Timed Entry Calls

{AI95-00345-01} {AI12-0439-1} [A timed_entry_call issues an entry call that is cancelled if the call (or a requeue-with-abort of the call) is not selected before the expiration time is reached. A procedure call may appear rather than an entry call for cases where the procedure can be implemented by an entry. ]

Syntax

Legality Rules

{AI95-00345-01} If a procedure_call_statement is used for a procedure_or_entry_call, the procedure_name or procedure_prefix of the procedure_call_statement shall statically denote an entry renamed as a procedure or (a view of) a primitive subprogram of a limited interface whose first parameter is a controlling parameter (see 3.9.2).

Reason: This would be a confusing way to call a procedure, so we only allow it when it is possible that the procedure is actually an entry. We could have allowed formal subprograms here, but we didn't because we'd have to allow all formal subprograms, and it would increase the difficulty of generic code sharing.

We say “statically denotes” because an access-to-subprogram cannot be primitive, and we don't have anything like access-to-entry. So only names of entries or procedures are possible.

Dynamic Semantics

{AI95-00345-01} For the execution of a timed_entry_call, the entry_name, procedure_name, or procedure_prefix, and any actual parameters are evaluated, as for a simple entry call (see 9.5.3) or procedure call (see 6.4). The expiration time (see 9.6) for the call is determined by evaluating the delay_expression of the delay_alternative. If the call is an entry call or a call on a procedure implemented by an entry, the entry call is then issued. Otherwise, the call proceeds as described in 6.4 for a procedure call, followed by the sequence_of_statements of the entry_call_alternative; the sequence_of_statements of the delay_alternative is ignored.

If the call is queued (including due to a requeue-with-abort), and not selected before the expiration time is reached, an attempt to cancel the call is made. If the call completes due to the cancellation, the optional sequence_of_statements of the delay_alternative is executed; if the entry call completes normally, the optional sequence_of_statements of the entry_call_alternative is executed.

Examples

Example of a timed entry call:

select
   Controller.Request(Medium)(Some_Item);
or
   delay 45.0;
   --  controller too busy, try something else
end select;

Wording Changes from Ada 83

{AI05-0299-1} This subclause comes before the one for Conditional Entry Calls, so we can define conditional entry calls in terms of timed entry calls.

Incompatibilities With Ada 95

{AI95-00345-01} {AI05-0005-1} A procedure call can be used as the entry_call_alternative in a timed or conditional entry call, if the procedure might actually be an entry. Since the fact that something is an entry could be used in resolving these calls in Ada 95, it is possible for timed or conditional entry calls that resolved in Ada 95 to be ambiguous in Ada 2005. That could happen if both an entry and procedure with the same name and profile exist, which should be rare.