D.1 Task Priorities
This subclause specifies the priority model for real-time
systems. In addition, the methods for specifying priorities are defined.
The aspect Priority is an expression
which shall be of type Integer.
The aspect Interrupt_Priority is an expression
which shall be of type Integer.
This paragraph was
If the Priority aspect is specified for a subprogram,
shall be static, and its value shall be in the range of System.Priority.
At most one of the Priority and Interrupt_Priority
aspects may be specified for a given entity.
Neither of the Priority or Interrupt_Priority aspects
shall be specified for a synchronized interface type.
The following declarations
exist in package System:
subtype Any_Priority is Integer range implementation-defined;
subtype Priority is Any_Priority
range Any_Priority'First .. implementation-defined;
subtype Interrupt_Priority is Any_Priority
range Priority'Last+1 .. Any_Priority'Last;
Default_Priority : constant Priority := (Priority'First + Priority'Last)/2;
The full range of priority values supported by an
implementation is specified by the subtype Any_Priority. The subrange
of priority values that are high enough to require the blocking of one
or more interrupts is specified by the subtype Interrupt_Priority. The
subrange of priority values below System.Interrupt_Priority'First is
specified by the subtype System.Priority.
This paragraph was
The Priority aspect has no effect if it is specified
for a subprogram other than the main subprogram; the Priority value is
not associated with any task.
is an integer value that indicates a degree of urgency
and is the basis for resolving competing demands of tasks for resources.
Unless otherwise specified, whenever tasks compete for processors or
other implementation-defined resources, the resources are allocated to
the task with the highest priority value. The base priority
a task is the priority with which it was created, or to which it was
later set by Dynamic_Priorities.Set_Priority (see D.5
At all times, a task also has an active priority
, which generally
reflects its base priority as well as any priority it inherits from other
sources. Priority inheritance
is the process by which the priority
of a task or other entity (e.g. a protected object; see D.3
is used in the evaluation of another task's active priority.
specified for the Priority or Interrupt_Priority aspect of a task type
is evaluated each time an object of the task type is created (see 9.1
For the Priority aspect, the value of the expression
is converted to the subtype Priority; for the Interrupt_Priority aspect,
this value is converted to the subtype Any_Priority. The priority value
is then associated with the task object.
Likewise, the priority value is associated with
the environment task if the aspect is specified for the main subprogram.
The initial value of a task's base priority is specified
by default or by means of a Priority or Interrupt_Priority aspect. After
a task is created, its base priority can be changed only by a call to
Dynamic_Priorities.Set_Priority (see D.5
The initial base priority of a task in the absence of an aspect is the
base priority of the task that creates it at the time of creation (see
). If the aspect Priority is not specified
for the main subprogram, the initial base priority of the environment
task is System.Default_Priority. The task's active priority is used when
the task competes for processors. Similarly, the task's active priority
is used to determine the task's position in any queue when Priority_Queuing
is specified (see D.4
At any time, the active
priority of a task is the maximum of all the priorities the task is inheriting
at that instant. For a task that is not held (see D.11
its base priority is a source of priority inheritance unless otherwise
specified for a particular task dispatching policy. Other sources of
priority inheritance are specified under the following conditions:
During activation, a task being activated inherits
the active priority that its activator (see 9.2
had at the time the activation was initiated.
During rendezvous, the task accepting the entry
call inherits the priority of the entry call (see 9.5.3
During a protected action on a protected object,
a task inherits the ceiling priority of the protected object (see 9.5
In all of these cases, the priority ceases to be
inherited as soon as the condition calling for the inheritance no longer
The range of System.Interrupt_Priority shall include
at least one value.
The range of System.Priority shall include at least
4 The priority expression can include references
to discriminants of the enclosing type.
5 It is a consequence of the active priority
rules that at the point when a task stops inheriting a priority from
another source, its active priority is re-evaluated. This is in addition
to other instances described in this Annex for such re-evaluation.
6 An implementation may provide a nonstandard
mode in which tasks inherit priorities under conditions other than those
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