13.3 Operational and Representation Attributes
The values of certain implementation-dependent characteristics can be obtained by interrogating appropriate operational or representation attributes. Some of these attributes are specifiable via an attribute_definition_clause.
- 1 Syntax
- 2 Name Resolution Rules
- 3 Legality Rules
- 4 Static Semantics
- 5 Erroneous Execution
- 6 Implementation Advice
- 7 Notes
- 8 Static Semantics
- 9 Erroneous Execution
- 10 Implementation Advice
- 11 Notes
- 12 Static Semantics
- 13 Implementation Advice
- 14 Static Semantics
- 15 Implementation Requirements
- 16 Implementation Advice
- 17 Notes
- 18 Static Semantics
- 19 Syntax
- 20 Name Resolution Rules
- 21 Dynamic Semantics
- 22 Static Semantics
- 23 Implementation Advice
- 24 Static Semantics
- 25 Implementation Requirements
- 26 Notes
- 27 Examples
- 28 Notes
attribute_definition_clause ::= for local_name'attribute_designator use expression; | for local_name'attribute_designator use name;
Name Resolution Rules
For an attribute_definition_clause that specifies an attribute that denotes a value, the form with an expression shall be used. Otherwise, the form with a name shall be used.
For an attribute_definition_clause that specifies an attribute that denotes a value or an object, the expected type for the expression or name is that of the attribute. For an attribute_definition_clause that specifies an attribute that denotes a subprogram, the expected profile for the name is the profile required for the attribute. For an attribute_definition_clause that specifies an attribute that denotes some other kind of entity, the name shall resolve to denote an entity of the appropriate kind.
An attribute_designator is allowed in an attribute_definition_clause only if this International Standard explicitly allows it, or for an implementation-defined attribute if the implementation allows it. Each specifiable attribute constitutes an operational aspect or aspect of representation.
For an attribute_definition_clause that specifies an attribute that denotes a subprogram, the profile shall be mode conformant with the one required for the attribute, and the convention shall be Ada. Additional requirements are defined for particular attributes.
A Size clause is an attribute_definition_clause whose attribute_designator is Size. Similar definitions apply to the other specifiable attributes.
A storage element is an addressable element of storage in the machine. A word is the largest amount of storage that can be conveniently and efficiently manipulated by the hardware, given the implementation's run-time model. A word consists of an integral number of storage elements.
The following representation attributes are defined: Address, Alignment, Size, Storage_Size, and Component_Size.
For a prefix X that denotes an object, program unit, or label:
- Denotes the address of the first of the storage elements allocated to X. For a program unit or label, this value refers to the machine code associated with the corresponding body or statement. The value of this attribute is of type System.Address.
- Address may be specified for stand-alone objects and for program units via an attribute_definition_clause.
If an Address is specified, it is the programmer's responsibility to ensure that the address is valid; otherwise, program execution is erroneous.
For an array X, X'Address should point at the first component of the array, and not at the array bounds.
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.
- An implementation should support Address clauses for imported subprograms.
- Objects (including subcomponents) that are aliased or of a by-reference type should be allocated on storage element boundaries.
- 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.
1 The specification of a link name in a pragma Export (see B.1) for a subprogram or object is an alternative to explicit specification of its link-time address, allowing a link-time directive to place the subprogram or object within memory.
2 The rules for the Size attribute imply, for an aliased object X, that if X'Size = Storage_Unit, then X'Address points at a storage element containing all of the bits of X, and only the bits of X.
For a prefix X that denotes a subtype or object:
- The Address of an object that is allocated under control of the implementation is an integral multiple of the Alignment of the object (that is, the Address modulo the Alignment is zero). The offset of a record component is a multiple of the Alignment of the component. For an object that is not allocated under control of the implementation (that is, one that is imported, that is allocated by a user-defined allocator, whose Address has been specified, or is designated by an access value returned by an instance of Unchecked_Conversion), the implementation may assume that the Address is an integral multiple of its Alignment. The implementation shall not assume a stricter alignment.
- The value of this attribute is of type universal_integer, and nonnegative; zero means that the object is not necessarily aligned on a storage element boundary.
- Alignment may be specified for first subtypes and stand-alone objects via an attribute_definition_clause; the expression of such a clause shall be static, and its value nonnegative. If the Alignment of a subtype is specified, then the Alignment of an object of the subtype is at least as strict, unless the object's Alignment is also specified. The Alignment of an object created by an allocator is that of the designated subtype.
- If an Alignment is specified for a composite subtype or object, this Alignment shall be equal to the least common multiple of any specified Alignments of the subcomponent subtypes, or an integer multiple thereof.
Program execution is erroneous if an Address clause is given that conflicts with the Alignment.
If the Alignment is specified for an object that is not allocated under control of the implementation, execution is erroneous if the object is not aligned according to the Alignment.
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:
- An implementation need not support specified Alignments for combinations of Sizes and Alignments that cannot be easily loaded and stored by available machine instructions.
- An implementation need not support specified Alignments that are greater than the maximum Alignment the implementation ever returns by default.
The recommended level of support for the Alignment attribute for objects is:
- Same as above, for subtypes, but in addition:
- 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.
3 Alignment is a subtype-specific attribute.
4 The Alignment of a composite object is always equal to the least common multiple of the Alignments of its components, or a multiple thereof.
5 A component_clause, Component_Size clause, or a pragma Pack can override a specified Alignment.
For a prefix X that denotes an object:
- Denotes the size in bits of the representation of the object. The value of this attribute is of the type universal_integer.
- Size may be specified for stand-alone objects via an attribute_definition_clause; the expression of such a clause shall be static and its value nonnegative.
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).
For every subtype S:
- If S is definite, denotes the size (in bits) that the implementation would choose for the following objects of subtype S:
- A record component of subtype S when the record type is packed.
- The formal parameter of an instance of Unchecked_Conversion that converts from subtype S to some other subtype.
- If S is indefinite, the meaning is implementation defined. The value of this attribute is of the type universal_integer. The Size of an object is at least as large as that of its subtype, unless the object's Size is determined by a Size clause, a component_clause, or a Component_Size clause. Size may be specified for first subtypes via an attribute_definition_clause; the expression of such a clause shall be static and its value nonnegative.
In an implementation, Boolean'Size shall be 1.
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).
- Unaliased components, unless the Size of the component is determined by a component_clause or Component_Size clause.
A Size clause on a composite subtype should not affect the internal layout of components.
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.
- 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.
6 Size is a subtype-specific attribute.
7 A component_clause or Component_Size clause can override a specified Size. A pragma Pack cannot.
For a prefix T that denotes a task object (after any implicit dereference):
- Denotes the number of storage elements reserved for the task. The value of this attribute is of the type universal_integer. The Storage_Size includes the size of the task's stack, if any. The language does not specify whether or not it includes other storage associated with the task (such as the task control block used by some implementations.) If a pragma Storage_Size is given, the value of the Storage_Size attribute is at least the value specified in the pragma.
A pragma Storage_Size specifies the amount of storage to be reserved for the execution of a task.
The form of a pragma Storage_Size is as follows:
A pragma Storage_Size is allowed only immediately within a task_definition.
Name Resolution Rules
The expression of a pragma Storage_Size is expected to be of any integer type.
A pragma Storage_Size is elaborated when an object of the type defined by the immediately enclosing task_definition is created. For the elaboration of a pragma Storage_Size, the expression is evaluated; the Storage_Size attribute of the newly created task object is at least the value of the expression.
At the point of task object creation, or upon task activation, Storage_Error is raised if there is insufficient free storage to accommodate the requested Storage_Size.
For a prefix X that denotes an array subtype or array object (after any implicit dereference):
- Denotes the size in bits of components of the type of X. The value of this attribute is of type universal_integer.
- Component_Size may be specified for array types via an attribute_definition_clause; the expression of such a clause shall be static, and its value nonnegative.
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.
- 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.
The following operational attribute is defined: External_Tag.
For every subtype S of a tagged type T (specific or class-wide):
- S'External_Tag denotes an external string representation for S'Tag; it is of the predefined type String. External_Tag may be specified for a specific tagged type via an attribute_definition_clause; the expression of such a clause shall be static. The default external tag representation is implementation defined. See 3.9.2 and 13.13.2. The value of External_Tag is never inherited; the default value is always used unless a new value is directly specified for a type.
In an implementation, the default external tag for each specific tagged type declared in a partition shall be distinct, so long as the type is declared outside an instance of a generic body. If the compilation unit in which a given tagged type is declared, and all compilation units on which it semantically depends, are the same in two different partitions, then the external tag for the type shall be the same in the two partitions. What it means for a compilation unit to be the same in two different partitions is implementation defined. At a minimum, if the compilation unit is not recompiled between building the two different partitions that include it, the compilation unit is considered the same in the two partitions.
8 The following language-defined attributes are specifiable, at least for some of the kinds of entities to which they apply: Address, Size, Component_Size, Alignment, External_Tag, Small, Bit_Order, Storage_Pool, Storage_Size, Write, Output, Read, Input, and Machine_Radix.
9 It follows from the general rules in 13.1 that if one writes for X'Size use Y; then the X'Size attribute_reference will return Y (assuming the implementation allows the Size clause). The same is true for all of the specifiable attributes except Storage_Size.
Examples of attribute definition clauses:
Byte : constant := 8;
Page : constant := 2**12;
type Medium is range 0 .. 65_000;
for Medium'Size use 2*Byte;
for Medium'Alignment use 2;
Device_Register : Medium;
for Device_Register'Size use Medium'Size;
for Device_Register'Address use System.Storage_Elements.To_Address(16#FFFF_0020#);
type Short is delta 0.01 range -100.0 .. 100.0;
for Short'Size use 15;
for Car_Name'Storage_Size use -- specify access type's storage pool size
2000*((Car'Size/System.Storage_Unit) +1); -- approximately 2000 cars
function My_Read(Stream : access Ada.Streams.Root_Stream_Type'Class)
for T'Read use My_Read; -- see 13.13.2
10 Notes on the examples: In the Size clause for Short, fifteen bits is the minimum necessary, since the type definition requires Short'Small <= 2**(-7).
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Copyright © 2000 The MITRE Corporation, Inc. Ada Reference Manual