command.

pragma Ident

Syntax:

pragma Ident (static_string_EXPRESSION);

This pragma provides a string identification in the generated object file, if the system supports the concept of this kind of identification string. The maximum permitted length of the string literal is 31 characters. This pragma is allowed only in the outermost declarative part or declarative items of a compilation unit. On OpenVMS systems, the effect of the pragma is identical to the effect of the DEC Ada 83 pragma of the same name.

pragma Import_Exception

Syntax:

pragma Import_Exception (
     [Internal =>] LOCAL_NAME,
  [, [External =>] EXTERNAL_SYMBOL,]
  [, [Form     =>] Ada | VMS]
  [, [Code     =>] static_integer_EXPRESSION]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION

This pragma is implemented only in the OpenVMS implementation of GNAT. It allows OpenVMS conditions (for example, from OpenVMS system services or other OpenVMS languages) to be propagated to Ada programs as Ada exceptions. The pragma specifies that the exception associated with an exception declaration in an Ada program be defined externally (in non-Ada code). For further details on this pragma, see the DEC Ada Language Reference Manual, section 13.9a.3.1.

Import_Function ...

Syntax:

pragma Import_Function (
     [Internal                 =>] LOCAL_NAME,
  [, [External                 =>] EXTERNAL_SYMBOL]
  [, [Parameter_Types          =>] PARAMETER_TYPES]
  [, [Result_Type              =>] SUBTYPE_MARK]
  [, [Mechanism                =>] MECHANISM]
  [, [Result_Mechanism         =>] MECHANISM_NAME]
  [, [First_Optional_Parameter =>] IDENTIFIER]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION

PARAMETER_TYPES ::=
  null
| SUBTYPE_MARK {, SUBTYPE_MARK}

MECHANISM ::=
  MECHANISM_NAME
| (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})

MECHANISM_ASSOCIATION ::=
  [formal_parameter_NAME =>] MECHANISM_NAME

MECHANISM_NAME ::=
  Value
| Reference
| Descriptor [([Class =>] CLASS_NAME)]

CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca

This pragma is used in conjunction with a pragma Import to specify additional information for an imported function. The pragma Import (or equivalent pragma Interface) must precede the Import_Function pragma and both must appear in the same declarative part as the function specification. The Internal_Name argument must uniquely designate the function to which the pragma applies. If more than one function name exists of this name in the declarative part you must use the Parameter_Types and Result_Type parameters to achieve the required unique designation. Subtype marks in these parameters must exactly match the subtypes in the corresponding function specification, using positional notation to match parameters with subtype marks. You may optionally use the Mechanism and Result_Mechanism parameters to specify passing mechanisms for the parameters and result. If you specify a single mechanism name, it applies to all parameters. Otherwise you may specify a mechanism on a parameter by parameter basis using either positional or named notation. If the mechanism is not specified, the default mechanism is used. Passing by descriptor is supported only on the to OpenVMS ports of GNAT First_Optional_Parameter applies only to OpenVMS ports of GNAT. It specifies that the designated parameter and all following parameters are optional, meaning that they are not passed at the generated code level (this is distinct from the notion of optional parameters in Ada where the parameters are passed anyway with the designated optional parameters). All optional parameters must be of mode IN and have default parameter values that are either known at compile time expressions, or uses of the 'Null_Parameter attribute.

pragma Import_Object

Syntax:

pragma Import_Object
     [Internal =>] LOCAL_NAME,
  [, [External =>] EXTERNAL_SYMBOL],
  [, [Size     =>] EXTERNAL_SYMBOL])

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION

This pragma designates an object as imported, and apart from the extended rules for external symbols, is identical in effect to the use of the normal Import pragma applied to an object. Unlike the subprogram case, you need not use a separate Import pragma, although you may do so (and probably should do so from a portability point of view). size is syntax checked, but otherwise ignored by GNAT.

pragma Import_Procedure

Syntax:

pragma Import_Procedure (
     [Internal                 =>] LOCAL_NAME,
  [, [External                 =>] EXTERNAL_SYMBOL]
  [, [Parameter_Types          =>] PARAMETER_TYPES]
  [, [Mechanism                =>] MECHANISM]
  [, [First_Optional_Parameter =>] IDENTIFIER]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION

PARAMETER_TYPES ::=
  null
| SUBTYPE_MARK {, SUBTYPE_MARK}

MECHANISM ::=
  MECHANISM_NAME
| (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})

MECHANISM_ASSOCIATION ::=
  [formal_parameter_NAME =>] MECHANISM_NAME

MECHANISM_NAME ::=
  Value
| Reference
| Descriptor [([Class =>] CLASS_NAME)]

CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca

This pragma is identical to Import_Function except that it applies to a procedure rather than a function and the parameters Result_Type and Result_Mechanism are not permitted.

pragma Import_Valued_Procedure ...

Syntax:

pragma Import_Valued_Procedure (
     [Internal                 =>] LOCAL_NAME,
  [, [External                 =>] EXTERNAL_SYMBOL]
  [, [Parameter_Types          =>] PARAMETER_TYPES]
  [, [Mechanism                =>] MECHANISM]
  [, [First_Optional_Parameter =>] IDENTIFIER]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION

PARAMETER_TYPES ::=
  null
| SUBTYPE_MARK {, SUBTYPE_MARK}

MECHANISM ::=
  MECHANISM_NAME
| (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})

MECHANISM_ASSOCIATION ::=
  [formal_parameter_NAME =>] MECHANISM_NAME

MECHANISM_NAME ::=
  Value
| Reference
| Descriptor [([Class =>] CLASS_NAME)]

CLASS_NAME ::= ubs | ubsb | uba | s | sb | a | nca

This pragma is identical to Import_Procedure except that the first parameter of local_name, which must be present, must be of mode OUT, and externally the subprogram is treated as a function with this parameter as the result of the function. The purpose of this capability is to allow the use of OUT and IN OUT parameters in interfacing to external functions (which are not permitted in Ada functions). You may optionally use the Mechanism parameters to specify passing mechanisms for the parameters. If you specify a single mechanism name, it applies to all parameters. Otherwise you may specify a mechanism on a parameter by parameter basis using either positional or named notation. If the mechanism is not specified, the default mechanism is used.

pragma Inline_Always

Syntax:

pragma Inline_Always (NAME [, NAME]);

Similar to pragma Inline except that inlining is not subject to the use of option -gnatn for inter-unit inlining.

pragma Inline_Generic

Syntax:

pragma Inline_Generic (generic_package_NAME)

This is implemented for compatibility with DEC Ada 83 and is recognized, but otherwise ignored, by GNAT. All generic instantiations are inlined by default when using GNAT.

pragma Interface_Name

Syntax:

pragma Interface_Name ( 
     [Entity        =>] LOCAL_NAME
  [, [External_Name =>] static_string_EXPRESSION]
  [, [Link_Name     =>] static_string_EXPRESSION]);

This pragma provides an alternative way of specifying the interface name for an interfaced subprogram, and is provided for compatibility with Ada 83 compilers that use the pragma for this purpose. You must provide at least one of External_Name or Link_Name.

pragma Linker_Alias

Syntax:

pragma Linker_Alias (
  [Entity =>] LOCAL_NAME
  [Alias  =>] static_string_EXPRESSION);

This pragma establishes a linker alias for the given named entity. For further details on the exact effect, consult the GCC manual.

pragma Linker_Section

Syntax:

pragma Linker_Section (
  [Entity  =>] LOCAL_NAME
  [Section =>] static_string_EXPRESSION);

This pragma specifies the name of the linker section for the given entity. For further details on the exact effect, consult the GCC manual.

pragma No_Runtime

Syntax:

pragma No_Runtime;

This is a configuration pragma that makes sure the user code does not use nor need anything from the GNAT run time. This is mostly useful in context where code certification is required. Please consult the GNORT product documentation for additional information.

pragma Normalize_Scalars

Syntax:

pragma Normalize_Scalars;

This is a language defined pragma which is fully implemented in GNAT. The effect is to cause all scalar objects that are not otherwise initialized to be initialized. The initial values are implementation dependent and are as follows:

Standard.Character

Objects whose root type is Standard.Character are initialized to Character'Last. This will be out of range of the subtype only if the subtype range excludes this value.

Standard.Wide_Character

Objects whose root type is Standard.Wide_Character are initialized to Wide_Character'Last. This will be out of range of the subtype only if the subtype range excludes this value.

Integer types

Objects of an integer type are initialized to base_type'First, where base_type is the base type of the object type. This will be out of range of the subtype only if the subtype range excludes this value. For example, if you declare the subtype:

subtype Ityp is integer range 1 .. 10;

then objects of type x will be initialized to Integer'First, a negative number that is certainly outside the range of subtype Ityp.

Real types

Objects of all real types (fixed and floating) are initialized to base_type'First, where base_Type is the base type of the object type. This will be out of range of the subtype only if the subtype range excludes this value.

Modular types

Objects of a modular type are initialized to typ'Last. This will be out of range of the subtype only if the subtype excludes this value.

Enumeration types

Objects of an enumeration type are initialized to all one-bits, i.e. to the value 2 ** typ'Size - 1. This will be out of range of the enumeration subtype in all cases except where the subtype contains exactly 2**8, 2**16, or 2**32.

pragma Long_Float

Syntax:

pragma Long_Float (FLOAT_FORMAT);

FLOAT_FORMAT ::= D_Float | G_Float

This pragma is implemented only in the OpenVMS implementation of GNAT. It allows control over the internal representation chosen for the predefined type Long_Float and for floating point type representations with digits specified in the range 7 .. 15. For further details on this pragma, see the DEC Ada Language Reference Manual, section 3.5.7b. Note that to use this pragma, the standard runtime libraries must be recompiled. See the description of the GNAT LIBRARY command in the OpenVMS version of the GNAT Users Guide for details on the use of this command.

pragma Machine_Attribute ...

Syntax:

pragma Machine_Attribute (
  [Attribute_Name =>] string_EXPRESSION,
  [Entity         =>] LOCAL_NAME);

Machine dependent attributes can be specified for types and/or declarations. Currently only subprogram entities are supported. This pragma is semantically equivalent to __attribute__(( string_expression)) in GNU C, where string_expression> is recognized by the GNU C macros VALID_MACHINE_TYPE_ATTRIBUTE and VALID_MACHINE_DECL_ATTRIBUTE which are defined in the configuration header file `tm.h' for each machine. See the GCC manual for further information.

pragma Main_Storage

Syntax:

pragma Main_Storage
  (MAIN_STORAGE_OPTION [, MAIN_STORAGE_OPTION]);

MAIN_STORAGE_OPTION ::=
  [WORKING_STORAGE =>] static_SIMPLE_EXPRESSION
| [TOP_GUARD       =>] static_SIMPLE_EXPRESSION

This pragma is provided for compatibility with OpenVMS Vax Systems. It has no effect in GNAT, other than being syntax checked. Note that the pragma also has no effect in DEC Ada 83 for OpenVMS Alpha Systems.

pragma No_Return

Syntax:

pragma No_Return (procedure_LOCAL_NAME);

procedure_local_NAME must refer to one or more procedure declarations in the current declarative part. A procedure to which this pragma is applied may not contain any explicit return statements, and also may not contain any implicit return statements from falling off the end of a statement sequence. One use of this pragma is to identify procedures whose only purpose is to raise an exception. Another use of this pragma is to suppress incorrect warnings about missing returns in functions, where the last statement of a function statement sequence is a call to such a procedure.

pragma Passive

Syntax:

pragma Passive ([Semaphore | No]);

Syntax checked, but otherwise ignored by GNAT. This is recognized for compatibility with DEC Ada 83 implementations, where it is used within a task definition to request that a task be made passive. If the argument Semaphore is present, or no argument is omitted, then DEC Ada 83 treats the pragma as an assertion that the containing task is passive and that optimization of context switch with this task is permitted and desired. If the argument No is present, the task must not be optimized. GNAT does not attempt to optimize any tasks in this manner (since protected objects are available in place of passive tasks).

pragma Polling

Syntax:

pragma Polling (ON | OFF);

This pragma controls the generation of polling code. This is normally off. If pragma Polling (ON) is used then periodic calls are generated to the routine Ada.Exceptions.Poll. This routine is a separate unit in the runtime library, and can be found in file a-excpol.adb. Pragma polling can appear as a configuration pragma (for example it can be placed in the gnat.adc file) to enable polling globally, or it can be used in the statement or declaration sequence to control polling more locally. A call to the polling routine is generated at the start of every loop and at the start of every subprogram call. This guarantees that the Poll routine is called frequently, and places an upper bound (determined by the complexity of the code) on the period between two Poll calls. The primary purpose of the polling interface is to enable asynchronous aborts on targets that cannot otherwise support it (for example Windows NT), but it may be used for any other purpose requiring periodic polling. The standard version is null, and can be replaced by a user program. This will require re-compilation of the Ada.Exceptions package that can be found in files a-except.ads/adb. A standard alternative unit (called 4wexcpol.adb in the standard GNAT distribution) is used to enable the asynchronous abort capability on targets that do not normally support the capability. The version of Poll in this file makes a call to the appropriate runtime routine to test for an abort condition. Note that polling can also be enabled by use of the -gnatP switch. See the GNAT User's Guide for details.

pragma Propagate_Exceptions

Syntax:

pragma Propagate_Exceptions (subprogram_LOCAL_NAME);

This pragma indicates that the given entity, which is the name of an imported foreign-langauge subprogram may receive an Ada exception, and that the exception should be propagated. It is relevant only if zero cost exception handling is in use, and is thus never needed if the alternative longjmp/setjmp implementation of exceptions is used (although it is harmless to use it in such cases). The implementation of fast exceptions always properly propagates exceptions through Ada code, as described in the Ada Reference Manual. However, this manual is silent about the propagation of exceptions through foreign code. For example, consider the situation where P1 calls P2, and P2 calls P3, where P1 and P3 are in Ada, but P2 is in C. P3 raises an Ada exception. The question is whether or not it will be propagated through P2 and can be handled in P1. For the longjmp/setjmp implementation of exceptions, the answer is always yes. For some targets on which zero cost exception handling is implemented, the answer is also always yes. However, there are some targets, notably in the current version all x86 architecture targets, in which the answer is that such propagation does not happen automatically. If such propagation is required on these targets, it is mandatory to use Propagate_Exceptions to name all foreign language routines through which Ada exceptions may be propagated.

pragma Psect_Object

Syntax:

pragma Psect_Object
     [Internal =>] LOCAL_NAME,
  [, [External =>] EXTERNAL_SYMBOL]
  [, [Size     =>] EXTERNAL_SYMBOL]

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION

This pragma is identical in effect to pragma Common_Object.

Pure_Function

Syntax:

pragma Pure_Function ([Entity =>] function_LOCAL_NAME);

This pragma appears in the same declarative part as a function declaration (or a set of function declarations if more than one overloaded declaration exists, in which case the pragma applies to all entities). If specifies that the function Entity is to be considered pure for the purposes of code generation. This means that the compiler can assume that there are no side effects, and in particular that two calls with identical arguments produce the same result. It also means that the function can be used in an address clause. Note that, quite deliberately, there are no static checks to try to ensure that this promise is met, so Pure_Function can be used with functions that are conceptually pure, even if they do modify global variables. For example, a square root function that is instrumented to count the number of times it is called is still conceptually pure, and can still be optimized, even though it modifies a global variable (the count). Memo functions are another example (where a table of previous calls is kept and consulted to avoid re-computation). Note: All functions in a Pure package are automatically pure, and there is no need to use pragma Pure_Function in this case. Note: If pragma Pure_Function is applied to a renamed function, it applies to the underlying renamed function. This can be used to disambiguate cases of overloading where some but not all functions in a set of overloaded functions are to be designated as pure.

pragma Ravenscar

Syntax:

pragma Ravenscar

A configuration pragma that establishes the following set of restrictions:

No_Abort_Statements

[RM D.7] There are no abort_statements, and there are no calls to Task_Identification.Abort_Task.

No_Select_Statements

There are no select_statements.

No_Task_Hierarchy

[RM D.7] All (non-environment) tasks depend directly on the environment task of the partition.

No_Task_Allocators

[RM D.7] There are no allocators for task types or types containing task subcomponents.

No_Dynamic_Priorities

[RM D.7] There are no semantic dependencies on the package Dynamic_Priorities.

No_Terminate_Alternatives

[RM D.7] There are no selective_accepts with terminate_alternatives

No_Nested_Finalization

[RM D.7] Objects with controlled parts access types that designate such objects shall be declared only at library level.

No_IO

[RM H.4] Semantic dependence on any of the library units Sequential_IO, Direct_IO, Text_IO, Wide_Text_IO, or Stream_IO is not allowed.

No_Streams

There are no semantic dependencies on Ada.Streams.

No_Exception_Handlers

There are no exception handlers. Raise statements are allowed.

No_Dynamic_Interrupts

There are no semantic dependencies on Ada.Interrupts.

No_Protected_Type_Allocators

There are no allocators for protected types or types containing protected subcomponents.

No_Local_Protected_Objects

Protected objects and access types that designate such objects shall be declared only at library level.

No_Requeue

Requeue statements are not allowed.

No_Calendar

There are no semantic dependencies on the package Ada.Calendar.

No_Relative_Delay

There are no delay_relative_statements.

No_Task_Attributes

There are no semantic dependencies on the Ada.Task_Attributes package and there are no references to the attributes Callable and Terminated [RM 9.9].

No_Enumeration_Maps

Similar to Discard_Names in C.5, but this is a restriction instead. For enumeration types with representation clauses the following attributes are not allowed (for subtype S with an enumeration clause):

• S'Pos
• S'Val
• S'Succ
• S'Pred
• S'Valid

Also not allowed are FOR loops where the index is of type S, nor arrays which are indexed by type S. Also not allowed are generic instantiations where the actual parameter is type S and the formal parameter is a discrete type.

Static_Storage_Size

The expression for pragma Storage_Size is static.

Boolean_Entry_Barriers

Entry barrier condition expressions shall be boolean objects which are declared in the protected type which contains the entry.

Max_Asynchronous_Select_Nesting = 0

[RM D.7] Specifies the maximum dynamic nesting level of asynchronous_selects. A value of zero prevents the use of any asynchronous_select.

Max_Task_Entries = 0

[RM D.7] Specifies the maximum number of entries per task. The bounds of every entry family of a task unit shall be static, or shall be defined by a discriminant of a subtype whose corresponding bound is static. A value of zero indicates that no rendezvous are possible. For the Ravenscar pragma, the value of Max_Task_Entries is always 0 (zero).

Max_Protected_Entries = 1

[RM D.7] Specifies the maximum number of entries per protected type. The bounds of every entry family of a protected unit shall be static, or shall be defined by a discriminant of a subtype whose corresponding bound is static. For the Ravenscar pragma the value of Max_Protected_Entries is always 1.

Max_Select_Alternatives = 0

[RM D.7] Specifies the maximum number of alternatives in a selective_accept. For the Ravenscar pragma the value if always 0.

No_Task_Termination

Tasks which terminate are erroneous.

No_Entry_Queue

No task can be queued on a protected entry. Note that this restrictions is checked at run time. The violation of this restriction generates a Program_Error exception.

This set of restrictions corresponds to the definition of the "Ravenscar Profile" for limited tasking, devised and published by the International Workshop On Real Time Ada", 1997. The above set is a superset of the restrictions provided by pragma Restricted_Run_Time, it includes eight additional restrictions (Boolean_Entry_Barriers, No_Select_Statements, No_Exception_Handlers, No_Calendar, Static_Storage_Size, No_Relative_Delay, No_Enumeration_Maps, and No_Task_Termination). This means that pragma Ravenscar, like the pragma Restricted_Run_Time, automatically causes the use of a simplified, more efficient version of the tasking run-time system.

pragma Restricted_Run_Time

Syntax:

pragma Restricted_Run_Time

A configuration pragma that establishes the following set of restrictions:

• No_Abort_Statements
• No_Entry_Queue
• No_Task_Hierarchy
• No_Task_Allocators
• No_Dynamic_Priorities
• No_Terminate_Alternatives
• No_Nested_Finalization
• No_IO
• No_Streams
• No_Dynamic_Interrupts
• No_Protected_Type_Allocators
• No_Local_Protected_Objects
• No_Requeue
• No_Task_Attributes
• Max_Asynchronous_Select_Nesting = 0
• Max_Task_Entries = 0
• Max_Protected_Entries = 1
• Max_Select_Alternatives = 0

This set of restrictions causes the automatic selection of a simplified version of the run time that provides improved performance for the limited set of tasking functionality permitted by this set of restrictions.

pragma Share_Generic

Syntax:

pragma Share_Generic (NAME {, NAME});

This pragma is recognized for compatibility with other Ada compilers but is ignored by GNAT. GNAT does not provide the capability for sharing of generic code. All generic instantiations result in making an inlined copy of the template with appropriate substitutions.

pragma Source_File_Name

Syntax:

pragma Source_File_Name (
  [Unit_Name   =>] unit_NAME,
  [FNAME_DESIG =>] static_string_EXPRESSION);

FNAME_DESIG => Body_File_Name | Spec_File_Name

Use this to override the normal naming convention. It is a configuration pragma, and so has the usual applicability of configuration pragmas (i.e. it applies to either an entire partition, or to all units in a compilation, or to a single unit, depending on how it is used. unit_name is mapped to file_name_literal. The identifier for the second argument is required, and indicates whether this is the file name for the spec or for the body.

pragma Source_Reference

Syntax:

pragma Source_Reference (INTEGER_LITERAL, STRING_LITERAL);

This pragma typically appears as the first line of a source file. integer_literal is the logical line number of the line following the pragma line (for use in error messages and debugging information). string_literal is a static string constant that specifies the file name to be used in error messages and debugging information. This is most notably used for the output of gnatchop with the `-r' switch, to make sure that the original unchopped source file is the one referred to. The second argument must be a string literal, it cannot be a static string expression other than a string literal. This is because its value is needed for error messages issued by all phases of the compiler.

pragma Stream_Convert

Syntax:

pragma Stream_Convert (
  [Entity =>] type_LOCAL_NAME,
  [Read   =>] function_NAME,
  [Write  =>] function NAME);

This pragma provides an efficient way of providing stream functions for types defined in packages. Not only is it simpler to use than declaring the necessary functions with attribute representation clauses, but more significantly, it allows the declaration to made in such a way that the stream packages are not loaded unless they are needed. The use of the Stream_Convert pragma adds no overhead at all, unless the stream attributes are actually used on the designated type. The first argument specifies the type for which stream functions are provided. The second parameter provides a function used to read values of this type. It must name a function whose argument type may be any subtype, and whose returned type must be the type given as the first argument to the pragma. The meaning of the Read parameter is that if a stream attribute directly or indirectly specifies reading of the type given as the first parameter, then a value of the type given as the argument to the Read function is read from the stream, and then the Read function is used to convert this to the required target type. Similarly the Write parameter specifies how to treat write attributes that directly or indirectly apply to the type given as the first parameter. It must have an input parameter of the type specified by the first parameter, and the return type must be the same as the input type of the Read function. The effect is to first call the Write function to convert to the given stream type, and then write the result type to the stream. The Read and Write functions must not be overloaded subprograms. If necessary renamings can be supplied to meet this requirement. The usage of this attribute is best illustrated by a simple example, taken from the GNAT implementation of package Ada.Strings.Unbounded:

function To_Unbounded (S : String) return Unbounded_String
  renames To_Unbounded_String;

pragma Stream_Convert
  (Unbounded_String, To_Unbounded, To_String);

The specifications of the referenced functions, as given in the Ada 95 Reference Manual are:

function To_Unbounded_String (Source : String)
  return Unbounded_String;

function To_String (Source : Unbounded_String)
  return String;

The effect is that if the value of an unbounded string is written to a stream, then the representation of the item in the stream is in the same format used for Standard.String, and this same representation is expected when a value of this type is read from the stream.

pragma Subtitle

Syntax:

pragma Subtitle ([Subtitle =>] STRING_LITERAL);

This pragma is recognized for compatibility with other Ada compilers but is ignored by GNAT.

pragma Suppress_All

Syntax:

pragma Suppress_All;

This pragma can only appear immediately following a compilation unit. The effect is to apply Suppress (All_Checks) to the unit which it follows. This pragma is implemented for compatibility with DEC Ada 83 usage. The use of pragma Suppress (All_Checks) as a normal configuration pragma is the preferred usage in GNAT.

pragma Suppress_Initialization

Syntax:

pragma Suppress_Initialization ([Entity =>] type_Name);

This pragma suppresses any implicit or explicit initialization associated with the given type name for all variables of this type.

pragma Task_Info

Syntax

pragma Task_Info (EXPRESSION);

This pragma appears within a task definition (like pragma Priority) and applies to the task in which it appears. The argument must be of type System.Task_Info.Task_Info_Type. The Task_Info pragma provides system dependent control over aspect of tasking implementation, for example, the ability to map tasks to specific processors. For details on the facilities available for the version of GNAT that you are using, see the documentation in the specification of package System.Task_Info in the runtime library.

pragma Task_Storage

Syntax:

pragma Task_Storage
  [Task_Type =>] LOCAL_NAME,
  [Top_Guard =>] static_integer_EXPRESSION);

This pragma specifies the length of the guard area for tasks. The guard area is an additional storage area allocated to a task. A value of zero means that either no guard area is created or a minimal guard area is created, depending on the target. This pragma can appear anywhere a Storage_Size attribute definition clause is allowed for a task type.

pragma Time_Slice

Syntax:

pragma Time_Slice (static_duration_EXPRESSION);

For implementations of GNAT on operating systems where it is possible to supply a time slice value, this pragma may be used for this purpose. It is ignored if it is used in a system that does not allow this control, or if it appears in other than the main program unit. Note that the effect of this pragma is identical to the effect of the DEC Ada 83 pragma of the same name when operating under OpenVMS systems.

pragma Title

Syntax:

pragma Title (TITLING_OPTION [, TITLING OPTION]);

TITLING_OPTION ::=
  [Title    =>] STRING_LITERAL,
| [Subtitle =>] STRING_LITERAL

Syntax checked but otherwise ignored by GNAT. This is a listing control pragma used in DEC Ada 83 implementations to provide a title and/or subtitle for the program listing. The program listing generated by GNAT does not have titles or subtitles. Unlike other pragmas, the full flexibility of named notation is allowed for this pragma, i.e. the parameters may be given in any order if named notation is used, and named and positional notation can be mixed following the normal rules for procedure calls in Ada.

pragma Unchecked_Union

Syntax:

pragma Unchecked_Union (first_subtype_LOCAL_NAME)

This pragma is used to declare that the specified type should be represented in a manner equivalent to a C union type, and is intended only for use in interfacing with C code that uses union types. In Ada terms, the named type must obey the following rules:

• It is a non-tagged non-limited record type.
• It has a single discrete discriminant with a default value.
• The component list consists of a single variant part.
• Each variant has a component list with a single component.
• No nested variants are allowed.
• No component has an explicit default value.
• No component has a non-static constraint.

In addition, given a type that meets the above requirements, the following restrictions apply to its use throughout the program:

• The discriminant name can be mentioned only in an aggregate.
• No subtypes may be created of this type.
• The type may not be constrained by giving a discriminant value.
• The type cannot be passed as the actual for a generic formal with a discriminant.

Equality and inequality operations on unchecked_unions are not available, since there is no discriminant to compare and the compiler does not even know how many bits to compare. It is implementation dependent whether this is detected at compile time as an illegality or whether it is undetected and considered to be an erroneous construct. In GNAT, a direct comparison is illegal, but GNAT does not attempt to catch the composite case (where two composites are compared that contain an unchecked union component), so such comparisons are simply considered erroneous. The layout of the resulting type corresponds exactly to a C union, where each branch of the union corresponds to a single variant in the Ada record. The semantics of the Ada program is not changed in any way by the pragma, i.e. provided the above restrictions are followed, and no erroneous incorrect references to fields or erroneous comparisons occur, the semantics is exactly as described by the Ada reference manual. Pragma Suppress (Discriminant_Check) applies implicitly to the type and the default convention is C

pragma Unimplemented_Unit

Syntax:

pragma Unimplemented_Unit;

If this pragma occurs in a unit that is processed by the compiler, GNAT aborts with the message `xxx not implemented', where xxx is the name of the current compilation unit. This pragma is intended to allow the compiler to handle unimplemented library units in a clean manner. The abort only happens if code is being generated. Thus you can use specs of unimplemented packages in syntax or semantic checking mode.

pragma Unreserve_All_Interrupts

Syntax:

pragma Unreserve_All_Interrupts;

Normally certain interrupts are reserved to the implementation. Any attempt to attach an interrupt causes Program_Error to be raised, as described in RM C.3.2(22). A typical example is the SIGINT interrupt used in many systems for an Ctrl-C interrupt. Normally this interrupt is reserved to the implementation, so that Ctrl-C can be used to interrupt execution. If the pragma Unreserve_All_Interrupts appears anywhere in any unit in a program, then all such interrupts are unreserved. This allows the program to handle these interrupts, but disables their standard functions. For example, if this pragma is used, then pressing Ctrl-C will not automatically interrupt execution. However, a program can then handle the SIGINT interrupt as it chooses. For a full list of the interrupts handled in a specific implementation, see the source code for the specification of Ada.Interrupts.Names in file s-intnam.ads. This is a target dependent file that contains the list of interrupts recognized for a given target. The documentation in this file also specifies what interrupts are affected by the use of the Unreserve_All_Interrupts pragma.

pragma Unsuppress

Syntax:

pragma Unsuppress (IDENTIFIER [, [On =>] NAME]);

This pragma undoes the effect of a previous pragma Suppress. If there is no corresponding pragma Suppress in effect, it has no effect. The range of the effect is the same as for pragma Suppress. The meaning of the arguments is identical to that used in pragma Suppress. One important application is to ensure that checks are on in cases where code depends on the checks for its correct functioning, so that the code will compile correctly even if the compiler switches are set to suppress checks.

pragma Use_VADS_Size

Syntax:

pragma Use_VADS_Size;

This is a configuration pragma. In a unit to which it applies, any use of the 'Size attribute is automatically interpreted as a use of the 'VADS_Size attribute. Note that this may result in incorrect semantic processing of valid Ada 95 programs. This is intended to aid in the handling of legacy code which depends on the interpretation of Size as implemented in the VADS compiler. See description of the VADS_Size attribute for further details.

pragma Volatile

Syntax:

pragma Volatile (local_NAME)

This pragma is defined by the Ada 95 Reference Manual, and the GNAT implementation is fully conformant with this definition. The reason it is mentioned in this section is that a pragma of the same name was supplied in some Ada 83 compilers, including DEC Ada 83. The Ada 95 implementation of pragma Volatile is upwards compatible with the implementation in Dec Ada 83.

pragma Warnings

Syntax:

pragma Warnings (On | Off [, LOCAL_NAME]);

Normally warnings are enabled, with the output being controlled by the command line switch. Warnings (Off) turns off generation of warnings until a Warnings (On) is encountered or the end of the current unit. If generation of warnings is turned off using this pragma, then no warning messages are output, regardless of the setting of the command line switches. The form with a single argument is a configuration pragma. If the local_name parameter is present, warnings are suppressed for the specified entity. This suppression is effective from the point where it occurs till the end of the extended scope of the variable (similar to the scope of Suppress).

pragma Weak_External

Syntax:

pragma Weak_External ([Entity =>] LOCAL_NAME);

This pragma specifies that the given entity should be marked as a weak external (one that does not have to be resolved) for the linker. For further details, consult the GCC manual.

Implementation Defined Attributes

Ada 95 defines (throughout the Ada 95 reference manual, summarized in annex K), a set of attributes that provide useful additional functionality in all areas of the language. These language defined attributes are implemented in GNAT and work as described in the Ada 95 Reference Manual.

In addition, Ada 95 allows implementations to define additional attributes whose meaning is defined by the implementation. GNAT provides a number of these implementation-dependent attributes which can be used to extend and enhance the functionality of the compiler. This section of the GNAT reference manual describes these additional attributes.

Note that any program using these attributes may not be portable to other compilers (although GNAT implements this set of attributes on all platforms). Therefore if portability to other compilers is an important consideration, you should minimize the use of these attributes.

Abort_Signal

Standard'Abort_Signal (Standard is the only allowed prefix) provides the entity for the special exception used to signal task abort or asynchronous transfer of control. Normally this attribute should only be used in the tasking runtime (it is highly peculiar, and completely outside the normal semantics of Ada, for a user program to intercept the abort exception).

Address_Size

Standard'Address_Size (Standard is the only allowed prefix) is a static constant giving the number of bits in an Address. It is used primarily for constructing the definition of Memory_Size in package Standard, but may be freely used in user programs.

Asm_Input

The Asm_Input attribute denotes a function that takes two parameters. The first is a string, the second is an expression of the type designated by the prefix. The first (string) argument is required to be a static expression, and is the constraint for the parameter, (e.g. what kind of register is required). The second argument is the value to be used as the input argument. The possible values for the constant are the same as those used in the RTL, and are dependent on the configuration file used to built the GCC back end. section Machine Code Insertions

Asm_Output

The Asm_Output attribute denotes a function that takes two parameters. The first is a string, the second is the name of a variable of the type designated by the attribute prefix. The first (string) argument is required to be a static expression and designates the constraint for the parameter (e.g. what kind of register is required). The second argument is the variable to be updated with the result. The possible values for constraint are the same as those used in the RTL, and are dependent on the configuration file used to build the GCC back end. If there are no output operands, then this argument may either be omitted, or explicitly given as No_Output_Operands. section Machine Code Insertions

AST_Entry

This attribute is implemented only in OpenVMS versions of GNAT. Applied to the name of an entry, it yields a value of the predefined type AST_Handler (declared in the predefined package System, as extended by the use of pragma Extend_System (Aux_DEC)). This value enables the given entry to be called when an AST occurs. For further details, refer to the DEC Ada Language Reference Manual, section 9.12a.

Bit

obj'Bit, where obj is any object, yields the bit offset within the storage unit (byte) that contains the first bit of storage allocated for the object. The value of this attribute is of the type Universal_Integer, and is always a non-negative number not exceeding the value of System.Storage_Unit. For an object that is a variable or a constant allocated in a register, the value is zero. (The use of this attribute does not force the allocation of a variable to memory). For an object that is a formal parameter, this attribute applies to either the matching actual parameter or to a copy of the matching actual parameter. For an access object the value is zero. Note that obj.all'Bit is subject to an Access_Check for the designated object. Similarly for a record component X.C'Bit is subject to a discriminant check and X(I).Bit and X(I1..I2)'Bit are subject to index checks. This attribute is designed to be compatible with the DEC Ada 83 definition and implementation of the Bit attribute.

Bit_Position

R.C'Bit, where R is a record object and C is one of the fields of the record type, yields the bit offset within the record contains the first bit of storage allocated for the object. The value of this attribute is of the type Universal_Integer. The value depends only on the field C and is independent of the alignment of the containing record R.

Code_Address

The 'Address attribute may be applied to subprograms in Ada 95, but the intended effect from the Ada 95 reference manual seems to be to provide an address value which can be used to call the subprogram by means of an address clause as in the following example:

procedure K is ...

procedure L;
for L'Address use K'Address;
pragma Import (Ada, L);

A call to L is then expected to result in a call to K. In Ada 83, where there were no access-to-subprogram values, this was a common work around for getting the effect of an indirect call. GNAT implements the above use of Address and the technique illustrated by the example code works correctly. However, for some purposes, it is useful to have the address of the start of the generated code for the subprogram. On some architectures, this is not necessarily the same as the Address value described above. For example, the Address value may reference a subprogram descriptor rather than the subprogram itself. The 'Code_Address attribute, which can only be applied to subprogram entities, always returns the address of the start of the generated code of the specified subprogram, which may or may not be the same value as is returned by the corresponding 'Address attribute.

Default_Bit_Order

Standard'Default_Bit_Order (Standard is the only permissible prefix), provides the value System.Default_Bit_Order as a Pos value (0 for High_Order_First, 1 for Low_Order_First). This is used to construct the definition of Default_Bit_Order in package System.

Elaborated

The prefix of the 'Elaborated attribute must be a unit name. The value is a Boolean which indicates whether or not the given unit has been elaborated. This attribute is primarily intended for internal use by the generated code for dynamic elaboration checking, but it can also be used in user programs. The value will always be True once elaboration of all units has been completed.

Elab_Body

This attribute can only be applied to a program unit name. It returns the entity for the corresponding elaboration procedure for elaborating the body of the referenced unit. This is used in the main generated elaboration procedure by the binder and is not normally used in any other context. However, there may be specialized situations in which it is useful to be able to call this elaboration procedure from Ada code, e.g. if it is necessary to do selective re-elaboration to fix some error.

Elab_Spec

This attribute can only be applied to a program unit name. It returns the entity for the corresponding elaboration procedure for elaborating the specification of the referenced unit. This is used in the main generated elaboration procedure by the binder and is not normally used in any other context. However, there may be specialized situations in which it is useful to be able to call this elaboration procedure from Ada code, e.g. if it is necessary to do selective re-elaboration to fix some error.

Emax

The Emax attribute is provided for compatibility with Ada 83. See the Ada 83 reference manual for an exact description of the semantics of this attribute.

Enum_Rep

For every enumeration subtype S, S'Enum_Rep denotes a function with the following specification:

function S'Enum_Rep (Arg : S'Base) return Universal_Integer;

It is also allowable to apply Enum_Rep directly to an object of an enumeration type or to a non-overloaded enumeration literal. In this case S'Enum_Rep is equivalent to typ'Enum_Rep(S) where typ is the type of the enumeration literal or object. The function returns the representation value for the given enumeration value. This will be equal to value of the Pos attribute in the absence of an enumeration representation clause. This is a static attribute (i.e. the result is static if the argument is static). S'Enum_Rep can also be used with integer types and objects, in which case it simply returns the integer value. The reason for this is to allow it to be used for (<>) discrete formal arguments in a generic unit that can be instantiated with either enumeration types or integer types.

Epsilon

The Epsilon attribute is provided for compatibility with Ada 83. See the Ada 83 reference manual for an exact description of the semantics of this attribute.

Fixed_Value

For every fixed-point type S, S'Fixed_Value denotes a function with the following specification:

function S'Fixed_Value (Arg : Universal_Integer) return S;

The value returned is the fixed-point value V such that

V = Arg * S'Small

The effect is thus equivalent to first converting the argument to the integer type used to represent S, and then doing an unchecked conversion to the fixed-point type. This attribute is primarily intended for use in implementation of the input-output functions for fixed-point values.

Has_Discriminants

The prefix of the Has_Discriminants attribute is a type. The result is a Boolean value which is True if the type has discriminants, and False otherwise. The intended use of this attribute is in conjunction with generic definitions. If the attribute is applied to a generic private type, it indicates whether or not the corresponding actual type has discriminants.

Img

The Img attribute differs from Image in that it may be applied to objects as well as types, in which case it gives the Image for the subtype of the object. This is convenient for debugging:

Put_Line ("X = " & X'Img);

has the same meaning as the more verbose:

Put_Line ("X = " & type'Image (X));

where type is the subtype of the object X.

Integer_Value

For every integer type S, S'Integer_Value denotes a function with the following specification:

function S'Integer_Value (Arg : Universal_Fixed) return S;

The value returned is the integer value V, such that

Arg = V * type'Small

The effect is thus equivalent to first doing an unchecked convert from the fixed-point type to its corresponding implementation type, and then converting the result to the target integer type. This attribute is primarily intended for use in implementation of the standard input-output functions for fixed-point values.

Large

The Large attribute is provided for compatibility with Ada 83. See the Ada 83 reference manual for an exact description of the semantics of this attribute.

Machine_Size

This attribute is identical to the Object_Size attribute. It is provided for compatibility with the DEC Ada 83 attribute of this name.

Mantissa

The Mantissa attribute is provided for compatibility with Ada 83. See the Ada 83 reference manual for an exact description of the semantics of this attribute.

Max_Interrupt_Priority

Standard'Max_Interrupt_Priority (Standard is the only permissible prefix), provides the value System.Max_Interrupt_Priority and is intended primarily for constructing this definition in package System.

Max_Priority

Standard'Max_Priority (Standard is the only permissible prefix) provides the value System.Max_Priority and is intended primarily for constructing this definition in package System.

Maximum_Alignment

Standard'Maximum_Alignment (Standard is the only permissible prefix) provides the maximum useful alignment value for the target. This is a static value that can be used to specify the alignment for an object, guaranteeing that it is properly aligned in all cases. This is useful when an external object is imported and its alignment requirements are unknown.

Mechanism_Code

function'Mechanism_Code yields an integer code for the mechanism used for the result of function, and subprogram'Mechanism_Code (n) yields the mechanism used for formal parameter number n (a static integer value with 1 meaning the first parameter) of subprogram. The code returned is:

1

by copy (value)

2

by reference

3

by descriptor (default descriptor class)

4

by descriptor (UBS: unaligned bit string)

5

by descriptor (UBSB: aligned bit string with arbitrary bounds)

6

by descriptor (UBA: unaligned bit array)

7

by descriptor (S: string, also scalar access type parameter)

8

by descriptor (SB: string with arbitrary bounds)

9

by descriptor (A: contiguous array)

10

by descriptor (NCA: non-contiguous array)

Values from 3-10 are only relevant to Digital OpenVMS implementations.

Null_Parameter

A reference T'Null_Parameter denotes an imaginary object of type or subtype T allocated at machine address zero. The attribute is allowed only as the default expression of a formal parameter, or as an actual expression of a subprogram call. In either case, the subprogram must be imported. The identity of the object is represented by the address zero in the argument list, independent of the passing mechanism (explicit or default). This capability is needed to specify that a zero address should be passed for a record or other composite object passed by reference. There is no way of indicating this without the Null_Parameter attribute.

Object_Size

The size of an object is not necessarily the same as the size of the type of an object. This is because by default object sizes are increased to be a multiple of the alignment of the object. For example, Natural'Size is 31, but by default objects of type Natural will have a size of 32 bits. Similarly, a record containing an integer and a character:

type Rec is record
   I : Integer;
   C : Character;
end record;

will have a size of 40 (that is Rec'Size will be 40. The alignment will be 4, because of the integer field, and so the default size of record objects for this type will be 64 (8 bytes). The type'Object_Size attribute has been added to GNAT to allow the default object size of a type to be easily determined. For example, Natural'Object_Size is 32, and Rec'Object_Size (for the record type in the above example) will be 64. Note also that, unlike the situation with the Size attribute as defined in the Ada RM, the Object_Size attribute can be specified individually for different subtypes. For example:

type R is new Integer;
subtype R1 is R range 1 .. 10;
subtype R2 is R range 1 .. 10;
for R2'Object_Size use 8;

In this example, R'Object_Size and R1'Object_Size are both 32 since the default object size for a subtype is the same as the object size for the the parent subtype. This means that objects of type R or R1 will by default be 32 bits (four bytes). But objects of type R2 will be only 8 bits (one byte), since R2'Object_Size has been set to 8.

Passed_By_Reference

type'Passed_By_Reference for any subtype type returns a value of type Boolean value that is True if the type is normally passed by reference and False if the type is normally passed by copy in calls. For scalar types, the result is always False and is static. For non-scalar types, the result is non-static.

Range_Length

type'Range_Length for any discrete type type yields the number of values represented by the subtype (zero for a null range). The result is static for static subtypes. Range_Length applied to the index subtype of a one dimensional array always gives the same result as Range applied to the array itself.

Safe_Emax

The Safe_Emax attribute is provided for compatibility with Ada 83. See the Ada 83 reference manual for an exact description of the semantics of this attribute.

Safe_Large

The Safe_Large attribute is provided for compatibility with Ada 83. See the Ada 83 reference manual for an exact description of the semantics of this attribute.

Safe_Large

The Safe_Large attribute is provided for compatibility with Ada 83. See the Ada 83 reference manual for an exact description of the semantics of this attribute.

Small

The Small attribute is defined in Ada 95 only for fixed-point types. GNAT also allows this attribute to be applied to floating-point types for compatibility with Ada 83. See the Ada 83 reference manual for an exact description of the semantics of this attribute when applied to floating-point types.

Storage_Unit

Standard'Storage_Unit (Standard is the only permissible prefix) provides the value System.Storage_Unit and is intended primarily for constructing this definition in package System.

Tick

Standard'Tick (Standard is the only permissible prefix) provides the value of System.Tick and is intended primarily for constructing this definition in package System.

To_Address

The System'To_Address (System is the only permissible prefix) denotes a function identical to System.Storage_Elements.To_Address except that it is a static attribute. This means that if its argument is a static expression, then the result of the attribute is a static expression. The result is that such an expression can be used in contexts (e.g. preelaborable packages) which require a static expression and where the function call could not be used (since the function call is always non-static, even if its argument is static).

Type_Class

type'Type_Class for any type or subtype type yields the value of the type class for the full type of type. If type is a generic formal type, the value is the value for the corresponding actual subtype. The value of this attribute is of type System.Aux_DEC.Type_Class, which has the following definition:

  type Type_Class is
    (Type_Class_Enumeration,
     Type_Class_Integer,
     Type_Class_Fixed_Point,
     Type_Class_Floating_Point,
     Type_Class_Array,
     Type_Class_Record,
     Type_Class_Access,
     Type_Class_Task,
     Type_Class_Address);

Protected types yield the value Type_Class_Task, which thus applies to all concurrent types. This attribute is designed to be compatible with the DEC Ada 83 attribute of the same name.

UET_Address

The UET_Address attribute can only be used for a prefix which denotes a library package. It yields the address of the unit exception table when zero cost exception handling is used. This attribute is intended only for use within the GNAT implementation. See the unit Ada.Exceptions in files `a-except.ads,a-except.adb' for details on how this attribute is used in the implementation.

Universal_Literal_String

The prefix of Universal_Literal_String must be a named number. The static result is the string consisting of the characters of the number as defined in the original source. This allows the user program to access the actual text of named numbers without intermediate conversions and without the need to enclose the strings in quotes (which would preclude their use as numbers). This is used internally for the construction of values of the floating-point attributes from the file `ttypef.ads', but may also be used by user programs.

Unrestricted_Access

The Unrestricted_Access attribute is similar to Access except that all accessibility and aliased view checks are omitted. This is a user-beware attribute. It is similar to Address, for which it is a desirable replacement where the value desired is an access type. In other words, its effect is identical to first applying the Address attribute and then doing an unchecked conversion to a desired access type. In GNAT, but not necessarily in other implementations, the use of static chains for inner level subprograms means that Unrestricted_Access applied to a subprogram yields a value that can be called as long as the subprogram is in scope (normal Ada 95 accessibility rules restrict this usage).

VADS_Size

The 'VADS_Size attribute is intended to make it easier to port legacy code which relies on the semantics of 'Size as implemented by the VADS Ada 83 compiler. GNAT makes a best effort at duplicating the same semantic interpretation. In particular, 'VADS_Size applied to a predefined or other primitive type with no Size clause yields the Object_Size (for example, Natural'Size is 32 rather than 31 on typical machines). In addition 'VADS_Size applied to an object gives the result that would be obtained by applying the attribute to the corresponding type.

Value_Size

type'Value_Size is the number of bits required to represent a value of the given subtype. It is the same as type'Size, but, unlike Size, may be set for non-first subtypes.

Word_Size

Standard'Word_Size (Standard is the only permissible prefix) provides the value System.Word_Size and is intended primarily for constructing this definition in package System.

Implementation Advice

The main text of the Ada 95 Reference Manual describes the required behavior of all Ada 95 compilers, and the GNAT compiler conforms to these requirements.

In addition, there are sections throughout the Ada 95 reference manual headed by the phrase "implementation advice". These sections are not normative, i.e. they do not specify requirements that all compilers must follow. Rather they provide advice on generally desirable behavior. You may wonder why they are not requirements. The most typical answer is that they describe behavior that seems generally desirable, but cannot be provided on all systems, or which may be undesirable on some systems.

As far as practical, GNAT follows the implementation advice sections in the Ada 95 Reference Manual. This chapter contains a table giving the reference manual section number, paragraph number and several keywords for each advice. Each entry consists of the text of the advice followed by the GNAT interpretation of this advice. Most often, this simply says "followed", which means that GNAT follows the advice. However, in a number of cases, GNAT deliberately deviates from this advice, in which case the text describes what GNAT does and why.

1.1.3(20): Error Detection

If an implementation detects the use of an unsupported Specialized Needs Annex feature at run time, it should raise Program_Error if feasible. Not relevant. All specialized needs annex features are either supported, or diagnosed at compile time.

1.1.3(31): Child Units

If an implementation wishes to provide implementation-defined extensions to the functionality of a language-defined library unit, it should normally do so by adding children to the library unit. Followed.

1.1.5(12): Bounded Errors

If an implementation detects a bounded error or erroneous execution, it should raise Program_Error. Followed in all cases in which the implementation detects a bounded error or erroneous execution. Not all such situations are detected at runtime.

2.8(16): Pragmas

Normally, implementation-defined pragmas should have no semantic effect for error-free programs; that is, if the implementation-defined pragmas are removed from a working program, the program should still be legal, and should still have the same semantics. The following implementation defined pragmas are exceptions to this rule:

Abort_Defer

Affects semantics

Ada_83

Affects legality

Assert

Affects semantics

CPP_Class

Affects semantics

CPP_Constructor

Affects semantics

CPP_Destructor

Affects semantics

CPP_Virtual

Affects semantics

CPP_Vtable

Affects semantics

Debug

Affects semantics

Interface_Name

Affects semantics

Machine_Attribute

Affects semantics

Unimplemented_Unit

Affects legality

Unchecked_Union

Affects semantics

In each of the above cases, it is essential to the purpose of the pragma that this advice not be followed. For details see the separate section on implementation defined pragmas.

2.8(17-19): Pragmas

Normally, an implementation should not define pragmas that can make an illegal program legal, except as follows:

A pragma used to complete a declaration, such as a pragma Import;

A pragma used to configure the environment by adding, removing, or replacing library_items. See response to paragraph 16 of this same section.

3.5.2(5): Alternative Character Sets

If an implementation supports a mode with alternative interpretations for Character and Wide_Character, the set of graphic characters of Character should nevertheless remain a proper subset of the set of graphic characters of Wide_Character. Any character set "localizations" should be reflected in the results of the subprograms defined in the language-defined package Characters.Handling (see A.3) available in such a mode. In a mode with an alternative interpretation of Character, the implementation should also support a corresponding change in what is a legal identifier_letter. Not all wide character modes follow this advice, in particular the JIS and IEC modes reflect standard usage in Japan, and in these encoding, the upper half of the Latin-1 set is not part of the wide-character subset, since the most significant bit is used for wide character encoding. However, this only applies to the external forms. Internally there is no such restriction.

3.5.4(28): Integer Types

An implementation should support Long_Integer in addition to Integer if the target machine supports 32-bit (or longer) arithmetic. No other named integer subtypes are recommended for package Standard. Instead, appropriate named integer subtypes should be provided in the library package Interfaces (see B.2). Long_Integer is supported. Other standard integer types are supported so this advice is not fully followed. These types are supported for convenient interface to C, and so that all hardware types of the machine are easily available.

3.5.4(29): Integer Types

An implementation for a two's complement machine should support modular types with a binary modulus up to System.Max_Int*2+2. An implementation should support a non-binary modules up to Integer'Last. Followed.

3.5.5(8): Enumeration Values

For the evaluation of a call on S'Pos for an enumeration subtype, if the value of the operand does not correspond to the internal code for any enumeration literal of its type (perhaps due to an un-initialized variable), then the implementation should raise Program_Error. This is particularly important for enumeration types with noncontiguous internal codes specified by an enumeration_representation_clause. Followed.

3.5.7(17): Float Types

An implementation should support Long_Float in addition to Float if the target machine supports 11 or more digits of precision. No other named floating point subtypes are recommended for package Standard. Instead, appropriate named floating point subtypes should be provided in the library package Interfaces (see B.2). Short_Float and Long_Long_Float are also provided. The former provides improved compatibility with other implementations supporting this type. The latter corresponds to the highest precision floating-point type supported by the hardware. On most machines, this will be the same as Long_Float, but on some machines, it will correspond to the IEEE extended form. On the Silicon Graphics processors, which do not support IEEE extended form, Long_Long_Float is the same as Long_Float.

3.6.2(11): Multidimensional Arrays

An implementation should normally represent multidimensional arrays in row-major order, consistent with the notation used for multidimensional array aggregates (see 4.3.3). However, if a pragma Convention (Fortran, ...) applies to a multidimensional array type, then column-major order should be used instead (see B.5, "Interfacing with Fortran"). Followed.

9.6(30-31): Duration'Small

Whenever possible in an implementation, the value of Duration'Small should be no greater than 100 microseconds. Followed. (Duration'Small = 10**(-9)).

The time base for delay_relative_statements should be monotonic; it need not be the same time base as used for Calendar.Clock. Followed.

10.2.1(12): Consistent Representation

In an implementation, a type declared in a pre-elaborated package should have the same representation in every elaboration of a given version of the package, whether the elaborations occur in distinct executions of the same program, or in executions of distinct programs or partitions that include the given version. Followed, except in the case of tagged types. Tagged types involve implicit pointers to a local copy of a dispatch table, and these pointers have representations which thus depend on a particular elaboration of the package. It is not easy to see how it would be possible to follow this advice without severely impacting efficiency of execution.

11.4.1(19): Exception Information

Exception_Message by default and Exception_Information should produce information useful for debugging. Exception_Message should be short, about one line. Exception_Information can be long. Exception_Message should not include the Exception_Name. Exception_Information should include both the Exception_Name and the Exception_Message. Followed. For each exception that doesn't have a specified Exception_Message, the compiler generates one containing the location of the raise statement. This location has the form "file:line", where file is the short file name (without path information) and line is the line number in the file. Note that in the case of the Zero Cost Exception mechanism, these messages become redundant with the Exception_Information that contains a full backtrace of the calling sequence, so they are disabled. To disable explicitly the generation of the source location message, use the Pragma Discard_Names.

11.5(28): Suppression of Checks

The implementation should minimize the code executed for checks that have been suppressed. Followed.

13.1 (21-24): Representation Clauses

The recommended level of support for all representation items is qualified as follows:

An implementation need not support representation items containing non-static expressions, except that an implementation should support a representation item for a given entity if each non-static expression in the representation item is a name that statically denotes a constant declared before the entity. Followed. GNAT does not support non-static expressions in representation clauses unless they are constants declared before the entity. For example:

     X : typ;
     for X'Address use To_address (16#2000#); 

will be rejected, since the To_Address expression is non-static. Instead write:

     X_Address : constant Address : = 
     To_Address    ((16#2000#); 
     X : typ;
     for X'Address use X_Address;

An implementation need not support a specification for the Size for a given composite subtype, nor the size or storage place for an object (including a component) of a given composite subtype, unless the constraints on the subtype and its composite subcomponents (if any) are all static constraints. Followed. Size Clauses are not permitted on non-static components, as described above.

An aliased component, or a component whose type is by-reference, should always be allocated at an addressable location. Followed.

13.2(6-8): Packed Types

If a type is packed, then the implementation should try to minimize storage allocated to objects of the type, possibly at the expense of speed of accessing components, subject to reasonable complexity in addressing calculations.

The recommended level of support pragma Pack is:

For a packed record type, the components should be packed as tightly as possible subject to the Sizes of the component subtypes, and subject to any record_representation_clause that applies to the type; the implementation may, but need not, reorder components or cross aligned word boundaries to improve the packing. A component whose Size is greater than the word size may be allocated an integral number of words. Followed. Tight packing of arrays is supported for all component sizes up to 32-bits, which is the word size on typical implementations of GNAT.

An implementation should support Address clauses for imported subprograms. Followed.

13.3(14-19): Address Clauses

For an array X, X'Address should point at the first component of the array, and not at the array bounds. Followed.

The recommended level of support for the Address attribute is:

X'Address should produce a useful result if X is an object that is aliased or of a by-reference type, or is an entity whose Address has been specified. Followed. A valid address will be produced even if none of those conditions have been met. If necessary, the object is forced into memory to ensure the address is valid.

An implementation should support Address clauses for imported subprograms. Followed.

Objects (including subcomponents) that are aliased or of a by-reference type should be allocated on storage element boundaries. Followed.

If the Address of an object is specified, or it is imported or exported, then the implementation should not perform optimizations based on assumptions of no aliases. Followed.

13.3(29-35): Alignment Clauses

The recommended level of support for the Alignment attribute for subtypes is:

An implementation should support specified Alignments that are factors and multiples of the number of storage elements per word, subject to the following: Followed.

An implementation need not support specified Alignments for combinations of Sizes and Alignments that cannot be easily loaded and stored by available machine instructions. Followed.

An implementation need not support specified Alignments that are greater than the maximum Alignment the implementation ever returns by default. Followed.

The recommended level of support for the Alignment attribute for objects is:

Same as above, for subtypes, but in addition: Followed.

For stand-alone library-level objects of statically constrained subtypes, the implementation should support all Alignments supported by the target linker. For example, page alignment is likely to be supported for such objects, but not for subtypes. Followed.

13.3(42-43): Size Clauses

The recommended level of support for the Size attribute of objects is:

A Size clause should be supported for an object if the specified Size is at least as large as its subtype's Size, and corresponds to a size in storage elements that is a multiple of the object's Alignment (if the Alignment is nonzero). Followed.

13.3(50-56): Size Clauses

If the Size of a subtype is specified, and allows for efficient independent addressability (see 9.10) on the target architecture, then the Size of the following objects of the subtype should equal the Size of the subtype:

Aliased objects (including components). Followed.

Size clause on a composite subtype should not affect the internal layout of components. Followed.

The recommended level of support for the Size attribute of subtypes is:

The Size (if not specified) of a static discrete or fixed point subtype should be the number of bits needed to represent each value belonging to the subtype using an unbiased representation, leaving space for a sign bit only if the subtype contains negative values. If such a subtype is a first subtype, then an implementation should support a specified Size for it that reflects this representation. Followed.

For a subtype implemented with levels of indirection, the Size should include the size of the pointers, but not the size of what they point at. Followed.

13.3(71-73): Component Size Clauses

The recommended level of support for the Component_Size attribute is:

An implementation need not support specified Component_Sizes that are less than the Size of the component subtype. Followed.

An implementation should support specified Component_Sizes that are factors and multiples of the word size. For such Component_Sizes, the array should contain no gaps between components. For other Component_Sizes (if supported), the array should contain no gaps between components when packing is also specified; the implementation should forbid this combination in cases where it cannot support a no-gaps representation. Followed.

13.4(9-10): Enumeration Representation Clauses

The recommended level of support for enumeration representation clauses is:

An implementation need not support enumeration representation clauses for boolean types, but should at minimum support the internal codes in the range System.Min_Int.System.Max_Int. Followed.

13.5.1(17-22): Record Representation Clauses

The recommended level of support for record_representation_clauses is:

An implementation should support storage places that can be extracted with a load, mask, shift sequence of machine code, and set with a load, shift, mask, store sequence, given the available machine instructions and run-time model. Followed.

A storage place should be supported if its size is equal to the Size of the component subtype, and it starts and ends on a boundary that obeys the Alignment of the component subtype. Followed.

If the default bit ordering applies to the declaration of a given type, then for a component whose subtype's Size is less than the word size, any storage place that does not cross an aligned word boundary should be supported. Followed.

An implementation may reserve a storage place for the tag field of a tagged type, and disallow other components from overlapping that place. Followed. The storage place for the tag field is the beginning of the tagged record, and its size is Address'Size. GNAT will reject an explicit component clause for the tag field.

An implementation need not support a component_clause for a component of an extension part if the storage place is not after the storage places of all components of the parent type, whether or not those storage places had been specified. Followed. The above advice on record representation clauses is followed, and all mentioned features are implemented.

13.5.2(5): Storage Place Attributes

If a component is represented using some form of pointer (such as an offset) to the actual data of the component, and this data is contiguous with the rest of the object, then the storage place attributes should reflect the place of the actual data, not the pointer. If a component is allocated discontinuously from the rest of the object, then a warning should be generated upon reference to one of its storage place attributes. Followed. There are no such components in GNAT.

13.5.3(7-8): Bit Ordering

The recommended level of support for the non-default bit ordering is:

If Word_Size = Storage_Unit, then the implementation should support the non-default bit ordering in addition to the default bit ordering. Followed. Word size does not equal storage size in this implementation. Thus non-default bit ordering is not supported.

13.7(37): Address as Private

Address should be of a private type. Followed.

13.7.1(16): Address Operations

Operations in System and its children should reflect the target environment semantics as closely as is reasonable. For example, on most machines, it makes sense for address arithmetic to "wrap around." Operations that do not make sense should raise Program_Error. Followed. Address arithmetic is modular arithmetic that wraps around. No operation raises Program_Error, since all operations make sense.

13.9(14-17): Unchecked Conversion

The Size of an array object should not include its bounds; hence, the bounds should not be part of the converted data. Followed.

The implementation should not generate unnecessary run-time checks to ensure that the representation of S is a representation of the target type. It should take advantage of the permission to return by reference when possible. Restrictions on unchecked conversions should be avoided unless required by the target environment. Followed. There are no restrictions on unchecked conversion. A warning is generated if the source and target types do not have the same size since the semantics in this case may be target dependent.

The recommended level of support for unchecked conversions is:

Unchecked conversions should be supported and should be reversible in the cases where this clause defines the result. To enable meaningful use of unchecked conversion, a contiguous representation should be used for elementary subtypes, for statically constrained array subtypes whose component subtype is one of the subtypes described in this paragraph, and for record subtypes without discriminants whose component subtypes are described in this paragraph. Followed.

13.11(23-25): Implicit Heap Usage

An implementation should document any cases in which it dynamically allocates heap storage for a purpose other than the evaluation of an allocator. Followed, the only other points at which heap storage is dynamically allocated are as follows:

• At initial elaboration time, to allocate dynamically sized global objects.
• To allocate space for a task when a task is created.
• To extend the secondary stack dynamically when needed. The secondary stack is used for returning variable length results.

A default (implementation-provided) storage pool for an access-to- constant type should not have overhead to support de-allocation of individual objects. Followed.

A storage pool for an anonymous access type should be created at the point of an allocator for the type, and be reclaimed when the designated object becomes inaccessible. Followed.

13.11.2(17): Unchecked De-allocation

For a standard storage pool, Free should actually reclaim the storage. Followed.

13.13.2(17): Stream Oriented Attributes

If a stream element is the same size as a storage element, then the normal in-memory representation should be used by Read and Write for scalar objects. Otherwise, Read and Write should use the smallest number of stream elements needed to represent all values in the base range of the scalar type. Followed.

A.1(52): Implementation Advice

If an implementation provides additional named predefined integer types, then the names should end with `Integer' as in `Long_Integer'. If an implementation provides additional named predefined floating point types, then the names should end with `Float' as in `Long_Float'. Followed.

A.3.2(49): Ada.Characters.Handling

If an implementation provides a localized definition of Character or Wide_Character, then the effects of the subprograms in Characters.Handling should reflect the localizations. See also 3.5.2. Followed. GNAT provides no such localized definitions.

A.4.4(106): Bounded-Length String Handling

Bounded string objects should not be implemented by implicit pointers and dynamic allocation. Followed. No implicit pointers or dynamic allocation are used.

A.5.2(46-47): Random Number Generation

Any storage associated with an object of type Generator should be reclaimed on exit from the scope of the object. Followed.

If the generator period is sufficiently long in relation to the number of distinct initiator values, then each possible value of Initiator passed to Reset should initiate a sequence of random numbers that does not, in a practical sense, overlap the sequence initiated by any other value. If this is not possible, then the mapping between initiator values and generator states should be a rapidly varying function of the initiator value. Followed. The generator period is sufficiently long for the first condition here to hold true.

A.10.7(23): Get_Immediate

The Get_Immediate procedures should be implemented with unbuffered input. For a device such as a keyboard, input should be available if a key has already been typed, whereas for a disk file, input should always be available except at end of file. For a file associated with a keyboard-like device, any line-editing features of the underlying operating system should be disabled during the execution of Get_Immediate. Followed.

B.1(39-41): Pragma Export

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. Followed.

Automatic elaboration of pre-elaborated packages should be provided when pragma Export is supported. Followed when the main program is in Ada. If the main program is in a foreign language, then adainit must be called to elaborate pre-elaborated packages.

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. Followed.

B.2(12-13): Package Interfaces

For each implementation-defined convention identifier, there should be a child package of package Interfaces with the corresponding name. This 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. Followed. An additional package not defined in the Ada 95 Reference Manual is Interfaces.CPP, used for interfacing to C++.

An implementation supporting an interface to C, COBOL, or Fortran should provide the corresponding package or packages described in the following clauses. Followed. GNAT provides all the packages described in this section.

B.3(63-71): Interfacing with C

An implementation should support the following interface correspondences between Ada and C. Followed.

An Ada procedure corresponds to a void-returning C function. Followed.

An Ada function corresponds to a non-void C function. Followed.

An Ada in scalar parameter is passed as a scalar argument to a C function. Followed.

An Ada in parameter of an access-to-object type with designated type T is passed as a t* argument to a C function, where t is the C type corresponding to the Ada type T. Followed.

An Ada access T parameter, or an Ada out or in out parameter of an elementary type T, is passed as a t* argument to a C function, where t is the C type corresponding to the Ada type T. In the case of an elementary out or in out parameter, a pointer to a temporary copy is used to preserve by-copy semantics. Followed.

An Ada parameter of a record type T, of any mode, is passed as a t* argument to a C function, where t is the C structure corresponding to the Ada type T. Followed. This convention may be overridden by the use of the C_Pass_By_Copy pragma, or Convention, or by explicitly specifying the mechanism for a given call using an extended import or export pragma.

An Ada parameter of an array type with component type T, of any mode, is passed as a t* argument to a C function, where t is the C type corresponding to the Ada type T. Followed.

An Ada parameter of an access-to-subprogram type is passed as a pointer to a C function whose prototype corresponds to the designated subprogram's specification. Followed.

B.4(95-98): Interfacing with COBOL

An Ada implementation should support the following interface correspondences between Ada and COBOL. Followed.

An Ada access T parameter is passed as a "BY REFERENCE" data item of the COBOL type corresponding to T. Followed.

An Ada in scalar parameter is passed as a "BY CONTENT" data item of the corresponding COBOL type. Followed.

Any other Ada parameter is passed as a "BY REFERENCE" data item of the COBOL type corresponding to the Ada parameter type; for scalars, a local copy is used if necessary to ensure by-copy semantics. Followed.

B.5(22-26): Interfacing with Fortran

An Ada implementation should support the following interface correspondences between Ada and Fortran: Followed.

An Ada procedure corresponds to a Fortran subroutine. Followed.

An Ada function corresponds to a Fortran function. Followed.

An Ada parameter of an elementary, array, or record type T is passed as a T argument to a Fortran procedure, where T 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. Followed.

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. Followed.

C.1(3-5): Access to Machine Operations

The machine code or intrinsic support should allow access to all operations normally available to assembly language programmers for the target environment, including privileged instructions, if any. Followed.

The interfacing pragmas (see Annex B) should support interface to assembler; the default assembler should be associated with the convention identifier Assembler. Followed.

If an entity is exported to assembly language, then the implementation should allocate it at an addressable location, and should ensure that it is retained by the linking process, even if not otherwise referenced from the Ada code. The implementation should assume that any call to a machine code or assembler subprogram is allowed to read or update every object that is specified as exported. Followed.

C.1(10-16): Access to Machine Operations

The implementation should ensure that little or no overhead is associated with calling intrinsic and machine-code subprograms. Followed for both intrinsics and machine-code subprograms.

It is recommended that intrinsic subprograms be provided for convenient access to any machine operations that provide special capabilities or efficiency and that are not otherwise available through the language constructs. Followed. A full set of machine operation intrinsic subprograms is provided.

Atomic read-modify-write operations -- e.g., test and set, compare and swap, decrement and test, enqueue/dequeue. Followed on any target supporting such operations.

Standard numeric functions -- e.g., sin, log. Followed on any target supporting such operations.

String manipulation operations -- e.g., translate and test. Followed on any target supporting such operations.

Vector operations -- e.g., compare vector against thresholds. Followed on any target supporting such operations.

Direct operations on I/O ports. Followed on any target supporting such operations.

C.3(28): Interrupt Support

If the Ceiling_Locking policy is not in effect, the implementation should provide means for the application to specify which interrupts are to be blocked during protected actions, if the underlying system allows for a finer-grain control of interrupt blocking. Followed. The underlying system does not allow for finer-grain control of interrupt blocking.

C.3.1(20-21): Protected Procedure Handlers

Whenever possible, the implementation should allow interrupt handlers to be called directly by the hardware. Followed on any target where the underlying operating system permits such direct calls.

Whenever practical, violations of any implementation-defined restrictions should be detected before run time. Followed. Compile time warnings are given when possible.

C.3.2(25): Package Interrupts

If implementation-defined forms of interrupt handler procedures are supported, such as protected procedures with parameters, then for each such form of a handler, a type analogous to Parameterless_Handler should be specified in a child package of Interrupts, with the same operations as in the predefined package Interrupts. Followed.

C.4(14): Pre-elaboration Requirements

It is recommended that pre-elaborated packages be implemented in such a way that there should be little or no code executed at run time for the elaboration of entities not already covered by the Implementation Requirements. Followed. Executable code is generated in some cases, e.g. loops to initialize large arrays.

C.5(8): Pragma Discard_Names

If the pragma applies to an entity, then the implementation should reduce the amount of storage used for storing names associated with that entity. Followed.

C.7.2(30): The Package Task_Attributes

Some implementations are targeted to domains in which memory use at run time must be completely deterministic. For such implementations, it is recommended that the storage for task attributes will be pre-allocated statically and not from the heap. This can be accomplished by either placing restrictions on the number and the size of the task's attributes, or by using the pre-allocated storage for the first N attribute objects, and the heap for the others. In the latter case, N should be documented. Not followed. This implementation is not targeted to such a domain.

D.3(17): Locking Policies

The implementation should use names that end with `_Locking' for locking policies defined by the implementation. Followed. No such implementation-defined locking policies exist.

D.4(16): Entry Queuing Policies

Names that end with `_Queuing' should be used for all implementation-defined queuing policies. Followed. No such implementation-defined queueing policies exist.

D.6(9-10): Preemptive Abort

Even though the abort_statement is included in the list of potentially blocking operations (see 9.5.1), it is recommended that this statement be implemented in a way that never requires the task executing the abort_statement to block. Followed.

On a multi-processor, the delay associated with aborting a task on another processor should be bounded; the implementation should use periodic polling, if necessary, to achieve this. Followed.

D.7(21): Tasking Restrictions

When feasible, the implementation should take advantage of the specified restrictions to produce a more efficient implementation. GNAT currently takes advantage of these restrictions by providing an optimized run time when the Ravenscar profile and the GNAT restricted run time set of restrictions are specified. See pragma Ravenscar and pragma Restricted_Run_Time for more details.

D.8(47-49): Monotonic Time

When appropriate, implementations should provide configuration mechanisms to change the value of Tick. Such configuration mechanisms are not appropriate to this implementation and are thus not supported.

It is recommended that Calendar.Clock and Real_Time.Clock be implemented as transformations of the same time base. Followed.

It is recommended that the best time base which exists in the underlying system be available to the application through Clock. Best may mean highest accuracy or largest range. Followed.

E.5(28-29): Partition Communication Subsystem

Whenever possible, the PCS on the called partition should allow for multiple tasks to call the RPC-receiver with different messages and should allow them to block until the corresponding subprogram body returns. Followed by GLADE, a separately supplied PCS that can be used with GNAT. For information on GLADE, contact Ada Core Technologies.

The Write operation on a stream of type Params_Stream_Type should raise Storage_Error if it runs out of space trying to write the Item into the stream. Followed by GLADE, a separately supplied PCS that can be used with GNAT. For information on GLADE, contact Ada Core Technologies.

F(7): COBOL Support

If COBOL (respectively, C) is widely supported in the target environment, implementations supporting the Information Systems Annex should provide the child package Interfaces.COBOL (respectively, Interfaces.C) specified in Annex B and should support a convention_identifier of COBOL (respectively, C) in the interfacing pragmas (see Annex B), thus allowing Ada programs to interface with programs written in that language. Followed.

F.1(2): Decimal Radix Support

Packed decimal should be used as the internal representation for objects of subtype S when S'Machine_Radix = 10. Not followed. GNAT ignores S'Machine_Radix and always uses binary representations.

G: Numerics

If Fortran (respectively, C) is widely supported in the target environment, implementations supporting the Numerics Annex should provide the child package Interfaces.Fortran (respectively, Interfaces.C) specified in Annex B and should support a convention_identifier of Fortran (respectively, C) in the interfacing pragmas (see Annex B), thus allowing Ada programs to interface with programs written in that language. Followed.

G.1.1(56-58): Complex Types

Because the usual mathematical meaning of multiplication of a complex operand and a real operand is that of the scaling of both components of the former by the latter, an implementation should not perform this operation by first promoting the real operand to complex type and then performing a full complex multiplication. In systems that, in the future, support an Ada binding to IEC 559:1989, the latter technique will not generate the required result when one of the components of the complex operand is infinite. (Explicit multiplication of the infinite component by the zero component obtained during promotion yields a NaN that propagates into the final result.) Analogous advice applies in the case of multiplication of a complex operand and a pure-imaginary operand, and in the case of division of a complex operand by a real or pure-imaginary operand. Not followed.

Similarly, because the usual mathematical meaning of addition of a complex operand and a real operand is that the imaginary operand remains unchanged, an implementation should not perform this operation by first promoting the real operand to complex type and then performing a full complex addition. In implementations in which the Signed_Zeros attribute of the component type is True (and which therefore conform to IEC 559:1989 in regard to the handling of the sign of zero in predefined arithmetic operations), the latter technique will not generate the required result when the imaginary component of the complex operand is a negatively signed zero. (Explicit addition of the negative zero to the zero obtained during promotion yields a positive zero.) Analogous advice applies in the case of addition of a complex operand and a pure-imaginary operand, and in the case of subtraction of a complex operand and a real or pure-imaginary operand. Not followed.

Implementations in which Real'Signed_Zeros is True should attempt to provide a rational treatment of the signs of zero results and result components. As one example, the result of the Argument function should have the sign of the imaginary component of the parameter X when the point represented by that parameter lies on the positive real axis; as another, the sign of the imaginary component of the Compose_From_Polar function should be the same as (respectively, the opposite of) that of the Argument parameter when that parameter has a value of zero and the Modulus parameter has a nonnegative (respectively, negative) value. Followed.

G.1.2(49): Complex Elementary Functions

Implementations in which Complex_Types.Real'Signed_Zeros is True should attempt to provide a rational treatment of the signs of zero results and result components. For example, many of the complex elementary functions have components that are odd functions of one of the parameter components; in these cases, the result component should have the sign of the parameter component at the origin. Other complex elementary functions have zero components whose sign is opposite that of a parameter component at the origin, or is always positive or always negative. Followed.

G.2.4(19): Accuracy Requirements

The versions of the forward trigonometric functions without a Cycle parameter should not be implemented by calling the corresponding version with a Cycle parameter of 2.0*Numerics.Pi, since this will not provide the required accuracy in some portions of the domain. For the same reason, the version of Log without a Base parameter should not be implemented by calling the corresponding version with a Base parameter of Numerics.e. Followed.

G.2.6(15): Complex Arithmetic Accuracy

The version of the Compose_From_Polar function without a Cycle parameter should not be implemented by calling the corresponding version with a Cycle parameter of 2.0*Numerics.Pi, since this will not provide the required accuracy in some portions of the domain. Followed.

Implementation Defined Characteristics

In addition to the implementation dependent pragmas and attributes, and the implementation advice, there are a number of other features of Ada 95 that are potentially implementation dependent. These are mentioned throughout the Ada 95 Reference Manual, and are summarized in annex M.

A requirement for conforming Ada compilers is that they provide documentation describing how the implementation deals with each of these issues. In this chapter, you will find each point in annex M listed followed by a description in italic font of how GNAT handles the implementation dependence.

You can use this chapter as a guide to minimizing implementation dependent features in your programs if portability to other compilers and other operating systems is an important consideration. The numbers in each section below correspond to the paragraph number in the Ada 95 Reference Manual.

2. Whether or not each recommendation given in Implementation Advice is followed. See 1.1.2(37). See section Implementation Advice.

3. Capacity limitations of the implementation. See 1.1.3(3). The complexity of programs that can be processed is limited only by the total amount of available virtual memory, and disk space for the generated object files.

4. Variations from the standard that are impractical to avoid given the implementation's execution environment. See 1.1.3(6). There are no variations from the standard.

5. Which code_statements cause external interactions. See 1.1.3(10). Any code_statement can potentially cause external interactions.

6. The coded representation for the text of an Ada program. See 2.1(4). See separate section on source representation.

7. The control functions allowed in comments. See 2.1(14). See separate section on source representation.

8. The representation for an end of line. See 2.2(2). See separate section on source representation.

9. Maximum supported line length and lexical element length. See 2.2(15). The maximum line length is 255 characters an the maximum length of a lexical element is also 255 characters.

10. Implementation defined pragmas. See 2.8(14).

See section Implementation Defined Pragmas.

11. Effect of pragma Optimize. See 2.8(27). Pragma Optimize, if given with a Time or Space parameter, checks that the optimization flag is set, and aborts if it is not.

12. The sequence of characters of the value returned by S'Image when some of the graphic characters of S'Wide_Image are not defined in Character. See 3.5(37). The sequence of characters is as defined by the wide character encoding method used for the source. See section on source representation for further details.

13. The predefined integer types declared in Standard. See 3.5.4(25).

Short_Short_Integer

8 bit signed

Short_Integer

(Short) 16 bit signed

Integer

32 bit signed

Long_Integer

32 bit signed

Long_Long_Integer

64 bit signed

14. Any nonstandard integer types and the operators defined for them. See 3.5.4(26). There are no nonstandard integer types.

15. Any nonstandard real types and the operators defined for them. See 3.5.6(8). There are no nonstandard real types.

16. What combinations of requested decimal precision and range are supported for floating point types. See 3.5.7(7). The precision and range is as defined by the IEEE standard.

17. The predefined floating point types declared in Standard. See 3.5.7(16).

Short_Float

32 bit IEEE short

Float

(Short) 32 bit IEEE short

Long_Float

64 bit IEEE long

Long_Long_Float

64 bit IEEE long

18. The small of an ordinary fixed point type. See 3.5.9(8). Fine_Delta is 2**(-63)

19. What combinations of small, range, and digits are supported for fixed point types. See 3.5.9(10). Any combinations are permitted that do not result in a small less than Fine_Delta and do not result in a mantissa larger than 63 bits.

20. The result of Tags.Expanded_Name for types declared within an unnamed block_statement. See 3.9(10). Block numbers of the form Bnnn, where nnn is a decimal integer are allocated.

21. Implementation-defined attributes. See 4.1.4(12). See section Implementation Defined Attributes.

22. Any implementation-defined time types. See 9.6(6). There are no implementation-defined time types.

23. The time base associated with relative delays. See 9.6(20). The time base used is that provided by the C library function gettimeofday.

24. The time base of the type Calendar.Time. See 9.6(23). The time base used is that provided by the C library function gettimeofday.

25. The time zone used for package Calendar operations. See 9.6(24). The time zone used by package Calendar is the current system time zone setting for local time, as accessed by the C library function localtime.

26. Any limit on delay_until_statements of select_statements. See 9.6(29). There are no such limits.

27. Whether or not two non overlapping parts of a composite object are independently addressable, in the case where packing, record layout, or Component_Size is specified for the object. See 9.10(1). Separate components are independently addressable if they do not share overlapping storage units.

28. The representation for a compilation. See 10.1(2). A compilation is represented by a sequence of files presented to the compiler in a single invocation of the `gcc' command.

29. Any restrictions on compilations that contain multiple compilation_units. See 10.1(4). No single file can contain more than one compilation unit, but any sequence of files can be presented to the compiler as a single compilation.

30. The mechanisms for creating an environment and for adding and replacing compilation units. See 10.1.4(3). See separate section on compilation model.

31. The manner of explicitly assigning library units to a partition. See 10.2(2). See separate section on binding and linking programs.

32. The implementation-defined means, if any, of specifying which compilation units are needed by a given compilation unit. See 10.2(2). See separate section on compilation unit.

33. The manner of designating the main subprogram of a partition. See 10.2(7). The main program is designated by providing the name of the corresponding ali file as the input parameter to the binder.

34. The order of elaboration of library_items. See 10.2(18). The first constraint on ordering is that it meets the requirements of chapter 10 of the Ada 95 Reference Manual. This still leaves some implementation dependent choices, which are resolved by first elaborating bodies as early as possible (i.e. in preference to specs where there is a choice), and second by evaluating the immediate with clauses of a unit to determine the probably best choice, and third by elaborating in alphabetical order of unit names where a choice still remains.

35. Parameter passing and function return for the main subprogram. See 10.2(21). The main program has no parameters. It may be a procedure, or a function returning an integer type. In the latter case, the returned integer value is the return code of the program.

36. The mechanisms for building and running partitions. See 10.2(24). GNAT itself supports programs with only a single partition. The GNATDIST tool provided with the GLADE package (which also includes an implementation of the PCS) provides a completely flexible method for building and running programs consisting of multiple partitions. See the separate GLADE manual for details.

37. The details of program execution, including program termination. See 10.2(25). See separate section on compilation model.

38. The semantics of any non-active partitions supported by the implementation. See 10.2(28). Passive partitions are supported on targets where shared memory is provided by the operating system. See the GLADE reference manual for further details.

39. The information returned by Exception_Message. See 11.4.1(10). Exception message returns the null string unless a specific message has been passed by the program.

40. The result of Exceptions.Exception_Name for types declared within an unnamed block_statement. See 11.4.1(12). Blocks have implementation defined names of the form Bnnn where nnn is an integer.

41. The information returned by Exception_Information. See 11.4.1(13). Exception_Information contains the expanded name of the exception in upper case, and no other information.

42. Implementation-defined check names. See 11.5(27). No implementation-defined check names are supported.

43. The interpretation of each aspect of representation. See 13.1(20). See separate section on data representations.

44. Any restrictions placed upon representation items. See 13.1(20). See separate section on data representations.

45. The meaning of Size for indefinite subtypes. See 13.3(48). Size for an indefinite subtype is the maximum possible size, except that for the case of a subprogram parameter, the size of the parameter object is the actual size.

46. The default external representation for a type tag. See 13.3(75). The default external representation for a type tag is the fully expanded name of the type in upper case letters.

47. What determines whether a compilation unit is the same in two different partitions. See 13.3(76). A compilation unit is the same in two different partitions if and only if it derives from the same source file.

48. Implementation-defined components. See 13.5.1(15). The only implementation defined component is the tag for a tagged type, which contains a pointer to the dispatching table.

49. If Word_Size = Storage_Unit, the default bit ordering. See 13.5.3(5). Word_Size (32) is not the same as Storage_Unit (8) for this implementation, so no non-default bit ordering is supported. The default bit ordering corresponds to the natural endianness of the target architecture.

50. The contents of the visible part of package System and its language-defined children. See 13.7(2). See the definition of these packages in files `system.ads' and `s-stoele.ads'.

51. The contents of the visible part of package System.Machine_Code, and the meaning of code_statements. See 13.8(7). See the definition and documentation in file `s-maccod.ads'.

52. The effect of unchecked conversion. See 13.9(11). Unchecked conversion between types of the same size and results in an uninterpreted transmission of the bits from one type to the other. If the types are of unequal sizes, then in the case of discrete types, a shorter source is first zero or sign extended as necessary, and a shorter target is simply truncated on the left. For all non-discrete types, the source is first copied if necessary to ensure that the alignment requirements of the target are met, then a pointer is constructed to the source value, and the result is obtained by dereferencing this pointer after converting it to be a pointer to the target type.

53. The manner of choosing a storage pool for an access type when Storage_Pool is not specified for the type. See 13.11(17). There are 3 different standard pools used by the compiler when Storage_Pool is not specified depending whether the type is local to a subprogram or defined at the library level and whether Storage_Sizeis specified or not. See documentation in the runtime library units System.Pool_Global, System.Pool_Size and System.Pool_Local in files `s-poosiz.ads', `s-pooglo.ads' and `s-pooloc.ads' for full details on the default pools used.

54. Whether or not the implementation provides user-accessible names for the standard pool type(s). See 13.11(17).

See documentation in the sources of the run time mentioned in paragraph 53 . All these pools are accessible by means of with'ing these units.

55. The meaning of Storage_Size. See 13.11(18). Storage_Size is measured in storage units, and refers to the total space available for an access type collection, or to the primary stack space for a task.

56. Implementation-defined aspects of storage pools. See 13.11(22). See documentation in the sources of the run time mentioned in paragraph 53 for details on GNAT-defined aspects of storage pools.

57. The set of restrictions allowed in a pragma Restrictions. See 13.12(7). All RM defined Restriction identifiers are implemented. The following additional restriction identifiers are provided:

No_Implementation_Attributes

This restriction checks at compile time that no GNAT-defined attributes are present. With this restriction, the only attributes that can be used are those defined in the Ada 95 Reference Manual.

No_Implementation_Pragmas

This restriction checks at compile time that no GNAT-defined pragmas are present. With this restriction, the only pragmas that can be used are those defined in the Ada 95 Reference Manual.

No_Elaboration_Code

This restriction ensures at compile time that no elaboration code is generated. Note that this is not the same condition as is enforced by pragma Preelaborate. There are cases in which pragma Preelaborate still permits code to be generated (e.g. code to initialize a large array to all zeroes), and there are cases of units which do not meet the requirements for pragma Preelaborate, but for which no elaboration code is generated. Generally, it is the case that preelaborable units will meet the restrictions, with the exception of large aggregates initialized by others.

No_Complex_Barriers

This restriction ensures at compile time that barriers in entry declarations for protected types are restricted to references to simple boolean variables defined in the private part of the protected type. No other form of entry barriers is permitted. This is one of the restrictions of the Ravenscar profile for limited tasking (see also pragma Ravenscar).

No_Select_Statements

This restriction ensures at compile time no select statements of any kind are permitted, that is the keyword select may not appear. This is one of the restrictions of the Ravenscar profile for limited tasking (see also pragma Ravenscar).

58. The consequences of violating limitations on Restrictions pragmas. See 13.12(9). Restrictions that can be checked at compile time result in illegalities if violated. Currently there are no other consequences of violating restrictions.

59. The representation used by the Read and Write attributes of elementary types in terms of stream elements. See 13.13.2(9). The representation is the in-memory representation of the base type of the type, using the number of bits corresponding to the type'Size value, and the natural ordering of the machine.

60. The names and characteristics of the numeric subtypes declared in the visible part of package Standard. See A.1(3). See items describing the integer and floating-point types supported.

61. The accuracy actually achieved by the elementary functions. See A.5.1(1). The elementary functions correspond to the functions available in the C library. Only fast math mode is implemented.

62. The sign of a zero result from some of the operators or functions in Numerics.Generic_Elementary_Functions, when Float_Type'Signed_Zeros is True. See A.5.1(46). The sign of zeroes follows the requirements of the IEEE 754 standard on floating-point.

63. The value of Numerics.Float_Random.Max_Image_Width. See A.5.2(27). Maximum image width is 649, see library file `a-numran.ads'.

64. The value of Numerics.Discrete_Random.Max_Image_Width. See A.5.2(27). Maximum image width is 80, see library file `a-nudira.ads'.

65. The algorithms for random number generation. See A.5.2(32). The algorithm is documented in the source files `a-numran.ads' and `a-numran.adb'.

66. The string representation of a random number generator's state. See A.5.2(38). See the documentation contained in the file `a-numran.adb'.

67. The minimum time interval between calls to the time-dependent Reset procedure that are guaranteed to initiate different random number sequences. See A.5.2(45). The minimum period between reset calls to guarantee distinct series of random numbers is one microsecond.

68. The values of the Model_Mantissa, Model_Emin, Model_Epsilon, Model, Safe_First, and Safe_Last attributes, if the Numerics Annex is not supported. See A.5.3(72). See the source file `ttypef.ads' for the values of all numeric attributes.

69. Any implementation-defined characteristics of the input-output packages. See A.7(14). There are no special implementation defined characteristics for these packages.

70. The value of Buffer_Size in Storage_IO. See A.9(10). All type representations are contiguous, and the Buffer_Size is the value of type'Size rounded up to the next storage unit boundary.

71. External files for standard input, standard output, and standard error See A.10(5). These files are mapped onto the files provided by the C streams libraries. See source file `i-cstrea.ads' for further details.

72. The accuracy of the value produced by Put. See A.10.9(36). If more digits are requested in the output than are represented by the precision of the value, zeroes are output in the corresponding least significant digit positions.

73. The meaning of Argument_Count, Argument, and Command_Name. See A.15(1). These are mapped onto the argv and argc parameters of the main program in the natural manner.

74. Implementation-defined convention names. See B.1(11). The following convention names are supported

Ada

Ada

Asm

Assembly language

Assembler

Assembly language

C

C

C_Pass_By_Copy

Treated like C, except for record types

COBOL

COBOL

CPP

C++

Default

Treated the same as C

DLL

DLL (used for Windows implementations only) is handled like the Stdcall convention. This convention is used to access variables and functions (with Stdcall convention) in a DLL.

External

Treated the same as C

Fortran

Fortran

Intrinsic

Intrinsic

Stdcall

Stdcall (used for Windows implementations only). This convention correspond to the WINAPI (previously called Pascal convention) C/C++ convention under Windows. A function with this convention clean the stack before exit.

In addition, all otherwise unrecognized convention names are also treated as being synonymous with convention C. In all implementations except for VMS, use of such other names results in a warning. In VMS implementations, these names are accepted silently.

75. The meaning of link names. See B.1(36). Link names are the actual names used by the linker.

76. The manner of choosing link names when neither the link name nor the address of an imported or exported entity is specified. See B.1(36). The default linker name is that which would be assigned by the relevant external language, interpreting the Ada name as being in all lower case letters.

77. The effect of pragma Linker_Options. See B.1(37). The string passed to Linker_Options is presented uninterpreted as an argument to the link command.

78. The contents of the visible part of package Interfaces and its language-defined descendants. See B.2(1). See files with prefix `i-' in the distributed library.

79. Implementation-defined children of package Interfaces. The contents of the visible part of package Interfaces. See B.2(11). See files with prefix `i-' in the distributed library.

80. The types Floating, Long_Floating, Binary, Long_Binary, Decimal_ Element, and COBOL_Character; and the initialization of the variables Ada_To_COBOL and COBOL_To_Ada, in Interfaces.COBOL. See B.4(50).

Floating

Float

Long_Floating

(Floating) Long_Float

Binary

Integer

Long_Binary

Long_Long_Integer

Decimal_Element

Character

COBOL_Character

Character

For initialization, see the file `i-cobol.ads' in the distributed library.

81. Support for access to machine instructions. See C.1(1). See documentation in file `s-maccod.ads' in the distributed library.

82. Implementation-defined aspects of access to machine operations. See C.1(9). See documentation in file `s-maccod.ads' in the distributed library.

83. Implementation-defined aspects of interrupts. See C.3(2). Interrupts are mapped to signals or conditions as appropriate. See definition of unit Ada.Interrupt_Names in source file `a-intnam.ads' for details on the interrupts supported on a particular target.

84. Implementation-defined aspects of pre-elaboration. See C.4(13). GNAT does not permit a partition to be restarted without reloading, except under control of the debugger.

85. The semantics of pragma Discard_Names. See C.5(7). Pragma Discard_Names causes names of enumeration literals to be suppressed. In the presence of this pragma, the Image attribute provides the image of the Pos of the literal, and Value accepts Pos values.

86. The result of the Task_Identification.Image attribute. See C.7.1(7). The result of this attribute is an 8-digit hexadecimal string representing the virtual address of the task control block.

87. The value of Current_Task when in a protected entry or interrupt handler. See C.7.1(17). Protected entries or interrupt handlers can be executed by any convenient thread, so the value of Current_Task is undefined.

88. The effect of calling Current_Task from an entry body or interrupt handler. See C.7.1(19). The effect of calling Current_Task from an entry body or interrupt handler is to return the identification of the task currently executing the code.

89. Implementation-defined aspects of Task_Attributes. See C.7.2(19). There are no implementation-defined aspects of Task_Attributes.

90. Values of all Metrics. See D(2). Information on metrics is not yet available.

91. The declarations of Any_Priority and Priority. See D.1(11). See declarations in file `system.ads'.

92. Implementation-defined execution resources. See D.1(15). There are no implementation-defined execution resources.

93. Whether, on a multiprocessor, a task that is waiting for access to a protected object keeps its processor busy. See D.2.1(3). On a multi-processor, a task that is waiting for access to a protected object does not keep its processor busy.

94. The affect of implementation defined execution resources on task dispatching. See D.2.1(9). Tasks map to threads in the threads package used by GNAT. Where possible and appropriate, these threads correspond to native threads of the underlying operating system.

95. Implementation-defined policy_identifiers allowed in a pragma Task_Dispatching_Policy. See D.2.2(3). There are no implementation-defined policy-identifiers allowed in this pragma.

96. Implementation-defined aspects of priority inversion. See D.2.2(16). Execution of a task cannot be preempted by the implementation processing of delay expirations for lower priority tasks.

97. Implementation defined task dispatching. See D.2.2(18). The policy is the same as that of the underlying threads implementation.

98. Implementation-defined policy_identifiers allowed in a pragma Locking_Policy. See D.3(4). There are no implementation defined policy identifiers allowed in this pragma.

99. Default ceiling priorities. See D.3(10). The ceiling priority of protected objects of the type System.Interrupt_Priority'Last as described in the Ada 95 Reference Manual D.3(10),

100. The ceiling of any protected object used internally by the implementation. See D.3(16). The ceiling priority of internal protected objects is System.Priority'Last.

101. Implementation-defined queuing policies. See D.4(1). There are no implementation-defined queueing policies.

102. On a multiprocessor, any conditions that cause the completion of an aborted construct to be delayed later than what is specified for a single processor. See D.6(3). The semantics for abort on a multi-processor is the same as on a single processor, there are no further delays.

103. Any operations that implicitly require heap storage allocation. See D.7(8). The only operation that implicitly requires heap storage allocation is task creation.

104. Implementation-defined aspects of pragma Restrictions. See D.7(20). There are no such implementation-defined aspects.

105. Implementation-defined aspects of package Real_Time. See D.8(17). There are no implementation defined aspects of package Real_Time.

106. Implementation-defined aspects of delay_statements. See D.9(8). Any difference greater than one microsecond will cause the task to be delayed (see D.9(7)).

107. The upper bound on the duration of interrupt blocking caused by the implementation. See D.12(5). The upper bound is determined by the underlying operating system. In no cases is it more than 10 milliseconds.

108. The means for creating and executing distributed programs. See E(5). The GLADE package provides a utility GNATDIST for creating and executing distributed programs. See the GLADE reference manual for further details.

109. Any events that can result in a partition becoming inaccessible. See E.1(7). See the GLADE reference manual for full details on such events.

110. The scheduling policies, treatment of priorities, and management of shared resources between partitions in certain cases. See E.1(11). See the GLADE reference manual for full details on these aspects of multi-partition execution.

111. Events that cause the version of a compilation unit to change. See E.3(5). Editing the source file of a compilation unit, or the source files of any units on which it is dependent in a significant way cause the version to change. No other actions cause the version number to change. All changes are significant except those which affect only layout, capitalization or comments.

112. Whether the execution of the remote subprogram is immediately aborted as a result of cancellation. See E.4(13). See the GLADE reference manual for details on the effect of abort in a distributed application.

113. Implementation-defined aspects of the PCS. See E.5(25). See the GLADE reference manual for a full description of all implementation defined aspects of the PCS.

114. Implementation-defined interfaces in the PCS. See E.5(26). See the GLADE reference manual for a full description of all implementation defined interfaces.

115. The values of named numbers in the package Decimal. See F.2(7).

Max_Scale

+18

Min_Scale

-18

Min_Delta

1.0E-18

Max_Delta

1.0E+18

Max_Decimal_Digits

18

116. The value of Max_Picture_Length in the package Text_IO.Editing. See F.3.3(16). 64

117. The value of Max_Picture_Length in the package Wide_Text_IO.Editing. See F.3.4(5). 64

118. The accuracy actually achieved by the complex elementary functions and by other complex arithmetic operations. See G.1(1). Standard library functions are used for the complex arithmetic operations. Only fast math mode is currently supported.

119. The sign of a zero result (or a component thereof) from any operator or function in Numerics.Generic_Complex_Types, when Real'Signed_Zeros is True. See G.1.1(53). The signs of zero values are as recommended by the relevant implementation advice.

120. The sign of a zero result (or a component thereof) from any operator or function in Numerics.Generic_Complex_Elementary_Functions, when Real'Signed_Zeros is True. See G.1.2(45). The signs of zero values are as recommended by the relevant implementation advice.

121. Whether the strict mode or the relaxed mode is the default. See G.2(2). The strict mode is the default. There is no separate relaxed mode. GNAT provides a highly efficient implementation of strict mode.

122. The result interval in certain cases of fixed-to-float conversion. See G.2.1(10). For cases where the result interval is implementation dependent, the accuracy is that provided by performing all operations in 64-bit IEEE floating-point format.

123. The result of a floating point arithmetic operation in overflow situations, when the Machine_Overflows attribute of the result type is False. See G.2.1(13). Infinite and Nan values are produced as dictated by the IEEE floating-point standard.

124. The result interval for division (or exponentiation by a negative exponent), when the floating point hardware implements division as multiplication by a reciprocal. See G.2.1(16). Not relevant, division is IEEE exact.

125. The definition of close result set, which determines the accuracy of certain fixed point multiplications and divisions. See G.2.3(5). Operations in the close result set are performed using IEEE long format floating-point arithmetic. The input operands are converted to floating-point, the operation is done in floating-point, and the result is converted to the target type.

126. Conditions on a universal_real operand of a fixed point multiplication or division for which the result shall be in the perfect result set. See G.2.3(22). The result is only defined to be in the perfect result set if the result can be computed by a single scaling operation involving a scale factor representable in 64-bits.

127. The result of a fixed point arithmetic operation in overflow situations, when the Machine_Overflows attribute of the result type is False. See G.2.3(27). Not relevant, Machine_Overflows is True for fixed-point types.

128. The result of an elementary function reference in overflow situations, when the Machine_Overflows attribute of the result type is False. See G.2.4(4). IEEE infinite and Nan values are produced as appropriate.

129. The value of the angle threshold, within which certain elementary functions, complex arithmetic operations, and complex elementary functions yield results conforming to a maximum relative error bound. See G.2.4(10). Information on this subject is not yet available.

130. The accuracy of certain elementary functions for parameters beyond the angle threshold. See G.2.4(10). Information on this subject is not yet available.

131. The result of a complex arithmetic operation or complex elementary function reference in overflow situations, when the Machine_Overflows attribute of the corresponding real type is False. See G.2.6(5). IEEE infinite and Nan values are produced as appropriate.

132. The accuracy of certain complex arithmetic operations and certain complex elementary functions for parameters (or components thereof) beyond the angle threshold. See G.2.6(8). Information on those subjects is not yet available.

133. Information regarding bounded errors and erroneous execution. See H.2(1). Information on this subject is not yet available.

134. Implementation-defined aspects of pragma Inspection_Point. See H.3.2(8). Pragma Inspection_Point ensures that the variable is live and can be examined by the debugger at the inspection point.

135. Implementation-defined aspects of pragma Restrictions. See H.4(25). There are no implementation-defined aspects of pragma Restrictions. The use of pragma Restrictions [No_Exceptions] has no effect on the generated code. Checks must suppressed by use of pragma Suppress.

136. Any restrictions on pragma Restrictions. See H.4(27). There are no restrictions on pragma Restrictions.

Standard Library Routines

The Ada 95 Reference Manual contains in Annex A a full description of an extensive set of standard library routines that can be used in any Ada program, and which must be provided by all Ada compilers. They are analogous to the standard C library used by C programs.

GNAT implements all of the facilities described in annex A, and for most purposes the description in the Ada 95 reference manual, or appropriate Ada text book, will be sufficient for making use of these facilities.

In the case of the input-output facilities, See section The Implementation of Standard I/O, gives details on exactly how GNAT interfaces to the file system. For the remaining packages, the Ada 95 reference manual should be sufficient. The following is a list of the packages included, together with a brief description of the functionality that is provided.

For completeness, references are included to other predefined library routines defined in other sections of the Ada 95 reference manual (these are cross-indexed from annex A).

Ada (A.2)

This is a parent package for all the standard library packages. It is usually included implicitly in your program, and itself contains no useful data or routines.

Ada.Calendar (9.6)

Calendar provides time of day access, and routines for manipulating times and durations.

Ada.Characters (A.3.1)

This is a dummy parent package that contains no useful entities

Ada.Characters.Handling (A.3.2)

This package provides some basic character handling capabilities, including classification functions for classes of characters (e.g. test for letters, or digits).

Ada.Characters.Latin_1 (A.3.3)

This package includes a complete set of definitions of the characters that appear in type CHARACTER. It is useful for writing programs that will run in international environments. For example, if you want an upper case E with an acute accent in a string, it is often better to use the definition of UC_E_Acute in this package. Then your program will print in an understandable manner even if your environment does not support these extended characters.

Ada.Command_Line (A.15)

This package provides access to the command line parameters and the name of the current program (analogous to the use of argc and argv in C), and also allows the exit status for the program to be set in a system-independent manner.

Ada.Decimal (F.2)

This package provides constants describing the range of decimal numbers implemented, and also a decimal divide routine (analogous to the COBOL verb DIVIDE .. GIVING .. REMAINDER ..)

Ada.Direct_IO (A.8.4)

This package provides input-output using a model of a set of records of fixed-length, containing an arbitrary definite Ada type, indexed by an integer record number.

Ada.Dynamic_Priorities (D.5)

This package allows the priorities of a task to be adjusted dynamically as the task is running.

Ada.Exceptions (11.4.1)

This package provides additional information on exceptions, and also contains facilities for treating exceptions as data objects, and raising exceptions with associated messages.

Ada.Finalization (7.6)

This package contains the declarations and subprograms to support the use of controlled types, providing for automatic initialization and finalization (analogous to the constructors and destructors of C++)

Ada.Interrupts (C.3.2)

This package provides facilities for interfacing to interrupts, which includes the set of signals or conditions that can be raised and recognized as interrupts.

Ada.Interrupts.Names (C.3.2)

This package provides the set of interrupt names (actually signal or condition names) that can be handled by GNAT.

Ada.IO_Exceptions (A.13)

This package defines the set of exceptions that can be raised by use of the standard IO packages.

Ada.Numerics

This package contains some standard constants and exceptions used throughout the numerics packages. Note that the constants pi and e are defined here, and it is better to use these definitions than rolling your own.

Ada.Numerics.Complex_Elementary_Functions

Provides the implementation of standard elementary functions (such as log and trigonometric functions) operating on complex numbers using the standard Float and the Complex and Imaginary types created by the package Numerics.Complex_Types.

Ada.Numerics.Complex_Types

This is a predefined instantiation of Numerics.Generic_Complex_Types using Standard.Float to build the type Complex and Imaginary.

Ada.Numerics.Discrete_Random

This package provides a random number generator suitable for generating random integer values from a specified range.

Ada.Numerics.Float_Random

This package provides a random number generator suitable for generating uniformly distributed floating point values.

Ada.Numerics.Generic_Complex_Elementary_Functions

This is a generic version of the package that provides the implementation of standard elementary functions (such as log an trigonometric functions) for an arbitrary complex type. The following predefined instantiations of this package exist

Short_Float

Ada.Numerics.Short_Complex_Elementary_Functions

Float

Ada.Numerics.Complex_Elementary_Functions

Long_Float

Ada.Numerics.Long_Complex_Elementary_Functions

Ada.Numerics.Generic_Complex_Types

This is a generic package that allows the creation of complex types, with associated complex arithmetic operations. The following predefined instantiations of this package exist

Short_Float

Ada.Numerics.Short_Complex_Complex_Types

Float

Ada.Numerics.Complex_Complex_Types

Long_Float

Ada.Numerics.Long_Complex_Complex_Types

Ada.Numerics.Generic_Elementary_Functions

This is a generic package that provides the implementation of standard elementary functions (such as log an trigonometric functions) for an arbitrary float type. The following predefined instantiations of this package exist

Short_Float

Ada.Numerics.Short_Elementary_Functions

Float

Ada.Numerics.Elementary_Functions

Long_Float

Ada.Numerics.Long_Elementary_Functions

Ada.Real_Time (D.8)

This package provides facilities similar to those of Calendar, but operating with a finer clock suitable for real time control.

Ada.Sequential_IO (A.8.1)

This package provides input-output facilities for sequential files, which can contain a sequence of values of a single type, which can be any Ada type, including indefinite (unconstrained) types.

Ada.Storage_IO (A.9)

This package provides a facility for mapping arbitrary Ada types to and from a storage buffer. It is primarily intended for the creation of new IO packages.

Ada.Streams (13.13.1)

This is a generic package that provides the basic support for the concept of streams as used by the stream attributes (Input, Output, Read and Write).

Ada.Streams.Stream_IO (A.12.1)

This package is a specialization of the type Streams defined in package Streams together with a set of operations providing Stream_IO capability. The Stream_IO model permits both random and sequential access to a file which can contain an arbitrary set of values of one or more Ada types.

Ada.Strings (A.4.1)

This package provides some basic constants used by the string handling packages.

Ada.Strings.Bounded (A.4.4)

This package provides facilities for handling variable length strings. The bounded model requires a maximum length. It is thus somewhat more limited than the unbounded model, but avoids the use of dynamic allocation or finalization.

Ada.Strings.Fixed (A.4.3)

This package provides facilities for handling fixed length strings.

Ada.Strings.Maps (A.4.2)

This package provides facilities for handling character mappings and arbitrarily defined subsets of characters. For instance it is useful in defining specialized translation tables.

Ada.Strings.Maps.Constants (A.4.6)

This package provides a standard set of predefined mappings and predefined character sets. For example, the standard upper to lower case conversion table is found in this package. Note that upper to lower case conversion is non-trivial if you want to take the entire set of characters, including extended characters like E with an acute accent, into account. You should use the mappings in this package (rather than adding 32 yourself) to do case mappings.

Ada.Strings.Unbounded (A.4.5)

This package provides facilities for handling variable length strings. The unbounded model allows arbitrary length strings, but requires the use of dynamic allocation and finalization.

Ada.Strings.Wide_Bounded (A.4.7)

Ada.Strings.Wide_Fixed (A.4.7)

Ada.Strings.Wide_Maps (A.4.7)

Ada.Strings.Wide_Maps.Constants (A.4.7)

Ada.Strings.Wide_Unbounded (A.4.7)

These package provide analogous capabilities to the corresponding packages without `Wide_' in the name, but operate with the types Wide_String and Wide_Character instead of String and Character.

Ada.Synchronous_Task_Control (D.10)

This package provides some standard facilities for controlling task communication in a synchronous manner.

Ada.Tags

This package contains definitions for manipulation of the tags of tagged values.

Ada.Task_Attributes

This package provides the capability of associating arbitrary task-specific data with separate tasks.

Ada.Text_IO

This package provides basic text input-output capabilities for character, string and numeric data. The subpackages of this package are listed next.

Ada.Text_IO.Decimal_IO

Provides input-output facilities for decimal fixed-point types

Ada.Text_IO.Enumeration_IO

Provides input-output facilities for enumeration types.

Ada.Text_IO.Fixed_IO

Provides input-output facilities for ordinary fixed-point types.

Ada.Text_IO.Float_IO

Provides input-output facilities for float types. The following predefined instantiations of this generic package are available:

Short_Float

Short_Float_Text_IO

Float

Float_Text_IO

Long_Float

Long_Float_Text_IO

Ada.Text_IO.Integer_IO

Provides input-output facilities for integer types. The following predefined instantiations of this generic package are available:

Short_Short_Integer

Ada.Short_Short_Integer_Text_IO

Short_Integer

Ada.Short_Integer_Text_IO

Integer

Ada.Integer_Text_IO

Long_Integer

Ada.Long_Integer_Text_IO

Long_Long_Integer

Ada.Long_Long_Integer_Text_IO

Ada.Text_IO.Modular_IO

Provides input-output facilities for modular (unsigned) types

Ada.Text_IO.Complex_IO (G.1.3)

This package provides basic text input-output capabilities for complex data.

Ada.Text_IO.Editing (F.3.3)

This package contains routines for edited output, analogous to the use of pictures in COBOL. The picture formats used by this package are a close copy of the facility in COBOL.

Ada.Text_IO.Text_Streams (A.12.2)

This package provides a facility that allows Text_IO files to be treated as streams, so that the stream attributes can be used for writing arbitrary data, including binary data, to Text_IO files.

Ada.Unchecked_Conversion (13.9)

This generic package allows arbitrary conversion from one type to another of the same size, providing for breaking the type safety in special circumstances.

Ada.Unchecked_Deallocation (13.11.2)

This generic package allows explicit freeing of storage previously allocated by use of an allocator.

Ada.Wide_Text_IO (A.11)

This package is similar to Ada.Text_IO, except that the external file supports wide character representations, and the internal types are Wide_Character and Wide_String instead of Character and String. It contains generic subpackages listed next.

Ada.Wide_Text_IO.Decimal_IO

Provides input-output facilities for decimal fixed-point types

Ada.Wide_Text_IO.Enumeration_IO

Provides input-output facilities for enumeration types.

Ada.Wide_Text_IO.Fixed_IO

Provides input-output facilities for ordinary fixed-point types.

Ada.Wide_Text_IO.Float_IO

Provides input-output facilities for float types. The following predefined instantiations of this generic package are available:

Short_Float

Short_Float_Wide_Text_IO

Float

Float_Wide_Text_IO

Long_Float

Long_Float_Wide_Text_IO

Ada.Wide_Text_IO.Integer_IO

Provides input-output facilities for integer types. The following predefined instantiations of this generic package are available:

Short_Short_Integer

Ada.Short_Short_Integer_Wide_Text_IO

Short_Integer

Ada.Short_Integer_Wide_Text_IO

Integer

Ada.Integer_Wide_Text_IO

Long_Integer

Ada.Long_Integer_Wide_Text_IO

Long_Long_Integer

Ada.Long_Long_Integer_Wide_Text_IO

Ada.Wide_Text_IO.Modular_IO

Provides input-output facilities for modular (unsigned) types

Ada.Wide_Text_IO.Complex_IO (G.1.3)

This package is similar to Ada.Text_IO.Complex_IO, except that the external file supports wide character representations.

Ada.Wide_Text_IO.Editing (F.3.4)

This package is similar to Ada.Text_IO.Editing, except that the types are Wide_Character and Wide_String instead of Character and String.

Ada.Wide_Text_IO.Streams (A.12.3)

This package is similar to Ada.Text_IO.Streams, except that the types are Wide_Character and Wide_String instead of Character and String.

The Implementation of Standard I/O

GNAT implements all the required input-output facilities described in A.6 through A.14. These sections of the Ada 95 reference manual describe the required behavior of these packages from the Ada point of view, and if you are writing a portable Ada program that does not need to know the exact manner in which Ada maps to the outside world when it comes to reading or writing external files, then you do not need to read this chapter. As long as your files are all regular files (not pipes or devices), and as long as you write and read the files only from Ada, the description in the Ada 95 reference manual is sufficient.

However, if you want to do input-output to pipes or other devices, such as the keyboard or screen, or if the files you are dealing with are either generated by some other language, or to be read by some other language, then you need to know more about the details of how the GNAT implementation of these input-output facilities behaves.

In this chapter we give a detailed description of exactly how GNAT interfaces to the file system. As always, the sources of the system are available to you for answering questions at an even more detailed level, but for most purposes the information in this chapter will suffice.

Another reason that you may need to know more about how input-output is implemented arises when you have a program written in mixed languages where, for example, files are shared between the C and Ada sections of the same program. GNAT provides some additional facilities, in the form of additional child library packages, that facilitate this sharing, and these additional facilities are also described in this chapter.

• Standard I/O Packages
• FORM Strings
• Direct_IO
• Sequential_IO
• Text_IO
• Wide_Text_IO
• Stream_IO
• Shared Files
• Open Modes
• Operations on C Streams
• Interfacing to C Streams

Standard I/O Packages

The Standard I/O packages described in Annex A for

• Ada.Text_IO
• Ada.Text_IO.Complex_IO
• Ada.Text_IO.Text_Streams,
• Ada.Wide_Text_IO
• Ada.Wide_Text_IO.Complex_IO,
• Ada.Wide_Text_IO.Text_Streams
• Ada.Stream_IO
• Ada.Sequential_IO
• Ada.Direct_IO

are implemented using the C library streams facility; where

• All files are opened using fopen.
• All input/output operations use fread/fwrite.

There is no internal buffering of any kind at the Ada library level. The only buffering is that provided at the system level in the implementation of the C library routines that support streams. This facilitates shared use of these streams by mixed language programs.

FORM Strings

The format of a FORM string in GNAT is:

"keyword=value,keyword=value,...,keyword=value"

where letters may be in upper or lower case, and there are no spaces between values. The order of the entries is not important. Currently there are two keywords defined.

SHARED=[YES|NO]
WCEM=[n|h|u|s\e]

The use of these parameters is described later in this section.

Direct_IO

Direct_IO can only be instantiated for definite types. This is a restriction of the Ada language, which means that the records are fixed length (the length being determined by type'Size, rounded up to the next storage unit boundary if necessary).

The records of a Direct_IO file are simply written to the file in index sequence, with the first record starting at offset zero, and subsequent records following. There is no control information of any kind. For example, if 32-bit integers are being written, each record takes 4-bytes, so the record at index K starts at offset (K - 1)*4.

There is no limit on the size of Direct_IO files, they are expanded as necessary to accommodate whatever records are written to the file.

Sequential_IO

Sequential_IO may be instantiated with either a definite (constrained) or indefinite (unconstrained) type.

For the definite type case, the elements written to the file are simply the memory images of the data values with no control information of any kind. The resulting file should be read using the same type, no validity checking is performed on input.

For the indefinite type case, the elements written consist of two parts. First is the size of the data item, written as the memory image of a Interfaces.C.size_t value, followed by the memory image of the data value. The resulting file can only be read using the same (unconstrained) type. Normal assignment checks are performed on these read operations, and if these checks fail, Data_Error is raised. In particular, in the array case, the lengths must match, and in the variant record case, if the variable for a particular read operation is constrained, the discriminants must match.

Note that it is not possible to use Sequential_IO to write variable length array items, and then read the data back into different length arrays. For example, the following will raise Data_Error:

 package IO is new Sequential_IO (String);
 F : IO.File_Type;
 S : String (1..4);
 ...
 IO.Create (F)
 IO.Write (F, "hello!")
 IO.Reset (F, Mode=>In_File);
 IO.Read (F, S);
 Put_Line (S);

On some Ada implementations, this will print `hell', but the program is clearly incorrect, since there is only one element in the file, and that element is the string `hello!'.

In Ada 95, this kind of behavior can be legitimately achieved using Stream_IO, and this is the preferred mechanism. In particular, the above program fragment rewritten to use Stream_IO will work correctly.

Text_IO

Text_IO files consist of a stream of characters containing the following special control characters:

LF (line feed, 16#0A#) Line Mark
FF (form feed, 16#0C#) Page Mark

A canonical Text_IO file is defined as one in which the following conditions are met:

• The character LF is used only as a line mark, i.e. to mark the end of the line.
• The character FF is used only as a page mark, i.e. to mark the end of a page and consequently can appear only immediately following a LF (line mark) character.
• The file ends with either LF (line mark) or LF-FF (line mark, page mark). In the former case, the page mark is implicitly assumed to be present.

A file written using Text_IO will be in canonical form provided that no explicit LF or FF characters are written using Put or Put_Line. There will be no FF character at the end of the file unless an explicit New_Page operation was performed before closing the file.

A canonical Text_IO file that is a regular file, i.e. not a device or a pipe, can be read using any of the routines in Text_IO. The semantics in this case will be exactly as defined in the Ada 95 reference manual and all the routines in Text_IO are fully implemented.

A text file that does not meet the requirements for a canonical Text_IO file has one of the following:

• The file contains FF characters not immediately following a LF character.
• The file contains LF or FF characters written by Put or Put_Line, which are not logically considered to be line marks or page marks.
• The file ends in a character other than LF or FF, i.e. there is no explicit line mark or page mark at the end of the file.

Text_IO can be used to read such non-standard text files but subprograms to do with line or page numbers do not have defined meanings. In particular, a FF character that does not follow a LF character may or may not be treated as a page mark from the point of view of page and line numbering. Every LF character is considered to end a line, and there is an implied LF character at the end of the file.

• Text_IO Stream Pointer Positioning
• Text_IO Reading and Writing Non-Regular Files
• Get_Immediate
• Treating Text_IO Files as Streams

Stream Pointer Positioning

Ada.Text_IO has a definition of current position for a file that is being read. No internal buffering occurs in Text_IO, and usually the physical position in the stream used to implement the file corresponds to this logical position defined by Text_IO. There are two exceptions:

• After a call to End_Of_Page that returns True, the stream is positioned past the LF (line mark) that precedes the page mark. Text_IO maintains an internal flag so that subsequent read operations properly handle the logical position which is unchanged by the End_Of_Page call.
• After a call to End_Of_File that returns True, if the Text_IO file was positioned before the line mark at the end of file before the call, then the logical position is unchanged, but the stream is physically positioned right at the end of file (past the line mark, and past a possible page mark following the line mark. Again Text_IO maintains internal flags so that subsequent read operations properly handle the logical position.

These discrepancies have no effect on the observable behavior of Text_IO, but if a single Ada stream is shared between a C program and Ada program, or shared (using `shared=yes' in the form string) between two Ada files, then the difference may be observable in some situations.

Reading and Writing Non-Regular Files

A non-regular file is a device (such as a keyboard), or a pipe. Text_IO can be used for reading and writing. Writing is not affected and the sequence of characters output is identical to the normal file case, but for reading, the behavior of Text_IO is modified to avoid undesirable look-ahead as follows:

An input file that is not a regular file is considered to have no page marks. Any Ascii.FF characters (the character normally used for a page mark) appearing in the file are considered to be data characters. In particular:

• Get_Line and Skip_Line do not test for a page mark following a line mark. If a page mark appears, it will be treated as a data character.
• This avoids the need to wait for an extra character to be typed or entered from the pipe to complete one of these operations.
• End_Of_Page always returns False
• End_Of_File will return False if there is a page mark at the end of the file.

Output to non-regular files is the same as for regular files. Page marks may be written to non-regular files using New_Page, but as noted above they will not be treated as page marks on input if the output is piped to another Ada program.

Another important discrepancy when reading non-regular files is that the end of file indication is not "sticky". If an end of file is entered, e.g. by pressing the EOT key, then end of file is signalled once (i.e. the test End_Of_File will yield True, or a read will raise End_Error), but then reading can resume to read data past that end of file indication, until another end of file indication is entered.

Get_Immediate

Get_Immediate returns the next character (including control characters) from the input file. In particular, Get_Immediate will return LF or FF characters used as line marks or page marks. Such operations leave the file positioned past the control character, and it is thus not treated as having its normal function. This means that page, line and column counts after this kind of Get_Immediate call are set as though the mark did not occur. In the case where a Get_Immediate leaves the file positioned between the line mark and page mark (which is not normally possible), it is undefined whether the FF character will be treated as a page mark.

Treating Text_IO Files as Streams

The package Text_IO.Streams allows a Text_IO file to be treated as a stream. Data written to a Text_IO file in this stream mode is binary data. If this binary data contains bytes 16#0A# (LF) or 16#0C# (FF), the resulting file may have non-standard format. Similarly if read operations are used to read from a Text_IO file treated as a stream, then LF and FF characters may be skipped and the effect is similar to that described above for Get_Immediate.

Wide_Text_IO

Wide_Text_IO is similar in most respects to Text_IO, except that both input and output files may contain special sequences that represent wide character values. The encoding scheme for a given file may be specified using a FORM parameter:

WCEM=x

as part of the FORM string (WCEM = wide character encoding method), where x is one of the following characters

`h'

Hex ESC encoding

`u'

Upper half encoding

`s'

Shift-JIS encoding

`e'

EUC Encoding

`8'

UTF-8 encoding

`b'

Brackets encoding

The encoding methods match those that can be used in a source program, but there is no requirement that the encoding method used for the source program be the same as the encoding method used for files, and different files may use different encoding methods.

The default encoding method for the standard files, and for opened files for which no WCEM parameter is given in the FORM string matches the wide character encoding specified for the main program (the default being brackets encoding if no coding method was specified with -gnatW).

Hex Coding

In this encoding, a wide character is represented by a five character sequence:

ESC a b c d

where a, b, c, d are the four hexadecimal characters (using upper case letters) of the wide character code. For example, ESC A345 is used to represent the wide character with code 16#A345#. This scheme is compatible with use of the full Wide_Character set.

Upper Half Coding

The wide character with encoding 16#abcd#, where the upper bit is on (i.e. a is in the range 8-F) is represented as two bytes 16#ab# and 16#cd#. The second byte may never be a format control character, but is not required to be in the upper half. This method can be also used for shift-JIS or EUC where the internal coding matches the external coding.

Shift JIS Coding

A wide character is represented by a two character sequence 16#ab# and 16#cd#, with the restrictions described for upper half encoding as described above. The internal character code is the corresponding JIS character according to the standard algorithm for Shift-JIS conversion. Only characters defined in the JIS code set table can be used with this encoding method.

EUC Coding

A wide character is represented by a two character sequence 16#ab# and 16#cd#, with both characters being in the upper half. The internal character code is the corresponding JIS character according to the EUC encoding algorithm. Only characters defined in the JIS code set table can be used with this encoding method.

UTF-8 Coding

A wide character is represented using UCS Transformation Format 8 (UTF-8) as defined in Annex R of ISO 10646-1/Am.2. Depending on the character value, the representation is a one, two, or three byte sequence:

16#0000#-16#007f#: 2#0xxxxxxx#
16#0080#-16#07ff#: 2#110xxxxx# 2#10xxxxxx#
16#0800#-16#ffff#: 2#1110xxxx# 2#10xxxxxx# 2#10xxxxxx#

where the xxx bits correspond to the left-padded bits of the the 16-bit character value. Note that all lower half ASCII characters are represented as ASCII bytes and all upper half characters and other wide characters are represented as sequences of upper-half (The full UTF-8 scheme allows for encoding 31-bit characters as 6-byte sequences, but in this implementation, all UTF-8 sequences of four or more bytes length will raise a Constraint_Error, as will all illegal UTF-8 sequences.)

Brackets Coding

In this encoding, a wide character is represented by the following eight character sequence:

[ " a b c d " ]

Where a, b, c, d are the four hexadecimal characters (using uppercase letters) of the wide character code. For example, ["A345"] is used to represent the wide character with code 16#A345#. This scheme is compatible with use of the full Wide_Character set. On input, brackets coding can also be used for upper half characters, e.g. ["C1"] for lower case a. However, on output, brackets notation is only used for wide characters with a code greater than 16#FF#.

For the coding schemes other than Hex and Brackets encoding, not all wide character values can be represented. An attempt to output a character that cannot be represented using the encoding scheme for the file causes Constraint_Error to be raised. An invalid wide character sequence on input also causes Constraint_Error to be raised.

• Wide_Text_IO Stream Pointer Positioning
• Wide_Text_IO Reading and Writing Non-Regular Files

Stream Pointer Positioning

Ada.Wide_Text_IO is similar to Ada.Text_IO in its handling of stream pointer positioning (see section Text_IO). There is one additional case:

If Ada.Wide_Text_IO.Look_Ahead reads a character outside the normal lower ASCII set (i.e. a character in the range:

Wide_Character'Val (16#0080#) .. Wide_Character'Val (16#FFFF#)

then although the logical position of the file pointer is unchanged by the Look_Ahead call, the stream is physically positioned past the wide character sequence. Again this is to avoid the need for buffering or backup, and all Wide_Text_IO routines check the internal indication that this situation has occurred so that this is not visible to a normal program using Wide_Text_IO. However, this discrepancy can be observed if the wide text file shares a stream with another file.

Reading and Writing Non-Regular Files

As in the case of Text_IO, when a non-regular file is read, it is assumed that the file contains no page marks (any form characters are treated as data characters), and End_Of_Page always returns False. Similarly, the end of file indication is not sticky, so it is possible to read beyond an end of file.

Stream_IO

A stream file is a sequence of bytes, where individual elements are written to the file as described in the Ada 95 reference manual. The type Stream_Element is simply a byte. There are two ways to read or write a stream file.

• The operations Read and Write directly read or write a sequence of stream elements with no control information.
• The stream attributes applied to a stream file transfer data in the manner described for stream attributes.

Shared Files

Section A.14 of the Ada 95 Reference Manual allows implementations to provide a wide variety of behavior if an attempt is made to access the same external file with two or more internal files.

To provide a full range of functionality, while at the same time minimizing the problems of portability caused by this implementation dependence, GNAT handles file sharing as follows:

• In the absence of a `shared=xxx' form parameter, an attempt to open two or more files with the same full name is considered an error and is not supported. The exception Use_Error will be raised. Note that a file that is not explicitly closed by the program remains open until the program terminates.
• If the form parameter `shared=no' appears in the form string, the file can be opened or created with its own separate stream identifier, regardless of whether other files sharing the same external file are opened. The exact effect depends on how the C stream routines handle multiple accesses to the same external files using separate streams.
• If the form parameter `shared=yes' appears in the form string for each of two or more files opened using the same full name, the same stream is shared between these files, and the semantics are as described in Ada 95 Reference Manual, Section A.14.

When a program that opens multiple files with the same name is ported from another Ada compiler to GNAT, the effect will be that Use_Error is raised.

The documentation of the original compiler and the documentation of the program should then be examined to determine if file sharing was expected, and `shared=xxx' parameters added to Open and Create calls as required.

When a program is ported from GNAT to some other Ada compiler, no special attention is required unless the `shared=xxx' form parameter is used in the program. In this case, you must examine the documentation of the new compiler to see if it supports the required file sharing semantics, and form strings modified appropriately. Of course it may be the case that the program cannot be ported if the target compiler does not support the required functionality. The best approach in writing portable code is to avoid file sharing (and hence the use of the `shared=xxx' parameter in the form string) completely.

One common use of file sharing in Ada 83 is the use of instantiations of Sequential_IO on the same file with different types, to achieve heterogenous input-output. Although this approach will work in GNAT if `shared=yes' is specified, it is preferable in Ada 95 to use Stream_IO for this purpose (using the stream attributes)

Open Modes

Open and Create calls result in a call to fopen using the mode shown in Table 6.1

Table 6-1 Open and Create Call Modes

                               OPEN            CREATE
Append_File                    "r+"             "w+"
In_File                        "r"              "w+"
Out_File (Direct_IO)           "r+"             "w"
Out_File (all other cases)     "w"              "w"
Inout_File                     "r+"             "w+"

If text file translation is required, then either `b' or `t' is added to the mode, depending on the setting of Text. Text file translation refers to the mapping of CR/LF sequences in an external file to LF characters internally. This mapping only occurs in DOS and DOS-like systems, and is not relevant to other systems.

A special case occurs with Stream_IO. As shown in the above table, the file is initially opened in `r' or `w' mode for the In_File and Out_File cases. If a Set_Mode operation subsequently requires switching from reading to writing or vice-versa, then the file is reopened in `r+' mode to permit the required operation.

Operations on C Streams

The package Interfaces.C_Streams provides an Ada program with direct access to the C library functions for operations on C streams:

package Interfaces.C_Streams is
  -- Note: the reason we do not use the types that are in
  -- Interfaces.C is that we want to avoid dragging in the
  -- code in this unit if possible.
  subtype chars is System.Address;
  -- Pointer to null-terminated array of characters
  subtype FILEs is System.Address;
  -- Corresponds to the C type FILE*
  subtype voids is System.Address;
  -- Corresponds to the C type void*
  subtype int is Integer;
  subtype long is Long_Integer;
  -- Note: the above types are subtypes deliberately, and it
  -- is part of this spec that the above correspondences are
  -- guaranteed. This means that it is legitimate to, for
  -- example, use Integer instead of int. We provide these
  -- synonyms for clarity, but in some cases it may be
  -- convenient to use the underlying types (for example to
  -- avoid an unnecessary dependency of a spec on the spec
  -- of this unit).
  type size_t is mod 2 ** Standard'Address_Size;
  NULL_Stream : constant FILEs;
  -- Value returned (NULL in C) to indicate an
  -- fdopen/fopen/tmpfile error
  ----------------------------------
  -- Constants Defined in stdio.h --
  ----------------------------------
  EOF : constant int;
  -- Used by a number of routines to indicate error or 
  -- end of file
  IOFBF : constant int;
  IOLBF : constant int;
  IONBF : constant int;
  -- Used to indicate buffering mode for setvbuf call
  SEEK_CUR : constant int;
  SEEK_END : constant int;
  SEEK_SET : constant int;
  -- Used to indicate origin for fseek call
  function stdin return FILEs;
  function stdout return FILEs;
  function stderr return FILEs;
  -- Streams associated with standard files
  --------------------------
  -- Standard C functions --
  --------------------------
  -- The functions selected below are ones that are
  -- available in DOS, OS/2, UNIX and Xenix (but not
  -- necessarily in ANSI C). These are very thin interfaces
  -- which copy exactly the C headers. For more
  -- documentation on these functions, see the Microsoft C 
  -- "Run-Time Library Reference" (Microsoft Press, 1990,
  -- ISBN 1-55615-225-6), which includes useful information
  -- on system compatibility.
  procedure clearerr (stream : FILEs);
  function fclose (stream : FILEs) return int;
  function fdopen (handle : int; mode : chars) return FILEs; 
  function feof (stream : FILEs) return int; 
  function ferror (stream : FILEs) return int; 
  function fflush (stream : FILEs) return int; 
  function fgetc (stream : FILEs) return int; 
  function fgets (strng : chars; n : int; stream : FILEs) 
      return chars; 
  function fileno (stream : FILEs) return int; 
  function fopen (filename : chars; Mode : chars) 
      return FILEs;
  -- Note: to maintain target independence, use
  -- text_translation_required, a boolean variable defined in
  -- a-sysdep.c to deal with the target dependent text
  -- translation requirement. If this variable is set, 
  -- then  b/t should be appended to the standard mode
  -- argument to set the text translation mode off or on 
  -- as required.
  function fputc (C : int; stream : FILEs) return int;
  function fputs (Strng : chars; Stream : FILEs) return int;
  function fread
     (buffer : voids;
      size : size_t;
      count : size_t;
      stream : FILEs)
      return size_t;
  function freopen
     (filename : chars;
      mode : chars;
      stream : FILEs)
      return FILEs;
  function fseek
     (stream : FILEs;
      offset : long;
      origin : int)
      return int;
  function ftell (stream : FILEs) return long;
  function fwrite
     (buffer : voids;
      size : size_t;
      count : size_t;
      stream : FILEs)
      return size_t; 
  function isatty (handle : int) return int;
  procedure mktemp (template : chars);
  -- The return value (which is just a pointer to template)
  -- is discarded
  procedure rewind (stream : FILEs);
  function rmtmp return int;
  function setvbuf
     (stream : FILEs;
      buffer : chars;
      mode : int;
      size : size_t)
      return int;

  function tmpfile return FILEs;
  function ungetc (c : int; stream : FILEs) return int;
  function unlink (filename : chars) return int;
  ---------------------
  -- Extra functions --
  ---------------------
  -- These functions supply slightly thicker bindings than
  -- those above. They are derived from functions in the 
  -- C Run-Time Library, but may do a bit more work than
  -- just directly calling one of the Library functions.
  function is_regular_file (handle : int) return int;
  -- Tests if given handle is for a regular file (result 1)
  -- or for a non-regular file (pipe or device, result 0).
  ---------------------------------
  -- Control of Text/Binary Mode --
  ---------------------------------
  -- If text_translation_required is true, then the following
  -- functions may be used to dynamically switch a file from
  -- binary to text mode or vice versa. These functions have
  -- no effect if text_translation_required is false (i.e. in
  -- normal UNIX mode). Use fileno to get a stream handle.
  procedure set_binary_mode (handle : int);
  procedure set_text_mode (handle : int);
  ----------------------------
  -- Full Path Name support --
  ----------------------------
  procedure full_name (nam : chars; buffer : chars);
  -- Given a NUL terminated string representing a file
  -- name, returns in buffer a NUL terminated string
  -- representing the full path name for the file name. 
  -- On systems where it is relevant the   drive is also
  -- part of the full path name. It is the responsibility 
  -- of the caller to pass an actual parameter for buffer
  -- that is big enough for any full path name. Use
  -- max_path_len given below as the size of buffer.
  max_path_len : integer;
  -- Maximum length of an allowable full path name on the
  -- system, including a terminating NUL character.
end Interfaces.C_Streams;

Interfacing to C Streams

The packages in this section permit interfacing Ada files to C Stream operations.

 with Interfaces.C_Streams;
 package Ada.Sequential_IO.C_Streams is
    function C_Stream (F : File_Type)  
       return Interfaces.C_Streams.FILEs;
    procedure Open
      (File : in out File_Type;
       Mode : in File_Mode;
       C_Stream : in Interfaces.C_Streams.FILEs;
       Form : in String := "");
 end Ada.Sequential_IO.C_Streams;

  with Interfaces.C_Streams;
  package Ada.Direct_IO.C_Streams is
     function C_Stream (F : File_Type) 
        return Interfaces.C_Streams.FILEs;
     procedure Open
       (File : in out File_Type;
        Mode : in File_Mode;
        C_Stream : in Interfaces.C_Streams.FILEs;
        Form : in String := "");
  end Ada.Direct_IO.C_Streams;

  with Interfaces.C_Streams;
  package Ada.Text_IO.C_Streams is
     function C_Stream (F : File_Type)
        return Interfaces.C_Streams.FILEs;
     procedure Open
       (File : in out File_Type;
        Mode : in File_Mode;
        C_Stream : in Interfaces.C_Streams.FILEs;
        Form : in String := "");
  end Ada.Text_IO.C_Streams;

  with Interfaces.C_Streams;
  package Ada.Wide_Text_IO.C_Streams is
     function C_Stream (F : File_Type)  
        return Interfaces.C_Streams.FILEs;
     procedure Open
       (File : in out File_Type;
        Mode : in File_Mode;
        C_Stream : in Interfaces.C_Streams.FILEs;
        Form : in String := "");
 end Ada.Wide_Text_IO.C_Streams;

 with Interfaces.C_Streams;
 package Ada.Stream_IO.C_Streams is
    function C_Stream (F : File_Type)
       return Interfaces.C_Streams.FILEs;
    procedure Open
      (File : in out File_Type;
       Mode : in File_Mode;
       C_Stream : in Interfaces.C_Streams.FILEs;
       Form : in String := "");
 end Ada.Stream_IO.C_Streams;

In each of these five packages, the C_Stream function obtains the FILE pointer from a currently opened Ada file. It is then possible to use the Interfaces.C_Streams package to operate on this stream, or the stream can be passed to a C program which can operate on it directly. Of course the program is responsible for ensuring that only appropriate sequences of operations are executed.

One particular use of relevance to an Ada program is that the setvbuf function can be used to control the buffering of the stream used by an Ada file. In the absence of such a call the standard default buffering is used.

The Open procedures in these packages open a file giving an existing C Stream instead of a file name. Typically this stream is imported from a C program, allowing an Ada file to operate on an existing C file.

Interfacing to Other Languages

The facilities in annex B of the Ada 95 Reference Manual are fully implemented in GNAT, and in addition, a full interface to C++ is provided.

• Interfacing to C
• Interfacing to C++
• Interfacing to COBOL
• Interfacing to Fortran
• Interfacing to non-GNAT Ada code

Interfacing to C

Interfacing to C with GNAT can use one of two approaches:

1. The types in the package Interfaces.C may be used.
2. Standard Ada types may be used directly. This may be less portable to other compilers, but will work on all GNAT compilers, which guarantee correspondence between the C and Ada types.

Pragma Convention C maybe applied to Ada types, but mostly has no effect, since this is the default. The following table shows the correspondence between Ada scalar types and the corresponding C types.

Integer

int

Short_Integer

short

Short_Short_Integer

signed char

Long_Integer

long

Long_Long_Integer

long long

Short_Float

float

Float

float

Long_Float

double

Long_Long_Float

This is the longest floating-point type supported by the hardware. Sometimes, this is the same as Long_Float, i.e. as the C type double. Otherwise, it is a wider type which is also available as long double in GNU C.

• Ada enumeration types map to C enumeration types directly if pragma Convention C is specified, which causes them to have int length. Without pragma Convention C, Ada enumeration types map to 8, 16, or 32 bits (i.e. C types signed char, short, int respectively) depending on the number of values passed. This is the only case in which pragma Convention C affects the representation of an Ada type.
• Ada access types map to C pointers, except for the case of pointers to unconstrained types in Ada, which have no direct C equivalent.
• Ada arrays map directly to C arrays.
• Ada records map directly to C structures.
• Packed Ada records map to C structures where all members are bit fields of the length corresponding to the type'Size value in Ada.

Interfacing to C++

The interface to C++ makes use of the following pragmas, which are primarily intended to be constructed automatically using a binding generator tool, although it is possible to construct them by hand. Ada Core Technologies does not currently supply a suitable binding generator tool.

Using these pragmas it is possible to achieve complete inter-operability between Ada tagged types and C class definitions. See section Implementation Defined Pragmas for more details.

pragma CPP_Class ([Entity =>] local_name)

The argument denotes an entity in the current declarative region that is declared as a tagged or untagged record type. It indicates that the type corresponds to an externally declared C++ class type, and is to be laid out the same way that C++ would lay out the type.

pragma CPP_Constructor ([Entity =>] local_name)

This pragma identifies an imported function (imported in the usual way with pragma Import) as corresponding to a C++ constructor.

pragma CPP_Vtable ...

One CPP_Vtable pragma can be present for each component of type CPP.Interfaces.Vtable_Ptr in a record to which pragma CPP_Class applies.

Interfacing to COBOL

Interfacing to COBOL is achieved as described in section B.4 of the Ada 95 reference manual.

Interfacing to Fortran

Interfacing to Fortran is achieved as described in section B.5 of the reference manual. The pragma Convention Fortran, applied to a multi- dimensional array causes the array to be stored in column-major order as required for convenient interface to Fortran.

Interfacing to non-GNAT Ada code

It is possible to specify the convention Ada in a pragma Import or pragma Export. However this refers to the calling conventions used by GNAT, which may or may not be similar enough to those used by some other Ada 83 or Ada 95 compiler to allow interoperation.

If arguments types are kept simple, and if the foreign compiler generally follows system calling conventions, then it may be possible to integrate files compiled by other Ada compilers, provided that the elaboration issues are adequately addressed (for example by eliminating the need for any load time elaboration).

In particular, GNAT running on VMS is designed to be highly compatible with the DEC Ada 83 compiler, so this is one case in which it is possible to import foreign units of this type, provided that the data items passed are restricted to simple scalar values or simple record types without variants, or simple array types with fixed bounds.

Machine Code Insertions

Package Machine_Code provides machine code support as described in the Ada 95 Reference Manual in two separate forms:

• Machine code statements, consisting of qualified expressions that fit the requirements of RM section 13.8.
• An intrinsic callable procedure, providing an alternative mechanism of including machine instructions in a subprogram.

The two features are similar, and both closely related to the mechanism provided by the asm instruction in the GNU C compiler. Full understanding and use of the facilities in this package requires understanding the asm instruction as described in Using and Porting GNU CC by Richard Stallman. Calls to the function Asm and the procedure Asm have identical semantic restrictions and effects as described below. Both are provided so that the procedure call can be used as a statement, and the function call can be used to form a code_statement.

The first example given in the GNU CC documentation is the C asm instruction:

   asm ("fsinx %1 %0" : "=f" (result) : "f" (angle));

The equivalent can be written for GNAT as:

   Asm ("fsinx %1 %0",
        My_Float'Asm_Output ("=f", result),
        My_Float'Asm_Input  ("f",  angle));

The first argument to Asm is the assembler template, and is identical to what is used in GNU CC. This string must be a static expression. The second argument is the output operand list. It is either a single Asm_Output attribute reference, or a list of such references enclosed in parentheses (technically an array aggregate of such references).

The Asm_Output attribute denotes a function that takes two parameters. The first is a string, the second is the name of a variable of the type designated by the attribute prefix. The first (string) argument is required to be a static expression and designates the constraint for the parameter (e.g. what kind of register is required). The second argument is the variable to be updated with the result. The possible values for constraint are the same as those used in the RTL, and are dependent on the configuration file used to build the GCC back end. If there are no output operands, then this argument may either be omitted, or explicitly given as No_Output_Operands.

The second argument of my_float'Asm_Output functions as though it were an out parameter, which is a little curious, but all names have the form of expressions, so there is no syntactic irregularity, even though normally functions would not be permitted out parameters. The third argument is the list of input operands. It is either a single Asm_Input attribute reference, or a list of such references enclosed in parentheses (technically an array aggregate of such references).

The Asm_Input attribute denotes a function that takes two parameters. The first is a string, the second is an expression of the type designated by the prefix. The first (string) argument is required to be a static expression, and is the constraint for the parameter, (e.g. what kind of register is required). The second argument is the value to be used as the input argument. The possible values for the constant are the same as those used in the RTL, and are dependent on the configuration file used to built the GCC back end.

If there are no input operands, this argument may either be omitted, or explicitly given as No_Input_Operands. The fourth argument, not present in the above example, is a list of register names, called the clobber argument. This argument, if given, must be a static string expression, and is a space or comma separated list of names of registers that must be considered destroyed as a result of the Asm call. If this argument is the null string (the default value), then the code generator assumes that no additional registers are destroyed.

The fifth argument, not present in the above example, called the volatile argument, is by default False. It can be set to the literal value True to indicate to the code generator that all optimizations with respect to the instruction specified should be suppressed, and that in particular, for an instruction that has outputs, the instruction will still be generated, even if none of the outputs are used. See the full description in the GCC manual for further details.

The Asm subprograms may be used in two ways. First the procedure forms can be used anywhere a procedure call would be valid, and correspond to what the RM calls "intrinsic" routines. Such calls can be used to intersperse machine instructions with other Ada statements. Second, the function forms, which return a dummy value of the limited private type Asm_Insn, can be used in code statements, and indeed this is the only context where such calls are allowed. Code statements appear as aggregates of the form:

   Asm_Insn'(Asm (...));
   Asm_Insn'(Asm_Volatile (...));

In accordance with RM rules, such code statements are allowed only within subprograms whose entire body consists of such statements. It is not permissible to intermix such statements with other Ada statements.

Typically the form using intrinsic procedure calls is more convenient and more flexible. The code statement form is provided to meet the RM suggestion that such a facility should be made available. The following is the exact syntax of the call to Asm (of course if named notation is used, the arguments may be given in arbitrary order, following the normal rules for use of positional and named arguments)

  ASM_CALL ::= Asm (
                   [Template =>] static_string_EXPRESSION
                 [,[Outputs  =>] OUTPUT_OPERAND_LIST      ]
                 [,[Inputs   =>] INPUT_OPERAND_LIST       ]
                 [,[Clobber  =>] static_string_EXPRESSION ]
                 [,[Volatile =>] static_boolean_EXPRESSION] )
  OUTPUT_OPERAND_LIST ::=
    No_Output_Operands
  | OUTPUT_OPERAND_ATTRIBUTE
  | (OUTPUT_OPERAND_ATTRIBUTE {,OUTPUT_OPERAND_ATTRIBUTE})
  OUTPUT_OPERAND_ATTRIBUTE ::=
    SUBTYPE_MARK'Asm_Output (static_string_EXPRESSION, NAME)
  INPUT_OPERAND_LIST ::=
    No_Input_Operands
  | INPUT_OPERAND_ATTRIBUTE
  | (INPUT_OPERAND_ATTRIBUTE {,INPUT_OPERAND_ATTRIBUTE})
  INPUT_OPERAND_ATTRIBUTE ::=
    SUBTYPE_MARK'Asm_Input (static_string_EXPRESSION, EXPRESSION)

GNAT Implementation of Tasking

• Mapping Ada Tasks onto the Underlying Kernel Threads
• Ensuring Compliance with the Real-Time Annex

Mapping Ada Tasks onto the Underlying Kernel Threads

GNAT run-time system comprises two layers:

• GNARL (GNAT Run-time Layer)
• GNULL (GNAT Low-level Library)

In GNAT, Ada's tasking services rely on a platform and OS independent layer known as GNARL. This code is responsible for implementing the correct semantics of Ada's task creation, rendez-vous, protected operations etc.

GNARL decomposes Ada's tasking semantics into simpler lower level operations such as create a thread, set the priority of a thread, yield, create a lock, lock/unlock, etc. The spec for these low-level operations constitutes GNULLI, the GNULL Interface. This interface is directly inspired from the POSIX real-time API.

If the underlying executive or OS implements the POSIX standard faithfully, the GNULL Interface maps as is to the services offered by the underlying kernel. Otherwise, some target dependent glue code maps the services offered by the underlying kernel to the semantics expected by GNARL.

Whatever the underlying OS (VxWorks, UNIX, OS/2, Windows NT, etc.) the key point is that each Ada task is mapped on a thread in the underlying kernel. For example, in the case of VxWorks

1 Ada task = 1 VxWorks task

In addition Ada task priorities map onto the underlying thread priorities. Mapping Ada tasks onto the underlying kernel threads has several advantages:

1. The underlying scheduler is used to schedule the Ada tasks. This makes Ada tasks as efficient as kernel threads from a scheduling standpoint.
2. Interaction with code written in C containing threads is eased since at the lowest level Ada tasks and C threads map onto the same underlying kernel concept.
3. When an Ada task is blocked during I/O the remaining Ada tasks are able to proceed.
4. On multi-processor systems Ada Tasks can execute in parallel.

Ensuring Compliance with the Real-Time Annex

The reader will be quick to notice that while mapping Ada tasks onto the underlying threads has significant advantages, it does create some complications when it comes to respecting the scheduling semantics specified in the real-time annex (Annex D).

For instance Annex D requires that for the FIFO_Within_Priorities scheduling policy we have:

9 When the active priority of a ready task that is not running changes, or the setting of its base priority takes effect, the task is removed from the ready queue for its old active priority and is added at the tail of the ready queue for its new active priority, except in the case where the active priority is lowered due to the loss of inherited priority, in which case the task is added at the head of the ready queue for its new active priority.

While most kernels do put tasks at the end of the priority queue when a task changes its priority, (which respects the main FIFO_Within_Priorities requirement), almost none keep a thread at the beginning of its priority queue when its priority drops from the loss of inherited priority.

As a result most vendors have provided incomplete Annex D implementations.

The GNAT run-time, has a nice cooperative solution to this problem which ensures that accurate FIFO_Within_Priorities semantics are respected.

The principle is as follows. When an Ada task T is about to start running, it checks whether some other Ada task R with the same priority as T has been suspended due to the loss of priority inheritance. If this is the case, T yields and is placed at the end of its priority queue. When R arrives at the front of the queue it executes.

Note that this simple scheme preserves the relative order of the tasks that were ready to execute in the priority queue where R has been placed at the end.

Specialized Needs Annexes

Ada 95 defines a number of specialized needs annexes, which are not required in all implementations. However, as described in this chapter, GNAT implements all of these special needs annexes:

Systems Programming (Annex C)

The systems programming annex is fully implemented.

Real-Time Systems (Annex D)

The real-time systems annex is fully implemented.

Distributed Systems (Annex E)

Stub generation is fully implemented, but no PCS is provided yet, so distributed systems cannot yet be constructed with this version of GNAT.

Information Systems (Annex F)

The information systems annex is fully implemented.

Numerics (Annex G)

The numerics annex is fully implemented.

Safety and Security (Annex H)

The safety and security annex is fully implemented.

Obsolescent Features (Annex I)

The obsolescent features annex is fully implemented.

Language Defined Attributes (Annex J)

The language defined attributes annex is fully implemented.

Language Defined Pragmas (Annex K)

The language defined pragmas annex is fully implemented.

Compatibility Guide

This chapter contains sections that describe compatibility issues between GNAT and other Ada 83 and Ada 95 compilation systems, to aid in porting applications developed in other Ada environments.

• Compatibility with Ada 83
• Compatibility with DEC Ada 83
• Compatibility with Other Ada 95 Systems
• Representation Clauses

Compatibility with Ada 83

Ada 95 is designed to be highly upwards compatible with Ada 83. In particular, the design intention is that the difficulties associated with moving from Ada 83 to Ada 95 should be no greater than those that occur when moving from one Ada 83 system to another.

However, there are a number of points at which there are minor incompatibilities. The Ada 95 Annotated Reference Manual contains full details of these issues, and should be consulted for a complete treatment. In practice the following are the most likely issues to be encountered.

Character range

The range of Standard.Character is now the full 256 characters of Latin-1, whereas in most Ada 83 implementations it was restricted to 128 characters. This may show up as compile time or runtime errors. The desirable fix is to adapt the program to accommodate the full character set, but in some cases it may be convenient to define a subtype or derived type of Character that covers only the restricted range.

New reserved words

The identifiers abstract, aliased, protected, requeue, tagged, and until are reserved in Ada 95. Existing Ada 83 code using any of these identifiers must be edited to use some alternative name.

Freezing rules

The rules in Ada 95 are slightly different with regard to the point at which entities are frozen, and representation pragmas and clauses are not permitted past the freeze point. This shows up most typically in the form of an error message complaining that a representation item appears too late, and the appropriate corrective action is to move the item nearer to the declaration of the entity to which it refers. A particular case is that representation pragmas (including the extended DEC Ada 83 compatibility pragmas such as Export_Procedure), cannot be applied to a subprogram body. If necessary, a separate subprogram declaration must be introduced to which the pragma can be applied.

Optional bodies for library packages

In Ada 83, a package that did not require a package body was nevertheless allowed to have one. This lead to certain surprises in compiling large systems (situations in which the body could be unexpectedly ignored). In Ada 95, if a package does not require a body then it is not permitted to have a body. To fix this problem, simply remove a redundant body if it is empty, or, if it is non-empty, introduce a dummy declaration into the spec that makes the body required. One approach is to add a private part to the package declaration (if necessary), and define a parameterless procedure called Requires_Body, which must then be given a dummy procedure body in the package body, which then becomes required.

Numeric_Error is now the same as Constraint_Error

In Ada 95, the exception Numeric_Error is a renaming of Constraint_Error. This means that it is illegal to have separate exception handlers for the two exceptions. The fix is simply to remove the handler for the Numeric_Error case (since even in Ada 83, a compiler was free to raise Constraint_Error in place of Numeric_Error in all cases).

Indefinite subtypes in generics

In Ada 83, it was permissible to pass an indefinite type (e.g. String) as the actual for a generic formal private type, but then the instantiation would be illegal if there were any instances of declarations of variables of this type in the generic body. In Ada 95, to avoid this clear violation of the contract model, the generic declaration clearly indicates whether or not such instantiations are permitted. If a generic formal parameter has explicit unknown discriminants, indicated by using (<>) after the type name, then it can be instantiated with indefinite types, but no variables can be declared of this type. Any attempt to declare a variable will result in an illegality at the time the generic is declared. If the (<>) notation is not used, then it is illegal to instantiate the generic with an indefinite type. This will show up as a compile time error, and the fix is usually simply to add the (<>) to the generic declaration.

All implementations of GNAT provide a switch that causes GNAT to operate in Ada 83 mode. In this mode, some but not all compatibility problems of the type described above are handled automatically. For example, the new Ada 95 protected keywords are not recognized in this mode. However, in practice, it is usually advisable to make the necessary modifications to the program to remove the need for using this switch.

Compatibility with Other Ada 95 Systems

Providing that programs avoid the use of implementation dependent and implementation defined features of Ada 95, as documented in the Ada 95 reference manual, there should be a high degree of portability between GNAT and other Ada 95 systems. The following are specific items which have proved troublesome in moving GNAT programs to other Ada 95 compilers, but do not affect porting code to GNAT.

Ada 83 Pragmas and Attributes

Ada 95 compilers are allowed, but not required, to implement the missing Ada 83 pragmas and attributes that are no longer defined in Ada 95. GNAT implements all such pragmas and attributes, eliminating this as a compatibility concern, but some other Ada 95 compilers reject these pragmas and attributes.

Special-needs Annexes

GNAT implements the full set of special needs annexes. At the current time, it is the only Ada 95 compiler to do so. This means that programs making use of these features may not be portable to other Ada 95 compilation systems.

Representation Clauses

Some other Ada 95 compilers implement only the minimal set of representation clauses required by the Ada 95 reference manual. GNAT goes far beyond this minimal set, as described in the next section.

Representation Clauses

The Ada 83 reference manual was quite vague in describing both the minimal required implementation of representation clauses, and also their precise effects. The Ada 95 reference manual is much more explicit, but the minimal set of capabilities required in Ada 95 is quite limited.

GNAT implements the full required set of capabilities described in the Ada 95 reference manual, but also goes much beyond this, and in particular an effort has been made to be compatible with existing Ada 83 usage to the greatest extent possible.

A few cases exist in which Ada 83 compiler behavior is incompatible with requirements in the Ada 95 reference manual. These are instances of intentional or accidental dependence on specific implementation dependent characteristics of these Ada 83 compilers. The following is a list of the cases most likely to arise in existing legacy Ada 83 code.

Implicit Packing

Some Ada 83 compilers allowed a Size specification to cause implicit packing of an array or record. This is specifically disallowed by implementation advice in the Ada 83 reference manual (for good reason, this usage can cause expensive implicit conversions to occur in the code). The problem will show up as an error message rejecting the size clause. The fix is simply to provide the explicit pragma Pack.

Meaning of Size Attribute

The Size attribute in Ada 95 for discrete types is defined as being the minimal number of bits required to hold values of the type. For example, on a 32-bit machine, the size of Natural will typically be 31 and not 32 (since no sign bit is required). Some Ada 83 compilers gave 31, and some 32 in this situation. This problem will usually show up as a compile time error, but not always. It is a good idea to check all uses of the 'Size attribute when porting Ada 83 code. The GNAT specific attribute Object_Size can provide a useful way of duplicating the behavior of some Ada 83 compiler systems.

Size of Access Types

A common assumption in Ada 83 code is that an access type is in fact a pointer, and that therefore it will be the same size as a System.Address value. This assumption is true for GNAT in most cases with one exception. For the case of a pointer to an unconstrained array type (where the bounds may vary from one value of the access type to another), the default is to use a "fat pointer", which is represented as two separate pointers, one to the bounds, and one to the array. This representation has a number of advantages, including improved efficiency. However, it may cause some difficulties in porting existing Ada 83 code which makes the assumption that, for example, pointers fit in 32 bits on a machine with 32-bit addressing. To get around this problem, GNAT also permits the use of "thin pointers" for access types in this case (where the designated type is an unconstrained array type). These thin pointers are indeed the same size as a System.Address value. To specify a thin pointer, use a size clause for the type, for example:

type X is access all String;
for X'Size use System.Address'Size;

which will cause the type X to be represented using a single pointer. When using this representation, the bounds are right behind the array. This representation is slightly less efficient, and does not allow quite such flexibility in the use of foreign pointers or in using the Unrestricted_Access attribute to create pointers to non-aliased objects. But for any standard portable use of the access type it will work in a functionally correct manner and allow porting of existing code. Note that another way of forcing a thin pointer representation is to use a component size clause for the element size in an array, or a record representation clause for an access field in a record.

Compatibility with DEC Ada 83

The VMS version of GNAT fully implements all the pragmas and attributes provided by DEC Ada 83, as well as providing the standard DEC Ada 83 libraries, including Starlet. In addition, data layouts and parameter passing conventions are highly compatible. This means that porting existing DEC Ada 83 code to GNAT in VMS systems should be easier than most other porting efforts. The following are some of the most significant differences between GNAT and DEC Ada 83.

Default floating-point representation

In GNAT, the default floating-point format is IEEE, whereas in DEC Ada 83, it is VMS format. GNAT does implement the necessary pragmas (Long_Float, Float_Representation) for changing this default.

System

The package System in GNAT exactly corresponds to the definition in the Ada 95 reference manual, which means that it excludes many of the DEC Ada 83 extensions. However, a separate package Aux_DEC is provided that contains the additional definitions, and a special pragma, Extend_System allows this package to be treated transparently as an extension of package System.

To_Address

The definitions provided by Aux_DEC are exactly compatible with those in the DEC Ada 83 version of System, with one exception. DEC Ada provides the following declarations:

   TO_ADDRESS(INTEGER)
   TO_ADDRESS(UNSIGNED_LONGWORD)
   TO_ADDRESS(universal_integer)

The version of TO_ADDRESS taking a universal integer argument is in fact an extension to Ada 83 not strictly compatible with the reference manual. In GNAT, we are constrained to be exactly compatible with the standard, and this means we cannot provide this capability. In DEC Ada 83, the point of this definition is to deal with a call like:

   TO_ADDRESS (16#12777#);

Normally, according to the Ada 83 standard, one would expect this to be ambiguous, since it matches both the INTEGER and UNSIGNED_LONGWORD forms of TO_ADDRESS. However, in DEC Ada 83, there is no ambiguity, since the definition using universal_integer takes precedence. In GNAT, since the version with universal_integer cannot be supplied, it is not possible to be 100% compatible. Since there are many programs using numeric constants for the argument to TO_ADDRESS, the decision in GNAT was to change the name of the function in the UNSIGNED_LONGWORD case, so the declarations provided in the GNAT version of AUX_Dec are:

   function To_Address (X : Integer) return Address;
   pragma Pure_Function (To_Address);

   function To_Address_Long (X : Unsigned_Longword) return Address;
   pragma Pure_Function (To_Address_Long);

This means that programs using TO_ADDRESS for UNSIGNED_LONGWORD must change the name to TO_ADDRESS_LONG.

Task_Id values

The Task_Id values assigned will be different in the two systems, and GNAT does not provide a specified value for the Task_Id of the environment task, which in GNAT is treated like any other declared task.

For full details on these and other less significant compatibility issues, see appendix E of the Digital publication entitled "DEC Ada, Technical Overview and Comparison on DIGITAL Platforms".

For GNAT running on other than VMS systems, all the DEC Ada 83 pragmas and attributes are recognized, although only a subset of them can sensibly be implemented. The description of pragmas in this reference manual indicates whether or not they are applicable to non-VMS systems.

Index

a

b

c

d

e

f

g

h

i

l

m

n

o

p

r

s

t

u

v

w

z

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