9.7.2 Timed Entry Calls
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[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 might be implemented by an entry. {time-out:
See timed_entry_call} ]
Syntax
2
timed_entry_call ::=
select
entry_call_alternative
or
delay_alternative
end select;
3/2
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entry_call_alternative ::=
procedure_or_entry_call entry_call_statement [
sequence_of_statements]
3.1/2
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procedure_or_entry_call ::=
procedure_call_statement | entry_call_statementLegality Rules
3.2/2
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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). 3.a/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.
3.b/2
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.
Static Semantics
3.3/2
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If a procedure_call_statement
is used for a procedure_or_entry_call, and
the procedure is implemented by an entry, then the procedure_name,
or procedure_prefix and possibly the
first parameter of the procedure_call_statement,
determine the target object of the call and the entry to be called.
3.c/2
Discussion: The
above says “possibly the first parameter”, because Ada allows
entries to be renamed and passed as formal subprograms. In those cases,
the task or protected object is implicit in the name of the routine;
otherwise the object is an explicit parameter to the call.
Dynamic Semantics
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{execution (timed_entry_call) [partial]}
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.5
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.
5.a/2
This paragraph
was deleted.Ramification: {
AI-00345-01}
The fact that the syntax calls for an entry_call_statement
means that this fact is used in overload resolution. For example, if
there is a procedure X and an entry X (both with no parameters), then
"select X; ..." is legal, because overload resolution knows
that the entry is the one that was meant.
Examples
6
Example of a timed
entry call:
7
select
Controller.Request(Medium)(Some_Item);
or
delay 45.0;
-- controller too busy, try something else
end select;
Wording Changes from Ada 83
7.a
This clause 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
7.b/2
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{incompatibilities with Ada 95} A
procedure can be used as the 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.
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