Protected Subprograms and Protected Actions
Author: JTC1/SC22/WG9/ARG, by Randall Brukardt, ARG Editor

Ada Reference Manual — Legal Information

# 9.5.1 Protected Subprograms and Protected Actions

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A protected subprogram is a subprogram declared immediately within a protected_definition. Protected procedures provide exclusive read-write access to the data of a protected object; protected functions provide concurrent read-only access to the data.

#### Static Semantics

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Within the body of a protected function (or a function declared immediately within a protected_body), the current instance of the enclosing protected unit is defined to be a constant (that is, its subcomponents may be read but not updated). Within the body of a protected procedure (or a procedure declared immediately within a protected_body), and within an entry_body, the current instance is defined to be a variable (updating is permitted).

#### Dynamic Semantics

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For the execution of a call on a protected subprogram, the evaluation of the name or prefix and of the parameter associations, and any assigning back of in out or out parameters, proceeds as for a normal subprogram call (see 6.4). If the call is an internal call (see 9.5), the body of the subprogram is executed as for a normal subprogram call. If the call is an external call, then the body of the subprogram is executed as part of a new protected action on the target protected object; the protected action completes after the body of the subprogram is executed. A protected action can also be started by an entry call (see 9.5.3).
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A new protected action is not started on a protected object while another protected action on the same protected object is underway, unless both actions are the result of a call on a protected function. This rule is expressible in terms of the execution resource associated with the protected object:
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- Starting a protected action on a protected object corresponds to acquiring the execution resource associated with the protected object, either for concurrent read-only access if the protected action is for a call on a protected function, or for exclusive read-write access otherwise;

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- Completing the protected action corresponds to releasing the associated execution resource.

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After performing an operation on a protected object other than a call on a protected function, but prior to completing the associated protected action, the entry queues (if any) of the protected object are serviced (see 9.5.3).

#### Bounded (Run-Time) Errors

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During a protected action, it is a bounded error to invoke an operation that is potentially blocking. The following are defined to be potentially blocking operations:
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- a select_statement;

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- an accept_statement;

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- an entry_call_statement;

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- a delay_statement;

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- an abort_statement;

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- task creation or activation;

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- an external call on a protected subprogram (or an external requeue) with the same target object as that of the protected action;

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- a call on a subprogram whose body contains a potentially blocking operation.

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If the bounded error is detected, Program_Error is raised. If not detected, the bounded error might result in deadlock or a (nested) protected action on the same target object.
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Certain language-defined subprograms are potentially blocking. In particular, the subprograms of the language-defined input-output packages that manipulate files (implicitly or explicitly) are potentially blocking. Other potentially blocking subprograms are identified where they are defined. When not specified as potentially blocking, a language-defined subprogram is nonblocking.
NOTES
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18 If two tasks both try to start a protected action on a protected object, and at most one is calling a protected function, then only one of the tasks can proceed. Although the other task cannot proceed, it is not considered blocked, and it might be consuming processing resources while it awaits its turn. There is no language-defined ordering or queuing presumed for tasks competing to start a protected action — on a multiprocessor such tasks might use busy-waiting; for monoprocessor considerations, see D.3, “Priority Ceiling Locking”.
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19 The body of a protected unit may contain declarations and bodies for local subprograms. These are not visible outside the protected unit.
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20 The body of a protected function can contain internal calls on other protected functions, but not protected procedures, because the current instance is a constant. On the other hand, the body of a protected procedure can contain internal calls on both protected functions and procedures.
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21 From within a protected action, an internal call on a protected subprogram, or an external call on a protected subprogram with a different target object is not considered a potentially blocking operation.
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22 The pragma Detect_Blocking may be used to ensure that all executions of potentially blocking operations during a protected action raise Program_Error. See H.5.

#### Examples

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Examples of protected subprogram calls (see 9.4):
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Shared_Array.Set_Component(N, E);
E := Shared_Array.Component(M);
Control.Release;

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