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9.10 Shared Variables

Static Semantics

{AI05-0009-1} {AI05-0201-1} {AI05-0229-1} {AI05-0295-1} {AI12-0119-1} {AI12-0363-1} 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 tasks 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.
This paragraph was deleted.
Implementation Note: {AI05-0229-1} Independent addressability is the only high level semantic effect of aspect Pack. If two objects are independently addressable, the implementation should allocate them in such a way that each can be written by the hardware without writing the other. For example, unless the user asks for it, it is generally not feasible to choose a bit-packed representation on a machine without an atomic bit field insertion instruction, because there might be tasks that update neighboring subcomponents concurrently, and locking operations on all subcomponents is generally not a good idea.
{AI05-0229-1} Even if Pack or one of the other above-mentioned aspects is specified, subcomponents should still be updated independently if the hardware efficiently supports it. 
Ramification: {AI05-0009-1} {AI05-0201-1} {AI12-0001-1} An atomic object (including atomic components) is always independently addressable from any other nonoverlapping object. Aspect_specifications and representation items cannot change that fact Any aspect_specification or representation item which would prevent this from being true should be rejected, notwithstanding what this Reference Manual says elsewhere. Note, however, that the components of an atomic object are not necessarily atomic. 

Dynamic Semantics

{AI12-0119-1} [Separate logical threads of control tasks 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 tasks to allow, for example, meaningful communication by the direct updating and reading of variables shared between them the tasks.] The actions of two different logical threads of control 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:
If A1 and A2 are part of the execution of the same task, and the language rules require A1 to be performed before A2;
If A1 is the action of an activator that initiates the activation of a task, and A2 is part of the execution of the task that is activated;
If A1 is part of the activation of a task, and A2 is the action of waiting for completion of the activation;
If A1 is part of the execution of a task, and A2 is the action of waiting for the termination of the task;
{8652/0031} {AI95-00118-01} {AI05-0072-1} If A1 is the termination of a task T, and A2 is either an evaluation of the expression T'Terminated that results in True, or a call to Ada.Task_Identification.Is_Terminated with an actual parameter that identifies T and a result of True (see C.7.1);
{AI05-0262-1} If A1 is the action of issuing an entry call, and A2 is part of the corresponding execution of the appropriate entry_body or accept_statement;
Ramification: Evaluating the entry_index of an accept_statement is not synchronized with a corresponding entry call, nor is evaluating the entry barrier of an entry_body.
If A1 is part of the execution of an accept_statement or entry_body, and A2 is the action of returning from the corresponding entry call;
If A1 is part of the execution of a protected procedure body or entry_body for a given protected object, and A2 is part of a later execution of an entry_body for the same protected object; 
Reason: The underlying principle here is that for one action to “signal” a second, the second action has to follow a potentially blocking operation, whose blocking is dependent on the first action in some way. Protected procedures are not potentially blocking, so they can only be "signalers", they cannot be signaled.
Ramification: Protected subprogram calls are not defined to signal one another, which means that such calls alone cannot be used to synchronize access to shared data outside of a protected object. 
Reason: The point of this distinction is so that on multiprocessors with inconsistent caches, the caches only need to be refreshed at the beginning of an entry body, and forced out at the end of an entry body or protected procedure that leaves an entry open. Protected function calls, and protected subprogram calls for entryless protected objects do not require full cache consistency. Entryless protected objects are intended to be treated roughly like atomic objects — each operation is indivisible with respect to other operations (unless both are reads), but such operations cannot be used to synchronize access to other nonvolatile shared variables. 
If A1 signals some action that in turn signals A2. 
  {AI12-0298-1} 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.
{AI12-0267-1} 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 defined to be sequential are sequential if one of the following is true: 
One action signals the other;
{AI12-0119-1} Both actions occur as part of the execution of the same logical thread of control task
Reason: {AI12-0005-1} {AI12-0119-1} Any two actions of the same logical thread of control task are sequential, even if one does not signal the other because they can be executed in an “arbitrary” (but necessarily equivalent to some “sequential”) order. 
{AI12-0292-1} Both actions occur as part of protected actions on the same protected object, and at least most one of the actions is part of a call on an exclusive a protected operation function of the protected object. 
Reason: {AI12-0292-1} Because actions within protected actions do not always imply signaling, we have to mention them here explicitly to make sure that actions occurring within different protected actions of the same protected object are sequential with respect to one another (unless both are part of calls on nonexclusive protected functions). 
Ramification: {AI12-0292-1} It doesn't matter whether or not the variable being assigned is actually a subcomponent of the protected object; globals can be safely updated from within the bodies of protected procedures, protected or entries, or exclusive protected functions
{AI05-0229-1} Aspect Atomic or aspect Atomic_Components may also be specified to ensure that certain reads and updates are sequential — see C.6.
Ramification: {AI12-0119-1} If two actions are “sequential” it is known that their executions don't overlap in time, but it is not necessarily specified which occurs first. For example, all actions of a single logical thread of control task are sequential, even though the exact order of execution is not fully specified for all constructs. 
Discussion: Note that if two assignments to the same variable are sequential, but neither signals the other, then the program is not erroneous, but it is not specified which assignment ultimately prevails. Such a situation usually corresponds to a programming mistake, but in some (rare) cases, the order makes no difference, and for this reason this situation is not considered erroneous nor even a bounded error. In Ada 83, this was considered an “incorrect order dependence” if the “effect” of the program was affected, but “effect” was never fully defined. In Ada 95, this situation represents a potential nonportability, and a friendly compiler might want to warn the programmer about the situation, but it is not considered an error. An example where this would come up would be in gathering statistics as part of referencing some information, where the assignments associated with statistics gathering don't need to be ordered since they are just accumulating aggregate counts, sums, products, etc. 
{AI12-0267-1} Two actions that are not sequential are defined to be concurrent actions.
{AI12-0267-1} {AI12-0298-1} 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.
{AI12-0267-1} 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.]

Erroneous Execution

{AI12-0267-1} 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).

Wording Changes from Ada 95

{8652/0031} {AI95-00118-01} Corrigendum: Clarified that a task T2 can rely on values of variables that are updated by another task T1, if task T2 first verifies that T1'Terminated is True. 

Wording Changes from Ada 2005

{AI05-0009-1} {AI05-0201-1} Correction: Revised the definition of independent addressability to exclude conforming representation clauses and to require that atomic and independent objects always have independent addressability. This should not change behavior that the user sees for any Ada program, so it is not an inconsistency.
{AI05-0072-1} Correction: Corrected the wording of AI95-00118-01 to actually say what was intended (as described above). 

Wording Changes from Ada 2012

{AI12-0119-1} Added wording to support interaction of parallel constructs with tasks by changing various wording to talk about logical threads of control rather than purely about tasks.
{AI12-0267-1} {AI12-0298-1} Added wording to define potentially signalling actions and conflicting actions; these are used to define conflict policies.

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