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C.6 Shared Variable Control

1
[This clause specifies representation pragmas that control the use of shared variables.] 

Syntax

2
The form for pragmas Atomic, Volatile, Atomic_Components, and Volatile_Components is as follows: 
3
  pragma Atomic(local_name);
4
  pragma Volatile(local_name);
5
  pragma Atomic_Components(array_local_name);
6
  pragma Volatile_Components(array_local_name);
7/2
{AI95-00272-01} {atomic} An atomic type is one to which a pragma Atomic applies. An atomic object (including a component) is one to which a pragma Atomic applies, or a component of an array to which a pragma Atomic_Components applies, or any object of an atomic type, other than objects obtained by evaluating a slice.
7.a/2
Ramification: {AI95-00272-01} A slice of an atomic array object is not itself atomic. That's necessary as executing a read or write of a dynamic number of components in a single instruction is not possible on many targets.
8
{volatile} A volatile type is one to which a pragma Volatile applies. A volatile object (including a component) is one to which a pragma Volatile applies, or a component of an array to which a pragma Volatile_Components applies, or any object of a volatile type. In addition, every atomic type or object is also defined to be volatile. Finally, if an object is volatile, then so are all of its subcomponents [(the same does not apply to atomic)]. 

Name Resolution Rules

9
The local_name in an Atomic or Volatile pragma shall resolve to denote either an object_declaration, a non-inherited component_declaration, or a full_type_declaration. The array_local_name in an Atomic_Components or Volatile_Components pragma shall resolve to denote the declaration of an array type or an array object of an anonymous type.

Legality Rules

10
{indivisible} It is illegal to apply either an Atomic or Atomic_Components pragma to an object or type if the implementation cannot support the indivisible reads and updates required by the pragma (see below).
11
It is illegal to specify the Size attribute of an atomic object, the Component_Size attribute for an array type with atomic components, or the layout attributes of an atomic component, in a way that prevents the implementation from performing the required indivisible reads and updates.
12
If an atomic object is passed as a parameter, then the type of the formal parameter shall either be atomic or allow pass by copy [(that is, not be a nonatomic by-reference type)]. If an atomic object is used as an actual for a generic formal object of mode in out, then the type of the generic formal object shall be atomic. If the prefix of an attribute_reference for an Access attribute denotes an atomic object [(including a component)], then the designated type of the resulting access type shall be atomic. If an atomic type is used as an actual for a generic formal derived type, then the ancestor of the formal type shall be atomic or allow pass by copy. Corresponding rules apply to volatile objects and types.
13
If a pragma Volatile, Volatile_Components, Atomic, or Atomic_Components applies to a stand-alone constant object, then a pragma Import shall also apply to it. 
13.a
Ramification: Hence, no initialization expression is allowed for such a constant. Note that a constant that is atomic or volatile because of its type is allowed. 
13.b
Reason: Stand-alone constants that are explicitly specified as Atomic or Volatile only make sense if they are being manipulated outside the Ada program. From the Ada perspective the object is read-only. Nevertheless, if imported and atomic or volatile, the implementation should presume it might be altered externally. For an imported stand-alone constant that is not atomic or volatile, the implementation can assume that it will not be altered. 

Static Semantics

14
{representation pragma (Atomic) [partial]} {pragma, representation (Atomic) [partial]} {representation pragma (Volatile) [partial]} {pragma, representation (Volatile) [partial]} {representation pragma (Atomic_Components) [partial]} {pragma, representation (Atomic_Components) [partial]} {representation pragma (Volatile_Components) [partial]} {pragma, representation (Volatile_Components) [partial]} These pragmas are representation pragmas (see 13.1).

Dynamic Semantics

15
For an atomic object (including an atomic component) all reads and updates of the object as a whole are indivisible.
16
For a volatile object all reads and updates of the object as a whole are performed directly to memory. 
16.a
Implementation Note: This precludes any use of register temporaries, caches, and other similar optimizations for that object. 
17
{sequential (actions)} Two actions are sequential (see 9.10) if each is the read or update of the same atomic object.
18
{by-reference type (atomic or volatile) [partial]} If a type is atomic or volatile and it is not a by-copy type, then the type is defined to be a by-reference type. If any subcomponent of a type is atomic or volatile, then the type is defined to be a by-reference type.
19
If an actual parameter is atomic or volatile, and the corresponding formal parameter is not, then the parameter is passed by copy. 
19.a
Implementation Note: Note that in the case where such a parameter is normally passed by reference, a copy of the actual will have to be produced at the call-site, and a pointer to the copy passed to the formal parameter. If the actual is atomic, any copying has to use indivisible read on the way in, and indivisible write on the way out. 
19.b
Reason: It has to be known at compile time whether an atomic or a volatile parameter is to be passed by copy or by reference. For some types, it is unspecified whether parameters are passed by copy or by reference. The above rules further specify the parameter passing rules involving atomic and volatile types and objects.

Implementation Requirements

20
{external effect (volatile/atomic objects) [partial]} The external effect of a program (see 1.1.3) is defined to include each read and update of a volatile or atomic object. The implementation shall not generate any memory reads or updates of atomic or volatile objects other than those specified by the program.
20.a
Discussion: The presumption is that volatile or atomic objects might reside in an “active” part of the address space where each read has a potential side-effect, and at the very least might deliver a different value.
20.b
The rule above and the definition of external effect are intended to prevent (at least) the following incorrect optimizations, where V is a volatile variable:
20.c
20.d
20.e
20.f
20.g
20.h
20.i
21
If a pragma Pack applies to a type any of whose subcomponents are atomic, the implementation shall not pack the atomic subcomponents more tightly than that for which it can support indivisible reads and updates. 
21.a
Implementation Note: A warning might be appropriate if no packing whatsoever can be achieved. 

Implementation Advice

22/2
 {AI95-00259-01} A load or store of a volatile object whose size is a multiple of System.Storage_Unit and whose alignment is nonzero, should be implemented by accessing exactly the bits of the object and no others. 
22.a/2
Implementation Advice: A load or store of a volatile object whose size is a multiple of System.Storage_Unit and whose alignment is nonzero, should be implemented by accessing exactly the bits of the object and no others.
22.b/2
Reason: Since any object can be a volatile object, including packed array components and bit-mapped record components, we require the above only when it is reasonable to assume that the machine can avoid accessing bits outside of the object.
22.c/2
Ramification: This implies that the load or store of a volatile object that meets the above requirement should not be combined with that of any other object, nor should it access any bits not belonging to any other object. This means that the suitability of the implementation for memory-mapped I/O can be determined from its documentation, as any cases where the implementation does not follow Implementation Advice must be documented. 
23/2
 {AI95-00259-01} A load or store of an atomic object should, where possible, be implemented by a single load or store instruction.
23.a/2
Implementation Advice: A load or store of an atomic object should be implemented by a single load or store instruction.
NOTES
24
9  An imported volatile or atomic constant behaves as a constant (i.e. read-only) with respect to other parts of the Ada program, but can still be modified by an “external source.”

Incompatibilities With Ada 83

24.a
{incompatibilities with Ada 83} Pragma Atomic replaces Ada 83's pragma Shared. The name “Shared” was confusing, because the pragma was not used to mark variables as shared.

Wording Changes from Ada 95

24.b/2
{AI95-00259-01} Added Implementation Advice to clarify the meaning of Atomic and Volatile in machine terms. The documentation that this advice applies will make the use of Ada implementations more predictable for low-level (such as device register) programming.
24.c/2
{AI95-00272-01} Added wording to clarify that a slice of an object of an atomic type is not atomic, just like a component of an atomic type is not (necessarily) atomic. 

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