Ada Reference Manual (Ada 2022) — Legal Information

# 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.

#### Syntax

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asynchronous_select ::=
select
triggering_alternative
then abort
abortable_part
end select;
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triggering_alternative ::= triggering_statement [sequence_of_statements]
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triggering_statement ::= procedure_or_entry_call | delay_statement
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abortable_part ::= sequence_of_statements

#### Dynamic Semantics

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For the execution of an asynchronous_select whose triggering_statement is a procedure_or_entry_call, the entry_name, procedure_name, or procedure_prefix, and actual parameters are evaluated as for a simple entry call (see 9.5.3) or procedure call (see 6.4). If the call is an entry call or a call on a procedure implemented by an entry, 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. If the call is on a procedure that is not implemented by an entry, the call proceeds as described in 6.4, followed by the sequence_of_statements of the triggering_alternative; the abortable_part is never started.
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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.
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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.
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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.

#### Examples

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Example of a main command loop for a command interpreter:
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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;
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Example of a time-limited calculation:
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select
delay 5.0;
Put_Line("Calculation does not converge");
then abort
-- This calculation is expected to finish in 5.0 seconds;
-- if not, it is assumed to diverge.
Horribly_Complicated_Recursive_Function(X, Y);
end select;
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Note that these examples presume that there are abort completion points (see 9.8) within the execution of the abortable_part.

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