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9.7.3 Conditional Entry Calls

1
   [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).]
1.a
To be honest: In the case of an entry call on a protected object, it is OK if the entry is closed at the start of the corresponding protected action, so long as it opens and the call is selected before the end of that protected action (due to changes in the Count attribute).

Syntax

2
conditional_entry_call ::=
  select
   entry_call_alternative
  else
   sequence_of_statements
  end select;

Dynamic Semantics

3
   {execution (conditional_entry_call) [partial]} 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.
NOTES
4
37  A conditional_entry_call may briefly increase the Count attribute of the entry, even if the conditional call is not selected.

Examples

5
   Example of a conditional entry call:
6
procedure Spin(R : in Resource) is
begin
   loop
      select
         R.Seize;
         return;
      else
         null;  --  busy waiting
      end select;
   end loop;
end;

Wording Changes from Ada 83

6.a
This clause comes after the one for Timed Entry Calls, so we can define conditional entry calls in terms of timed entry calls. We do that so that an "expiration time" is defined for both, thereby simplifying the definition of what happens on a requeue-with-abort.

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