9.10 Shared Variables

If two different objects, including nonoverlapping parts of the same object, are independently addressable, they can be manipulated concurrently by two different logical threads of control without synchronization, unless both are subcomponents of the same full access object, and either is nonatomic (see C.6). Any two nonoverlapping objects are independently addressable if either object is specified as independently addressable (see C.6). Otherwise, two nonoverlapping objects are independently addressable except when they are both parts of a composite object for which a nonconfirming value is specified for any of the following representation aspects: (record) Layout, Component_Size, Pack, Atomic, or Convention; in this case it is unspecified whether the parts are independently addressable.

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

  Action A1 is defined to potentially signal action A2 if A1 signals A2, if action A1 and A2 occur as part of the execution of the same logical thread of control, and the language rules permit action A1 to precede action A2, or if action A1 potentially signals some action that in turn potentially signals A2.

Two actions are defined to be sequential if one of the following is true: 

Aspect Atomic or aspect Atomic_Components may also be specified to ensure that certain reads and updates are sequential — see C.6. 

Two actions that are not sequential are defined to be concurrent actions.

Two actions are defined to conflict if one action assigns to an object, and the other action reads or assigns to a part of the same object (or of a neighboring object if the two are not independently addressable). The action comprising a call on a subprogram or an entry is defined to potentially conflict with another action if the Global aspect (or Global'Class aspect in the case of a dispatching call) of the called subprogram or entry is such that a conflicting action would be possible during the execution of the call. Similarly, two calls are considered to potentially conflict if they each have Global (or Global'Class in the case of a dispatching call) aspects such that conflicting actions would be possible during the execution of the calls. Finally, two actions that conflict are also considered to potentially conflict.

A synchronized object is an object of a task or protected type, an atomic object (see C.6), a suspension object (see D.10), or a synchronous barrier (see D.10.1). Operations on such objects are necessarily sequential with respect to one another, and hence are never considered to conflict.

The execution of two concurrent actions is erroneous if the actions make conflicting uses of a shared variable (or neighboring variables that are not independently addressable).