Dynamic Priorities for Protected Objects
Author: JTC1/SC22/WG9/ARG, by Randall Brukardt, ARG Editor

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# D.5.2 Dynamic Priorities for Protected Objects

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This clause specifies how the priority of a protected object can be modified or queried at run time.

#### Static Semantics

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The following attribute is defined for a prefix P that denotes a protected object:
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P'Priority
Denotes a non-aliased component of the protected object P. This component is of type System.Any_Priority and its value is the priority of P. P'Priority denotes a variable if and only if P denotes a variable. A reference to this attribute shall appear only within the body of P.
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The initial value of this attribute is the initial value of the priority of the protected object, and can be changed by an assignment.

#### Dynamic Semantics

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If the locking policy Ceiling_Locking (see D.3) is in effect then the ceiling priority of a protected object P is set to the value of P'Priority at the end of each protected action of P.
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If the locking policy Ceiling_Locking is in effect, then for a protected object P with either an Attach_Handler or Interrupt_Handler pragma applying to one of its procedures, a check is made that the value to be assigned to P'Priority is in the range System.Interrupt_Priority. If the check fails, Program_Error is raised.

#### Metrics

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The implementation shall document the following metric:
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- The difference in execution time of calls to the following procedures in protected object P:

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protected P is
procedure Do_Not_Set_Ceiling (Pr : System.Any_Priority);
procedure Set_Ceiling (Pr : System.Any_Priority);
end P;
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protected body P is
procedure Do_Not_Set_Ceiling (Pr : System.Any_Priority) is
begin
null;
end;
procedure Set_Ceiling (Pr : System.Any_Priority) is
begin
P'Priority := Pr;
end;
end P;
NOTES
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31 Since P'Priority is a normal variable, the value following an assignment to the attribute immediately reflects the new value even though its impact on the ceiling priority of P is postponed until completion of the protected action in which it is executed.

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