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13.5.1 Record Representation Clauses

1
[A record_representation_clause specifies the storage representation of records and record extensions, that is, the order, position, and size of components (including discriminants, if any). ]

Language Design Principles

1.a/2
{AI95-00114-01} It should be feasible for an implementation to use negative offsets in the representation of composite types. However, no implementation should be forced to support negative offsets. Therefore, in the interest of uniformity, negative offsets should be disallowed in record_representation_clauses.

Syntax

2
record_representation_clause ::= 
    for first_subtype_local_name use
      record [mod_clause]
        {component_clause}
      end record;
3
component_clause ::= 
    component_local_name at position range first_bit .. last_bit;
4
position ::= static_expression
5
first_bit ::= static_simple_expression
6
last_bit ::= static_simple_expression
6.a
Reason: First_bit and last_bit need to be simple_expression instead of expression for the same reason as in range (see 3.5, “Scalar Types”). 

Name Resolution Rules

7
Each position, first_bit, and last_bit is expected to be of any integer type. 
7.a
Ramification: These need not have the same integer type. 

Legality Rules

8/2
{AI95-00436-01} The first_subtype_local_name of a record_representation_clause shall denote a specific record or record extension subtype. 
8.a
Ramification: As for all type-related representation items, the local_name is required to denote a first subtype. 
9
If the component_local_name is a direct_name, the local_name shall denote a component of the type. For a record extension, the component shall not be inherited, and shall not be a discriminant that corresponds to a discriminant of the parent type. If the component_local_name has an attribute_designator, the direct_name of the local_name shall denote either the declaration of the type or a component of the type, and the attribute_designator shall denote an implementation-defined implicit component of the type.
10
The position, first_bit, and last_bit shall be static expressions. The value of position and first_bit shall be nonnegative. The value of last_bit shall be no less than first_bit – 1. 
10.a
Ramification: A component_clause such as “X at 4 range 0..–1;” is allowed if X can fit in zero bits. 
10.1/2
   {AI95-00133-01} If the nondefault bit ordering applies to the type, then either: 
10.2/2
the value of last_bit shall be less than the size of the largest machine scalar; or
10.3/2
the value of first_bit shall be zero and the value of last_bit + 1 shall be a multiple of System.Storage_Unit. 
11
At most one component_clause is allowed for each component of the type, including for each discriminant (component_clauses may be given for some, all, or none of the components). Storage places within a component_list shall not overlap, unless they are for components in distinct variants of the same variant_part.
12
A name that denotes a component of a type is not allowed within a record_representation_clause for the type, except as the component_local_name of a component_clause.
12.a
Reason: It might seem strange to make the record_representation_clause part of the declarative region, and then disallow mentions of the components within almost all of the record_representation_clause. The alternative would be to treat the component_local_name like a formal parameter name in a subprogram call (in terms of visibility). However, this rule would imply slightly different semantics, because (given the actual rule) the components can hide other declarations. This was the rule in Ada 83, and we see no reason to change it. The following, for example, was and is illegal: 
12.b
type T is
    record
        X : Integer;
    end record;
X : constant := 31; -- Same defining name as the component.
for T use
    record
        X at 0 range 0..X; -- Illegal!
    end record;
    
12.c
The component X hides the named number X throughout the record_representation_clause.

Static Semantics

13/2
 {AI95-00133-01} A record_representation_clause (without the mod_clause) specifies the layout.
13.a/3
Aspect Description for Layout (record): Layout of record components. Specified by a record_representation_clause, not by an aspect_specification.
13.b/3
Aspect Description for Record layout: See Layout.
13.1/2
   {AI95-00133-01} If the default bit ordering applies to the type, the position, first_bit, and last_bit of each component_clause directly specify the position and size of the corresponding component.
13.2/3
   {AI95-00133-01} {AI05-0264-1} If the nondefault bit ordering applies to the type, then the layout is determined as follows: 
13.3/2
the component_clauses for which the value of last_bit is greater than or equal to the size of the largest machine scalar directly specify the position and size of the corresponding component;
13.4/2
for other component_clauses, all of the components having the same value of position are considered to be part of a single machine scalar, located at that position; this machine scalar has a size which is the smallest machine scalar size larger than the largest last_bit for all component_clauses at that position; the first_bit and last_bit of each component_clause are then interpreted as bit offsets in this machine scalar. 
13.c/2
This paragraph was deleted.{AI95-00133-01}
13.d
Ramification: A component_clause also determines the value of the Size attribute of the component, since this attribute is related to First_Bit and Last_Bit. 
14
[A record_representation_clause for a record extension does not override the layout of the parent part;] if the layout was specified for the parent type, it is inherited by the record extension. 

Implementation Permissions

15
An implementation may generate implementation-defined components (for example, one containing the offset of another component). An implementation may generate names that denote such implementation-defined components; such names shall be implementation-defined attribute_references. An implementation may allow such implementation-defined names to be used in record_representation_clauses. An implementation can restrict such component_clauses in any manner it sees fit. 
15.a
Implementation defined: Implementation-defined components.
15.b
Ramification: Of course, since the semantics of implementation-defined attributes is implementation defined, the implementation need not support these names in all situations. They might be purely for the purpose of component_clauses, for example. The visibility rules for such names are up to the implementation.
15.c
We do not allow such component names to be normal identifiers — that would constitute blanket permission to do all kinds of evil things. 
15.d
Discussion: Such implementation-defined components are known in the vernacular as “dope.” Their main purpose is for storing offsets of components that depend on discriminants. 
16
If a record_representation_clause is given for an untagged derived type, the storage place attributes for all of the components of the derived type may differ from those of the corresponding components of the parent type, even for components whose storage place is not specified explicitly in the record_representation_clause.
16.a
Reason: This is clearly necessary, since the whole record may need to be laid out differently. 

Implementation Advice

17
The recommended level of support for record_representation_clauses is: 
17.1/2
{AI95-00133-01} An implementation should support machine scalars that correspond to all of the integer, floating point, and address formats supported by the machine.
18
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.
19
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.
20/2
{AI95-00133-01} For a component with a subtype whose Size is less than the word size, any storage place that does not cross an aligned word boundary should be supported.
20.a
Reason: The above recommendations are sufficient to define interfaces to most interesting hardware. This causes less implementation burden than the definition in ACID, which requires arbitrary bit alignments of arbitrarily large components. Since the ACID definition is neither enforced by the ACVC, nor supported by all implementations, it seems OK for us to weaken it. 
21
An implementation may reserve a storage place for the tag field of a tagged type, and disallow other components from overlapping that place. 
21.a
Ramification: Similar permission for other dope is not granted. 
22
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. 
22.a
Reason: These restrictions are probably necessary if block equality operations are to be feasible for class-wide types. For block comparison to work, the implementation typically has to fill in any gaps with zero (or one) bits. If a “gap” in the parent type is filled in with a component in a type extension, then this won't work when a class-wide object is passed by reference, as is required. 
22.b/2
Implementation Advice: The recommended level of support for record_representation_clauses should be followed.
NOTES
23
14  If no component_clause is given for a component, then the choice of the storage place for the component is left to the implementation. If component_clauses are given for all components, the record_representation_clause completely specifies the representation of the type and will be obeyed exactly by the implementation. 
23.a
Ramification: The visibility rules prevent the name of a component of the type from appearing in a record_representation_clause at any place except for the component_local_name of a component_clause. However, since the record_representation_clause is part of the declarative region of the type declaration, the component names hide outer homographs throughout.
23.b/1
{8652/0009} {AI95-00137-01} A record_representation_clause cannot be given for a protected type, even though protected types, like record types, have components. The primary reason for this rule is that there is likely to be too much dope in a protected type — entry queues, bit maps for barrier values, etc. In order to control the representation of the user-defined components, simply declare a record type, give it a record_representation_clause, and give the protected type one component whose type is the record type. Alternatively, if the protected object is protecting something like a device register, it makes more sense to keep the thing being protected outside the protected object (possibly with a pointer to it in the protected object), in order to keep implementation-defined components out of the way. 

Examples

24
Example of specifying the layout of a record type: 
25
Word : constant := 4;  --  storage element is byte, 4 bytes per word
26
type State         is (A,M,W,P);
type Mode          is (Fix, Dec, Exp, Signif);
27
type Byte_Mask     is array (0..7)  of Boolean;
type State_Mask    is array (State) of Boolean;
type Mode_Mask     is array (Mode)  of Boolean;
28
type Program_Status_Word is
  record
      System_Mask        : Byte_Mask;
      Protection_Key     : Integer range 0 .. 3;
      Machine_State      : State_Mask;
      Interrupt_Cause    : Interruption_Code;
      Ilc                : Integer range 0 .. 3;
      Cc                 : Integer range 0 .. 3;
      Program_Mask       : Mode_Mask;
      Inst_Address       : Address;
end record;
29
for Program_Status_Word use
  record
      System_Mask      at 0*Word range 0  .. 7;
      Protection_Key   at 0*Word range 10 .. 11; -- bits 8,9 unused
      Machine_State    at 0*Word range 12 .. 15;
      Interrupt_Cause  at 0*Word range 16 .. 31;
      Ilc              at 1*Word range 0  .. 1;  -- second word
      Cc               at 1*Word range 2  .. 3;
      Program_Mask     at 1*Word range 4  .. 7;
      Inst_Address     at 1*Word range 8  .. 31;
  end record;
30
for Program_Status_Word'Size use 8*System.Storage_Unit;
for Program_Status_Word'Alignment use 8;
NOTES
31
15  Note on the example: The record_representation_clause defines the record layout. The Size clause guarantees that (at least) eight storage elements are used for objects of the type. The Alignment clause guarantees that aliased, imported, or exported objects of the type will have addresses divisible by eight. 

Wording Changes from Ada 83

31.a
The alignment_clause has been renamed to mod_clause and moved to Annex J, “Obsolescent Features”.
31.b
We have clarified that implementation-defined component names have to be in the form of an attribute_reference of a component or of the first subtype itself; surely Ada 83 did not intend to allow arbitrary identifiers.
31.c
The RM83-13.4(7) wording incorrectly allows components in nonvariant records to overlap. We have corrected that oversight.

Incompatibilities With Ada 95

31.d/2
{AI95-00133-01} Amendment Correction: The meaning of a record_representation_clause for the nondefault bit order is now clearly defined. Thus, such clauses can be portably written. In order to do that though, the equivalence of bit 1 in word 1 to bit 9 in word 0 (for a machine with Storage_Unit = 8) had to be dropped for the nondefault bit order. Any record_representation_clauses which depends on that equivalence will break (although such code would imply a noncontiguous representation for a component, and it seems unlikely that compilers were supporting that anyway). 

Extensions to Ada 95

31.e/2
{AI95-00436-01} Amendment Correction: The undocumented (and likely unintentional) incompatibility with Ada 83 caused by not allowing record_representation_clauses on limited record types is removed. 

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