9.7.1 Selective Accept
[This form of the
select_statement
allows a combination of waiting for, and selecting from, one or more
alternatives. The selection may depend on conditions associated with
each alternative of the
selective_accept.
{time-out: See selective_accept}
]
Syntax
terminate_alternative ::= terminate;
{else
part (of a selective_accept)} an
else
part (the reserved word
else followed by a
sequence_of_statements).
These three possibilities are mutually exclusive.
Legality Rules
Reason: This simplifies the implementation
and the description of the semantics.
Dynamic Semantics
{open alternative}
A
select_alternative
is said to be
open if it is not immediately preceded by a
guard,
or if the
condition
of its
guard
evaluates to True. It is said to be
closed otherwise.
{execution (selective_accept)
[partial]} For the execution of a
selective_accept,
any
guard
conditions
are evaluated; open alternatives are thus determined. For an open
delay_alternative,
the
delay_expression
is also evaluated. Similarly, for an open
accept_alternative
for an entry of a family, the
entry_index
is also evaluated. These evaluations are performed in an arbitrary order,
except that a
delay_expression
or
entry_index
is not evaluated until after evaluating the corresponding
condition,
if any. Selection and execution of one open alternative, or of the else
part, then completes the execution of the
selective_accept;
the rules for this selection are described below.
Open
accept_alternatives
are first considered. Selection of one such alternative takes place immediately
if the corresponding entry already has queued calls. If several alternatives
can thus be selected, one of them is selected according to the entry
queuing policy in effect (see
9.5.3 and
D.4).
When such an alternative is selected, the selected call is removed from
its entry queue and the
handled_sequence_of_statements
(if any) of the corresponding
accept_statement
is executed; after the rendezvous completes any subsequent
sequence_of_statements
of the alternative is executed.
{blocked
(execution of a selective_accept) [partial]} If
no selection is immediately possible (in the above sense) and there is
no else part, the task blocks until an open alternative can be selected.
Selection of the other
forms of alternative or of an else part is performed as follows:
Ramification: In the absence of a
requeue_statement,
the conditions stated are such that a
terminate_alternative
cannot be selected while there is a queued entry call for any entry of
the task. In the presence of requeues from a task to one of its subtasks,
it is possible that when a
terminate_alternative
of the subtask is selected, requeued calls (for closed entries only)
might still be queued on some entry of the subtask. Tasking_Error will
be propagated to such callers, as is usual when a task completes while
queued callers remain.
{Program_Error (raised
by failure of run-time check)} The exception
Program_Error is raised if all alternatives are closed and there is no
else part.
Examples
Example of a task
body with a selective accept:
task body Server is
Current_Work_Item : Work_Item;
begin
loop
select
accept Next_Work_Item(WI : in Work_Item) do
Current_Work_Item := WI;
end;
Process_Work_Item(Current_Work_Item);
or
accept Shut_Down;
exit; -- Premature shut down requested
or
terminate; -- Normal shutdown at end of scope
end select;
end loop;
end Server;
Wording Changes from Ada 83
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