Ada Reference Manual (Ada 2022)
9.7.3 Conditional Entry Calls
A conditional_entry_call issues an entry call that is then cancelled if it is not selected immediately (or if a requeue-with-abort of the call is not selected immediately). A procedure call may appear rather than an entry call for cases where the procedure can be implemented by an entry.
Syntax
Dynamic Semantics
The execution of a conditional_entry_call is defined to be equivalent to the execution of a timed_entry_call with a delay_alternative specifying an immediate expiration time and the same sequence_of_statements as given after the reserved word else.
NOTE A conditional_entry_call can briefly increase the Count attribute of the entry, even if the conditional call is not selected.
Examples
Example of a conditional entry call:
procedure Spin(R : in out Resource) is -- see 9.4
begin
loop
select
R.Seize;
return;
else
null; -- busy waiting
end select;
end loop;
end;
begin
loop
select
R.Seize;
return;
else
null; -- busy waiting
end select;
end loop;
end;
