9.7.4 Asynchronous Transfer of Control
An asynchronous select_statement provides asynchronous transfer of control upon completion of an entry call or the expiration of a delay.
asynchronous_select ::= select triggering_alternative then abort abortable_part end select;
triggering_alternative ::= triggering_statement [sequence_of_statements]
triggering_statement ::= entry_call_statement | delay_statement
abortable_part ::= sequence_of_statements
For the execution of an asynchronous_select whose triggering_statement is an entry_call_statement, the entry_name and actual parameters are evaluated as for a simple entry call (see 9.5.3), and the entry call is issued. If the entry call is queued (or requeued-with-abort), then the abortable_part is executed. If the entry call is selected immediately, and never requeued-with-abort, then the abortable_part is never started.
For the execution of an asynchronous_select whose triggering_statement is a delay_statement, the delay_expression is evaluated and the expiration time is determined, as for a normal delay_statement. If the expiration time has not already passed, the abortable_part is executed.
If the abortable_part completes and is left prior to completion of the triggering_statement, an attempt to cancel the triggering_statement is made. If the attempt to cancel succeeds (see 9.5.3 and 9.6), the asynchronous_select is complete.
If the triggering_statement completes other than due to cancellation, the abortable_part is aborted (if started but not yet completed -- see 9.8). If the triggering_statement completes normally, the optional sequence_of_statements of the triggering_alternative is executed after the abortable_part is left.
Example of a main command loop for a command interpreter:
loop select Terminal.Wait_For_Interrupt; Put_Line("Interrupted"); then abort -- This will be abandoned upon terminal interrupt Put_Line("-> "); Get_Line(Command, Last); Process_Command(Command(1..Last)); end select; end loop;
Example of a time-limited calculation:
select delay 5.0; Put_Line("Calculation does not converge"); then abort -- This calculation should finish in 5.0 seconds; -- if not, it is assumed to diverge. Horribly_Complicated_Recursive_Function(X, Y); end select;
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Copyright © 2000 The MITRE Corporation, Inc. Ada Reference Manual