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!standard D.7 (00)          02-09-02 AI95-00305/02
!class amendment 02-07-17
!status work item 02-07-17
!status received 02-07-17
!priority High
!difficulty Medium
!subject New pragma and additional restriction identifiers for Real-Time Systems
!summary
A new pragma and restrictions are proposed to enhance the ability to create highly-efficient and predictable tasking runtime systems.
!problem
Experience constructing the "highly efficient tasking runtime systems" of D.7 has shown that the set of restrictions is insufficient. In particular, the Ravenscar Profile is commonly used in Safety-Critical and High-Integrity applications to provide a highly efficient tasking runtime. However, the profile requires restrictions beyond those defined by the Standard, requiring users to fall back on vendor-defined extensions.
!proposal
This amendment introduces several new restriction identifiers to define runtime behaviors that are to be restricted when using the Ravenscar profile. These identifiers may be used to specify runtime behavior which is independent of the Ravenscar definition.
A new pragma is also defined to force an implementation to detect blocking within a protected operation.
!wording
The following new static restriction_identifiers are defined and inserted after D.7(10):
No_Calendar
There are no semantic dependencies on package Ada.Calendar.
No_Dynamic_Attachment
There is no call to any of the operations defined in package Ada.Interrupts (Is_Reserved, Is_Attached, Current_Handler, Attach_Handler, Exchange_Handler, Detach_Handler, Reference).
No_Local_Protected_Objects
All protected objects are created via library-level declarations.
No_Protected_Type_Allocators
There are no allocators for protected types or types containing protected type components.
No_Relative_Delay
Delay_relative statements are not allowed.
No_Requeue_Statements
Requeue statements are not allowed.
No_Select_Statements
Select_statements are not allowed.
No_Task_Attributes_Package
There are no semantic dependencies on package Ada.Task_Attributes.
Simple_Barriers
The Boolean expression in an entry barrier shall be either a Boolean literal value or the value of a Boolean component of the enclosing protected object.
The following new dynamic restrictions_identifier is defined and replaces D.7(15/1):
No_Task_Termination
All tasks are non-terminating. An implementation must define what happens if a task attempts to terminate.
The following new dynamic restrictions_parameter_identifier is defined and inserted after D.7(19/1):
Max_Entry_Queue_Length
Max_Entry_Queue_Length defines the maximum number of calls that are queued on an entry. Violation of this restriction results in the raising of Program_Error exception at the point of the call.
The following pragma is defined as a new subsection H.5:
H.5 Pragma Detect_Blocking
The following pragma forces an implementation to detect potentially blocking operations within a protected operation.
Syntax The form of a pragma Detect_Blocking is as follows: pragma Detect_Blocking;
Static Semantics The pragma is a configuration pragma.
Post-Compilation Rules An implementation is required to detect a potentially blocking operation within a protected operation, and to raise Program_Error as required under 9.5.1(17).
Implementation Permissions An operation that causes a task to be blocked within a foreign language domain is not defined to be potentially blocking, and may not be detected.
!example
!discussion
a) The pragma No_Task_Hierarchy must impose the constraint that all tasks depend directly on the Environment task as a result of all task objects being created by library level declarations. The restriction means that no support is needed for "masters" and "waiting for dependent tasks". This also matches the semantics for pragma No_Local_Protected_Objects.
b) No_Protected_Type_Allocators matches the existing restriction No_Task_Allocators.
c) No_Delay in H.4 is too strong for the Ravenscar Profile since we want to allow delay_until Ada.Real_Time.Time, but not relative delay (non- deterministic) nor package Ada.Calendar (too coarse).
d) No_Select_Statements excludes selective_accept, timed and conditional (protected) entry calls, and asynchronous_select.
e) Some restriction identifiers concerning tasking are actually defined in H.4. For reasons of minimum change it is not proposed to move them.
!ACATS test
!appendix

Editor's Note: This AI was split out of the Ravenscar AI, AI-249.

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