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C.3.2 The Package Interrupts

Static Semantics

1
The following language-defined packages exist: 
2
with System;
package Ada.Interrupts is
   type Interrupt_ID is implementation-defined;
   type Parameterless_Handler is
      access protected procedure;
3/1
This paragraph was deleted. 
4
   function Is_Reserved (Interrupt : Interrupt_ID)
      return Boolean;
5
   function Is_Attached (Interrupt : Interrupt_ID)
      return Boolean;
6
   function Current_Handler (Interrupt : Interrupt_ID)
      return Parameterless_Handler;
7
   procedure Attach_Handler
      (New_Handler : in Parameterless_Handler;
       Interrupt   : in Interrupt_ID);
8
   procedure Exchange_Handler
      (Old_Handler : out Parameterless_Handler;
       New_Handler : in Parameterless_Handler;
       Interrupt   : in Interrupt_ID);
9
   procedure Detach_Handler
      (Interrupt : in Interrupt_ID);
10
   function Reference(Interrupt : Interrupt_ID)
      return System.Address;
11
private
   ... -- not specified by the language
end Ada.Interrupts;
12
package Ada.Interrupts.Names is
   implementation-defined : constant Interrupt_ID :=
     implementation-defined;
      . . .
   implementation-defined : constant Interrupt_ID :=
     implementation-defined;
end Ada.Interrupts.Names;

Dynamic Semantics

13
The Interrupt_ID type is an implementation-defined discrete type used to identify interrupts.
14
The Is_Reserved function returns True if and only if the specified interrupt is reserved.
15
The Is_Attached function returns True if and only if a user-specified interrupt handler is attached to the interrupt.
16/1
 {8652/0069} {AI95-00166-01} The Current_Handler function returns a value that represents the attached handler of the interrupt. If no user-defined handler is attached to the interrupt, Current_Handler returns null a value that designates the default treatment; calling Attach_Handler or Exchange_Handler with this value restores the default treatment.
17
The Attach_Handler procedure attaches the specified handler to the interrupt, overriding any existing treatment (including a user handler) in effect for that interrupt. If New_Handler is null, the default treatment is restored. {Program_Error (raised by failure of run-time check)} If New_Handler designates a protected procedure to which the pragma Interrupt_Handler does not apply, Program_Error is raised. In this case, the operation does not modify the existing interrupt treatment.
18/1
 {8652/0069} {AI95-00166-01} The Exchange_Handler procedure operates in the same manner as Attach_Handler with the addition that the value returned in Old_Handler designates the previous treatment for the specified interrupt. If the previous treatment is not a user-defined handler, null is returned. 
18.a
Ramification: Calling Attach_Handler or Exchange_Handler with this value for New_Handler restores the previous handler.
18.a.1/1
{8652/0069} {AI95-00166-01} If the application uses only parameterless procedures as handlers (other types of handlers may be provided by the implementation, but are not required by the standard), then if Old_Handler is not null, it may be called to execute the previous handler. This provides a way to cascade application interrupt handlers. However, the default handler cannot be cascaded this way (Old_Handler must be null for the default handler). 
19
The Detach_Handler procedure restores the default treatment for the specified interrupt.
20
For all operations defined in this package that take a parameter of type Interrupt_ID, with the exception of Is_Reserved and Reference, a check is made that the specified interrupt is not reserved. {Program_Error (raised by failure of run-time check)} Program_Error is raised if this check fails.
21
If, by using the Attach_Handler, Detach_Handler, or Exchange_Handler procedures, an attempt is made to detach a handler that was attached statically (using the pragma Attach_Handler), the handler is not detached and Program_Error is raised. {Program_Error (raised by failure of run-time check)}
22/2
 {AI95-00434-01} The Reference function returns a value of type System.Address that can be used to attach a task entry, via an address clause (see J.7.1) to the interrupt specified by Interrupt. This function raises Program_Error if attaching task entries to interrupts (or to this particular interrupt) is not supported. {Program_Error (raised by failure of run-time check)}

Implementation Requirements

23
At no time during attachment or exchange of handlers shall the current handler of the corresponding interrupt be undefined.

Documentation Requirements

24/2
 {AI95-00434-01} If the Ceiling_Locking policy (see D.3) is in effect, the implementation shall document the default ceiling priority assigned to a protected object that contains either the Attach_Handler or Interrupt_Handler pragmas, but not the Interrupt_Priority pragma. [This default need not be the same for all interrupts.] 
24.a/2
Documentation Requirement: If the Ceiling_Locking policy is in effect, the default ceiling priority for a protected object that contains an interrupt handler pragma.

Implementation Advice

25
If implementation-defined forms of interrupt handler procedures are supported, such as protected procedures with parameters, then for each such form of a handler, a type analogous to Parameterless_Handler should be specified in a child package of Interrupts, with the same operations as in the predefined package Interrupts.
25.a/2
Implementation Advice: If implementation-defined forms of interrupt handler procedures are supported, then for each such form of a handler, a type analogous to Parameterless_Handler should be specified in a child package of Interrupts, with the same operations as in the predefined package Interrupts.
NOTES
26
8  The package Interrupts.Names contains implementation-defined names (and constant values) for the interrupts that are supported by the implementation.

Examples

27
Example of interrupt handlers: 
28
Device_Priority : constant
  array (1..5) of System.Interrupt_Priority := ( ... );
protected type Device_Interface
  (Int_ID : Ada.Interrupts.Interrupt_ID) is
  procedure Handler;
  pragma Attach_Handler(Handler, Int_ID);
  ...
  pragma Interrupt_Priority(Device_Priority(Int_ID));
end Device_Interface;
  ...
Device_1_Driver : Device_Interface(1);
  ...
Device_5_Driver : Device_Interface(5);
  ...

Wording Changes from Ada 95

28.a/2
{8652/0069} {AI95-00166-01} Corrigendum: Clarified that the value returned by Current_Handler and Exchange_Handler for the default treatment is null. 

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