Annotated Ada Reference Manual — Legal Information

9.5 Intertask Communication

{intertask communication} {critical section: See intertask communication} The primary means for intertask communication is provided by calls on entries and protected subprograms. Calls on protected subprograms allow coordinated access to shared data objects. Entry calls allow for blocking the caller until a given condition is satisfied (namely, that the corresponding entry is open — see 9.5.3), and then communicating data or control information directly with another task or indirectly via a shared protected object.

Static Semantics

{target object (of a call on an entry or a protected subprogram)} Any call on an entry or on a protected subprogram identifies a target object for the operation, which is either a task (for an entry call) or a protected object (for an entry call or a protected subprogram call). The target object is considered an implicit parameter to the operation, and is determined by the operation name (or prefix) used in the call on the operation, as follows:

If it is a direct_name or expanded name that denotes the declaration (or body) of the operation, then the target object is implicitly specified to be the current instance of the task or protected unit immediately enclosing the operation; {internal call} such a call is defined to be an internal call;

If it is a selected_component that is not an expanded name, then the target object is explicitly specified to be the task or protected object denoted by the prefix of the name; {external call} such a call is defined to be an external call;

4.a

Discussion: For example:

4.b

protected type Pt is
procedure Op1;
procedure Op2;
end Pt;

4.c

PO : Pt;
Other_Object : Some_Other_Protected_Type;

4.d

protected body Pt is
procedure Op1 is begin ... end Op1;

4.e

  procedure Op2 is
  begin
    Op1; -- An internal call.
    Pt.Op1; -- Another internal call.
    PO.Op1; -- An external call. It the current instance is PO, then
            -- this is a bounded error (see 9.5.1).
    Other_Object.Some_Op; -- An external call.
  end Op2;
end Pt;

If the name or prefix is a dereference (implicit or explicit) of an access-to-protected-subprogram value, then the target object is determined by the prefix of the Access attribute_reference that produced the access value originally, and the call is defined to be an external call;

If the name or prefix denotes a subprogram_renaming_declaration, then the target object is as determined by the name of the renamed entity.

{target object (of a requeue_statement)} {internal requeue} {external requeue} A corresponding definition of target object applies to a requeue_statement (see 9.5.4), with a corresponding distinction between an internal requeue and an external requeue.

Legality Rules

7.1/2

{AI95-00345-01} The view of the target protected object associated with a call of a protected procedure or entry shall be a variable.

Dynamic Semantics

Within the body of a protected operation, the current instance (see 8.6) of the immediately enclosing protected unit is determined by the target object specified (implicitly or explicitly) in the call (or requeue) on the protected operation.

8.a

To be honest: The current instance is defined in the same way within the body of a subprogram declared immediately within a protected_body.

Any call on a protected procedure or entry of a target protected object is defined to be an update to the object, as is a requeue on such an entry.

9.a

Reason: Read/write access to the components of a protected object is granted while inside the body of a protected procedure or entry. Also, any protected entry call can change the value of the Count attribute, which represents an update. Any protected procedure call can result in servicing the entries, which again might change the value of a Count attribute.

Wording Changes from Ada 95

9.b/2

{AI95-00345-01} Added a Legality Rule to make it crystal-clear that the protected object of an entry or procedure call must be a variable. This rule was implied by the Dynamic Semantics here, along with the Static Semantics of 3.3, but it is much better to explicitly say it. While many implementations have gotten this wrong, this is not an incompatibility — allowing updates of protected constants has always been wrong.