9.7.4 Asynchronous Transfer of Control

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An asynchronous select_statement provides asynchronous transfer of control upon completion of an entry call or the expiration of a delay.


asynchronous_select ::=
    then abort
    end select;

triggering_alternative ::= triggering_statement [sequence_of_statements]

triggering_statement ::= entry_call_statement | delay_statement

abortable_part ::= sequence_of_statements

Dynamic Semantics

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:

    then abort 
        -- This will be abandoned upon terminal interrupt
        Put_Line("-> ");
        Get_Line(Command, Last);
    end select;
end loop;

Example of a time-limited calculation:

    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