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D.1 Task Priorities
This clause specifies the priority model for real-time
systems. In addition, the methods for specifying priorities are defined.
The form of
a pragma Priority is as follows:
The form of
a pragma Interrupt_Priority is as
Name Resolution Rules
type for the expression
in a Priority
or Interrupt_Priority pragma is Integer.
A Priority pragma is allowed only immediately
within a task_definition, a protected_definition,
or the declarative_part of a subprogram_body.
An Interrupt_Priority pragma is allowed only immediately within a task_definition
or a protected_definition. At most
one such pragma shall appear within a given construct.
For a Priority pragma that appears in the declarative_part
of a subprogram_body, the expression
shall be static, and its value shall be in the range of System.Priority.
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.
The priority specified by a Priority or Interrupt_Priority
pragma is the value of the expression
in the pragma, if any. If there is no expression
in an Interrupt_Priority pragma, the priority value is Interrupt_Priority'Last.
A Priority pragma has no effect if it occurs
in the declarative_part of the subprogram_body
of a subprogram other than the main subprogram.
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.
The effect of specifying such a pragma in a protected_definition
is discussed in D.3
in a Priority or Interrupt_Priority pragma that appears in a task_definition
is evaluated for each task object (see 9.1
For a Priority pragma, the value of the expression
is converted to the subtype Priority; for an Interrupt_Priority pragma,
this value is converted to the subtype Any_Priority. The priority value
is then associated with the task object whose task_definition
contains the pragma.
Likewise, the priority value is associated with
the environment task if the pragma appears in the declarative_part
of 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
pragma. 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 a pragma is the
base priority of the task that creates it at the time of creation (see
). If a pragma Priority does not apply to
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 always a source of priority inheritance. 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 and D.3).
In all of these cases, the priority ceases to
be inherited as soon as the condition calling for the inheritance no
The range of System.Interrupt_Priority shall
include at least one value.
The range of System.Priority shall include at
least 30 values.
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
6 An implementation may
provide a non-standard mode in which tasks inherit priorities under conditions
other than those specified above.
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