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C.6 Shared Variable Control

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

Syntax

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The form for pragmas Atomic, Volatile, Atomic_Components, and Volatile_Components is as follows: 
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  pragma Atomic(local_name);
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  pragma Volatile(local_name);
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  pragma Atomic_Components(array_local_name);
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  pragma Volatile_Components(array_local_name);
7/2
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.
8
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
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. 

Static Semantics

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These pragmas are representation pragmas (see 13.1).

Dynamic Semantics

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For an atomic object (including an atomic component) all reads and updates of the object as a whole are indivisible.
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For a volatile object all reads and updates of the object as a whole are performed directly to memory. 
17
Two actions are sequential (see 9.10) if each is the read or update of the same atomic object.
18
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. 

Implementation Requirements

20
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. 
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. 

Implementation Advice

22/2
 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. 
23/2
 A load or store of an atomic object should, where possible, 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.”

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