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9.9 Elaboration Circularities ¶

An `elaboration circularity' occurs whenever the elaboration of a set of units enters a deadlocked state, where each unit is waiting for another unit to be elaborated. This situation may be the result of improper use of `with' clauses, elaboration-control pragmas, or invocations in elaboration code.

The following example exhibits an elaboration circularity.

with B; pragma Elaborate (B);
package A is
end A;

package B is
   procedure Force_Body;
end B;

with C;
package body B is
   procedure Force_Body is null;

   Elab : constant Integer := C.Func;
end B;

package C is
   function Func return Integer;
end C;

with A;
package body C is
   function Func return Integer is
   begin
      ...
   end Func;
end C;

The binder emits the following diagnostic:

error: Elaboration circularity detected
info:
info:    Reason:
info:
info:      unit "a (spec)" depends on its own elaboration
info:
info:    Circularity:
info:
info:      unit "a (spec)" has with clause and pragma Elaborate for unit "b (spec)"
info:      unit "b (body)" is in the closure of pragma Elaborate
info:      unit "b (body)" invokes a construct of unit "c (body)" at elaboration time
info:      unit "c (body)" has with clause for unit "a (spec)"
info:
info:    Suggestions:
info:
info:      remove pragma Elaborate for unit "b (body)" in unit "a (spec)"
info:      use the dynamic elaboration model (compiler switch -gnatE)

The diagnostic consist of the following sections:

• Reason

  This section provides a short explanation describing why the set of units could not be ordered.

• Circularity

  This section enumerates the units comprising the deadlocked set, along with their interdependencies.

• Suggestions

  This section enumerates various tactics for eliminating the circularity.