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9.10 Shared Variables

Static Semantics

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If two different objects, including nonoverlapping parts of the same object, are independently addressable, they can be manipulated concurrently by two different tasks without synchronization. Normally, any two nonoverlapping objects are independently addressable. However, if packing, record layout, or Component_Size is specified for a given composite object, then it is implementation defined whether or not two nonoverlapping parts of that composite object are independently addressable. 

Dynamic Semantics

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Separate tasks normally proceed independently and concurrently with one another. However, task interactions can be used to synchronize the actions of two or more tasks to allow, for example, meaningful communication by the direct updating and reading of variables shared between the tasks. The actions of two different tasks are synchronized in this sense when an action of one task signals an action of the other task; an action A1 is defined to signal an action A2 under the following circumstances:
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6.1/1
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Erroneous Execution

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Given an action of assigning to an object, and an action of reading or updating a part of the same object (or of a neighboring object if the two are not independently addressable), then the execution of the actions is erroneous unless the actions are sequential. Two actions are sequential if one of the following is true: 
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A pragma Atomic or Atomic_Components may also be used to ensure that certain reads and updates are sequential — see C.6.

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