source: rtems-docs/c_user/task_manager.rst @ 11e1a6f

Task Manager
############

.. index:: tasks

Introduction
============

The task manager provides a comprehensive set of directives to
create, delete, and administer tasks.  The directives provided
by the task manager are:

- ``rtems_task_create`` - Create a task

- ``rtems_task_ident`` - Get ID of a task

- ``rtems_task_self`` - Obtain ID of caller

- ``rtems_task_start`` - Start a task

- ``rtems_task_restart`` - Restart a task

- ``rtems_task_delete`` - Delete a task

- ``rtems_task_suspend`` - Suspend a task

- ``rtems_task_resume`` - Resume a task

- ``rtems_task_is_suspended`` - Determine if a task is suspended

- ``rtems_task_set_priority`` - Set task priority

- ``rtems_task_mode`` - Change current task’s mode

- ``rtems_task_wake_after`` - Wake up after interval

- ``rtems_task_wake_when`` - Wake up when specified

- ``rtems_iterate_over_all_threads`` - Iterate Over Tasks

- ``rtems_task_variable_add`` - Associate per task variable

- ``rtems_task_variable_get`` - Obtain value of a a per task variable

- ``rtems_task_variable_delete`` - Remove per task variable

Background
==========

Task Definition
---------------
.. index:: task, definition

Many definitions of a task have been proposed in computer literature.
Unfortunately, none of these definitions encompasses all facets of the
concept in a manner which is operating system independent.  Several of the
more common definitions are provided to enable each user to select a
definition which best matches their own experience and understanding of the
task concept:

- a "dispatchable" unit.

- an entity to which the processor is allocated.

- an atomic unit of a real-time, multiprocessor system.

- single threads of execution which concurrently compete for resources.

- a sequence of closely related computations which can execute
  concurrently with other computational sequences.

From RTEMS’ perspective, a task is the smallest thread of
execution which can compete on its own for system resources.  A
task is manifested by the existence of a task control block
(TCB).

Task Control Block
------------------

The Task Control Block (TCB) is an RTEMS defined data structure
which contains all the information that is pertinent to the
execution of a task.  During system initialization, RTEMS
reserves a TCB for each task configured.  A TCB is allocated
upon creation of the task and is returned to the TCB free list
upon deletion of the task.

The TCB’s elements are modified as a result of system calls made
by the application in response to external and internal stimuli.
TCBs are the only RTEMS internal data structure that can be
accessed by an application via user extension routines.  The TCB
contains a task’s name, ID, current priority, current and
starting states, execution mode, TCB user extension pointer,
scheduling control structures, as well as data required by a
blocked task.

A task’s context is stored in the TCB when a task switch occurs.
When the task regains control of the processor, its context is
restored from the TCB.  When a task is restarted, the initial
state of the task is restored from the starting context area in
the task’s TCB.

Task States
-----------
.. index:: task states

A task may exist in one of the following five states:

- *executing* - Currently scheduled to the CPU

- *ready* - May be scheduled to the CPU

- *blocked* - Unable to be scheduled to the CPU

- *dormant* - Created task that is not started

- *non-existent* - Uncreated or deleted task

An active task may occupy the executing, ready, blocked or
dormant state, otherwise the task is considered non-existent.
One or more tasks may be active in the system simultaneously.
Multiple tasks communicate, synchronize, and compete for system
resources with each other via system calls.  The multiple tasks
appear to execute in parallel, but actually each is dispatched
to the CPU for periods of time determined by the RTEMS
scheduling algorithm.  The scheduling of a task is based on its
current state and priority.

Task Priority
-------------
.. index:: task priority
.. index:: priority, task
.. index:: rtems_task_priority

A task’s priority determines its importance in relation to the
other tasks executing on the same processor.  RTEMS supports 255
levels of priority ranging from 1 to 255.  The data type``rtems_task_priority`` is used to store task
priorities.

Tasks of numerically
smaller priority values are more important tasks than tasks of
numerically larger priority values.  For example, a task at
priority level 5 is of higher privilege than a task at priority
level 10.  There is no limit to the number of tasks assigned to
the same priority.

Each task has a priority associated with it at all times.  The
initial value of this priority is assigned at task creation
time.  The priority of a task may be changed at any subsequent
time.

Priorities are used by the scheduler to determine which ready
task will be allowed to execute.  In general, the higher the
logical priority of a task, the more likely it is to receive
processor execution time.

Task Mode
---------
.. index:: task mode
.. index:: rtems_task_mode

A task’s execution mode is a combination of the following
four components:

- preemption

- ASR processing

- timeslicing

- interrupt level

It is used to modify RTEMS’ scheduling process and to alter the
execution environment of the task.  The data type``rtems_task_mode`` is used to manage the task
execution mode... index:: preemption

The preemption component allows a task to determine when control of the
processor is relinquished.  If preemption is disabled
(``RTEMS_NO_PREEMPT``), the task will retain control of the
processor as long as it is in the executing state – even if a higher
priority task is made ready.  If preemption is enabled
(``RTEMS_PREEMPT``)  and a higher priority task is made ready,
then the processor will be taken away from the current task immediately and
given to the higher priority task... index:: timeslicing

The timeslicing component is used by the RTEMS scheduler to determine how
the processor is allocated to tasks of equal priority.  If timeslicing is
enabled (``RTEMS_TIMESLICE``), then RTEMS will limit the amount
of time the task can execute before the processor is allocated to another
ready task of equal priority. The length of the timeslice is application
dependent and specified in the Configuration Table.  If timeslicing is
disabled (``RTEMS_NO_TIMESLICE``), then the task will be
allowed to execute until a task of higher priority is made ready.  If``RTEMS_NO_PREEMPT`` is selected, then the timeslicing
component is ignored by the scheduler.

The asynchronous signal processing component is used to determine when
received signals are to be processed by the task.
If signal processing is enabled (``RTEMS_ASR``), then signals
sent to the task will be processed the next time the task executes.  If
signal processing is disabled (``RTEMS_NO_ASR``), then all
signals received by the task will remain posted until signal processing is
enabled.  This component affects only tasks which have established a
routine to process asynchronous signals... index:: interrupt level, task

The interrupt level component is used to determine which
interrupts will be enabled when the task is executing.``RTEMS_INTERRUPT_LEVEL(n)``
specifies that the task will execute at interrupt level n.

- ``RTEMS_PREEMPT`` - enable preemption (default)

- ``RTEMS_NO_PREEMPT`` - disable preemption

- ``RTEMS_NO_TIMESLICE`` - disable timeslicing (default)

- ``RTEMS_TIMESLICE`` - enable timeslicing

- ``RTEMS_ASR`` - enable ASR processing (default)

- ``RTEMS_NO_ASR`` - disable ASR processing

- ``RTEMS_INTERRUPT_LEVEL(0)`` - enable all interrupts (default)

- ``RTEMS_INTERRUPT_LEVEL(n)`` - execute at interrupt level n

The set of default modes may be selected by specifying the``RTEMS_DEFAULT_MODES`` constant.

Accessing Task Arguments
------------------------
.. index:: task arguments
.. index:: task prototype

All RTEMS tasks are invoked with a single argument which is
specified when they are started or restarted.  The argument is
commonly used to communicate startup information to the task.
The simplest manner in which to define a task which accesses it
argument is:.. index:: rtems_task

.. code:: c

    rtems_task user_task(
    rtems_task_argument argument
    );

Application tasks requiring more information may view this
single argument as an index into an array of parameter blocks.

Floating Point Considerations
-----------------------------
.. index:: floating point

Creating a task with the ``RTEMS_FLOATING_POINT`` attribute
flag results
in additional memory being allocated for the TCB to store the state of the
numeric coprocessor during task switches.  This additional memory is*NOT* allocated for ``RTEMS_NO_FLOATING_POINT`` tasks. Saving
and restoring the context of a ``RTEMS_FLOATING_POINT`` task
takes longer than that of a ``RTEMS_NO_FLOATING_POINT`` task
because of the relatively large amount of time required for the numeric
coprocessor to save or restore its computational state.

Since RTEMS was designed specifically for embedded military applications
which are floating point intensive, the executive is optimized to avoid
unnecessarily saving and restoring the state of the numeric coprocessor.
The state of the numeric coprocessor is only saved when a``RTEMS_FLOATING_POINT`` task is dispatched and that task was
not the last task to utilize the coprocessor.  In a system with only one``RTEMS_FLOATING_POINT`` task, the state of the numeric
coprocessor will never be saved or restored.

Although the overhead imposed by ``RTEMS_FLOATING_POINT`` tasks
is minimal, some applications may wish to completely avoid the overhead
associated with ``RTEMS_FLOATING_POINT`` tasks and still
utilize a numeric coprocessor.  By preventing a task from being preempted
while performing a sequence of floating point operations, a``RTEMS_NO_FLOATING_POINT`` task can utilize the numeric
coprocessor without incurring the overhead of a``RTEMS_FLOATING_POINT`` context switch.  This approach also
avoids the allocation of a floating point context area.  However, if this
approach is taken by the application designer, NO tasks should be created
as ``RTEMS_FLOATING_POINT`` tasks.  Otherwise, the floating
point context will not be correctly maintained because RTEMS assumes that
the state of the numeric coprocessor will not be altered by``RTEMS_NO_FLOATING_POINT`` tasks.

If the supported processor type does not have hardware floating
capabilities or a standard numeric coprocessor, RTEMS will not provide
built-in support for hardware floating point on that processor.  In this
case, all tasks are considered ``RTEMS_NO_FLOATING_POINT``
whether created as ``RTEMS_FLOATING_POINT`` or``RTEMS_NO_FLOATING_POINT`` tasks.  A floating point emulation
software library must be utilized for floating point operations.

On some processors, it is possible to disable the floating point unit
dynamically.  If this capability is supported by the target processor, then
RTEMS will utilize this capability to enable the floating point unit only
for tasks which are created with the ``RTEMS_FLOATING_POINT``
attribute.  The consequence of a ``RTEMS_NO_FLOATING_POINT``
task attempting to access the floating point unit is CPU dependent but will
generally result in an exception condition.

Per Task Variables
------------------
.. index:: per task variables

Per task variables are deprecated, see the warning below.

Per task variables are used to support global variables whose value
may be unique to a task. After indicating that a variable should be
treated as private (i.e. per-task) the task can access and modify the
variable, but the modifications will not appear to other tasks, and
other tasks’ modifications to that variable will not affect the value
seen by the task.  This is accomplished by saving and restoring the
variable’s value each time a task switch occurs to or from the calling task.

The value seen by other tasks, including those which have not added the
variable to their set and are thus accessing the variable as a common
location shared among tasks, cannot be affected by a task once it has
added a variable to its local set.  Changes made to the variable by
other tasks will not affect the value seen by a task which has added the
variable to its private set.

This feature can be used when a routine is to be spawned repeatedly as
several independent tasks.  Although each task will have its own stack,
and thus separate stack variables, they will all share the same static and
global variables.  To make a variable not shareable (i.e. a "global" variable
that is specific to a single task), the tasks can call``rtems_task_variable_add`` to make a separate copy of the variable
for each task, but all at the same physical address.

Task variables increase the context switch time to and from the
tasks that own them so it is desirable to minimize the number of
task variables.  One efficient method is to have a single task
variable that is a pointer to a dynamically allocated structure
containing the task’s private "global" data.

A critical point with per-task variables is that each task must separately
request that the same global variable is per-task private.

*WARNING*: Per-Task variables are inherently broken on SMP systems. They
only work correctly when there is one task executing in the system and
that task is the logical owner of the value in the per-task variable’s
location. There is no way for a single memory image to contain the
correct value for each task executing on each core. Consequently,
per-task variables are disabled in SMP configurations of RTEMS.
Instead the application developer should
consider the use of POSIX Keys or Thread Local Storage (TLS). POSIX Keys
are not enabled in all RTEMS configurations.

Building a Task Attribute Set
-----------------------------
.. index:: task attributes, building

In general, an attribute set is built by a bitwise OR of the
desired components.  The set of valid task attribute components
is listed below:

- ``RTEMS_NO_FLOATING_POINT`` - does not use coprocessor (default)

- ``RTEMS_FLOATING_POINT`` - uses numeric coprocessor

- ``RTEMS_LOCAL`` - local task (default)

- ``RTEMS_GLOBAL`` - global task

Attribute values are specifically designed to be mutually
exclusive, therefore bitwise OR and addition operations are
equivalent as long as each attribute appears exactly once in the
component list.  A component listed as a default is not required
to appear in the component list, although it is a good
programming practice to specify default components.  If all
defaults are desired, then ``RTEMS_DEFAULT_ATTRIBUTES`` should be used.

This example demonstrates the attribute_set parameter needed to
create a local task which utilizes the numeric coprocessor.  The
attribute_set parameter could be ``RTEMS_FLOATING_POINT`` or``RTEMS_LOCAL | RTEMS_FLOATING_POINT``.
The attribute_set parameter can be set to``RTEMS_FLOATING_POINT`` because ``RTEMS_LOCAL`` is the default for all created
tasks.  If the task were global and used the numeric
coprocessor, then the attribute_set parameter would be``RTEMS_GLOBAL | RTEMS_FLOATING_POINT``.

Building a Mode and Mask
------------------------
.. index:: task mode, building

In general, a mode and its corresponding mask is built by a
bitwise OR of the desired components.  The set of valid mode
constants and each mode’s corresponding mask constant is
listed below:

- ``RTEMS_PREEMPT`` is masked by``RTEMS_PREEMPT_MASK`` and enables preemption

- ``RTEMS_NO_PREEMPT`` is masked by``RTEMS_PREEMPT_MASK`` and disables preemption

- ``RTEMS_NO_TIMESLICE`` is masked by``RTEMS_TIMESLICE_MASK`` and disables timeslicing

- ``RTEMS_TIMESLICE`` is masked by``RTEMS_TIMESLICE_MASK`` and enables timeslicing

- ``RTEMS_ASR`` is masked by``RTEMS_ASR_MASK`` and enables ASR processing

- ``RTEMS_NO_ASR`` is masked by``RTEMS_ASR_MASK`` and disables ASR processing

- ``RTEMS_INTERRUPT_LEVEL(0)`` is masked by``RTEMS_INTERRUPT_MASK`` and enables all interrupts

- ``RTEMS_INTERRUPT_LEVEL(n)`` is masked by``RTEMS_INTERRUPT_MASK`` and sets interrupts level n

Mode values are specifically designed to be mutually exclusive, therefore
bitwise OR and addition operations are equivalent as long as each mode
appears exactly once in the component list.  A mode component listed as a
default is not required to appear in the mode component list, although it
is a good programming practice to specify default components.  If all
defaults are desired, the mode ``RTEMS_DEFAULT_MODES`` and the
mask ``RTEMS_ALL_MODE_MASKS`` should be used.

The following example demonstrates the mode and mask parameters used with
the ``rtems_task_mode``
directive to place a task at interrupt level 3 and make it
non-preemptible.  The mode should be set to``RTEMS_INTERRUPT_LEVEL(3)  |
RTEMS_NO_PREEMPT`` to indicate the desired preemption mode and
interrupt level, while the mask parameter should be set to``RTEMS_INTERRUPT_MASK |
RTEMS_NO_PREEMPT_MASK`` to indicate that the calling task’s
interrupt level and preemption mode are being altered.

Operations
==========

Creating Tasks
--------------

The ``rtems_task_create``
directive creates a task by allocating a task
control block, assigning the task a user-specified name,
allocating it a stack and floating point context area, setting a
user-specified initial priority, setting a user-specified
initial mode, and assigning it a task ID.  Newly created tasks
are initially placed in the dormant state.  All RTEMS tasks
execute in the most privileged mode of the processor.

Obtaining Task IDs
------------------

When a task is created, RTEMS generates a unique task ID and
assigns it to the created task until it is deleted.  The task ID
may be obtained by either of two methods.  First, as the result
of an invocation of the ``rtems_task_create``
directive, the task ID is
stored in a user provided location.  Second, the task ID may be
obtained later using the ``rtems_task_ident``
directive.  The task ID is
used by other directives to manipulate this task.

Starting and Restarting Tasks
-----------------------------

The ``rtems_task_start``
directive is used to place a dormant task in the
ready state.  This enables the task to compete, based on its
current priority, for the processor and other system resources.
Any actions, such as suspension or change of priority, performed
on a task prior to starting it are nullified when the task is
started.

With the ``rtems_task_start``
directive the user specifies the task’s
starting address and argument.  The argument is used to
communicate some startup information to the task.  As part of
this directive, RTEMS initializes the task’s stack based upon
the task’s initial execution mode and start address.  The
starting argument is passed to the task in accordance with the
target processor’s calling convention.

The ``rtems_task_restart``
directive restarts a task at its initial
starting address with its original priority and execution mode,
but with a possibly different argument.  The new argument may be
used to distinguish between the original invocation of the task
and subsequent invocations.  The task’s stack and control block
are modified to reflect their original creation values.
Although references to resources that have been requested are
cleared, resources allocated by the task are NOT automatically
returned to RTEMS.  A task cannot be restarted unless it has
previously been started (i.e. dormant tasks cannot be
restarted).  All restarted tasks are placed in the ready state.

Suspending and Resuming Tasks
-----------------------------

The ``rtems_task_suspend``
directive is used to place either the caller or
another task into a suspended state.  The task remains suspended
until a ``rtems_task_resume``
directive is issued.  This implies that a
task may be suspended as well as blocked waiting either to
acquire a resource or for the expiration of a timer.

The ``rtems_task_resume``
directive is used to remove another task from
the suspended state. If the task is not also blocked, resuming
it will place it in the ready state, allowing it to once again
compete for the processor and resources.  If the task was
blocked as well as suspended, this directive clears the
suspension and leaves the task in the blocked state.

Suspending a task which is already suspended or resuming a
task which is not suspended is considered an error.
The ``rtems_task_is_suspended`` can be used to
determine if a task is currently suspended.

Delaying the Currently Executing Task
-------------------------------------

The ``rtems_task_wake_after`` directive creates a sleep timer
which allows a task to go to sleep for a specified interval.  The task is
blocked until the delay interval has elapsed, at which time the task is
unblocked.  A task calling the ``rtems_task_wake_after``
directive with a delay
interval of ``RTEMS_YIELD_PROCESSOR`` ticks will yield the
processor to any other ready task of equal or greater priority and remain
ready to execute.

The ``rtems_task_wake_when``
directive creates a sleep timer which allows
a task to go to sleep until a specified date and time.  The
calling task is blocked until the specified date and time has
occurred, at which time the task is unblocked.

Changing Task Priority
----------------------

The ``rtems_task_set_priority``
directive is used to obtain or change the
current priority of either the calling task or another task.  If
the new priority requested is``RTEMS_CURRENT_PRIORITY`` or the task’s
actual priority, then the current priority will be returned and
the task’s priority will remain unchanged.  If the task’s
priority is altered, then the task will be scheduled according
to its new priority.

The ``rtems_task_restart``
directive resets the priority of a task to its
original value.

Changing Task Mode
------------------

The ``rtems_task_mode``
directive is used to obtain or change the current
execution mode of the calling task.  A task’s execution mode is
used to enable preemption, timeslicing, ASR processing, and to
set the task’s interrupt level.

The ``rtems_task_restart``
directive resets the mode of a task to its
original value.

Task Deletion
-------------

RTEMS provides the ``rtems_task_delete``
directive to allow a task to
delete itself or any other task.  This directive removes all
RTEMS references to the task, frees the task’s control block,
removes it from resource wait queues, and deallocates its stack
as well as the optional floating point context.  The task’s name
and ID become inactive at this time, and any subsequent
references to either of them is invalid.  In fact, RTEMS may
reuse the task ID for another task which is created later in the
application.

Unexpired delay timers (i.e. those used by``rtems_task_wake_after`` and``rtems_task_wake_when``) and
timeout timers associated with the task are
automatically deleted, however, other resources dynamically
allocated by the task are NOT automatically returned to RTEMS.
Therefore, before a task is deleted, all of its dynamically
allocated resources should be deallocated by the user.  This may
be accomplished by instructing the task to delete itself rather
than directly deleting the task.  Other tasks may instruct a
task to delete itself by sending a "delete self" message, event,
or signal, or by restarting the task with special arguments
which instruct the task to delete itself.

Transition Advice for Obsolete Directives
-----------------------------------------

Notepads
~~~~~~~~.. index:: rtems_task_get_note
.. index:: rtems_task_set_note

Task notepads and the associated directives``rtems_task_get_note`` and``rtems_task_set_note`` were removed after the 4.11 Release
Series. These were never thread-safe to access and subject to conflicting
use of the notepad index by libraries which were designed independently.

It is recommended that applications be modified to use services
which are thread safe and not subject to issues with multiple applications
conflicting over the key (e.g. notepad index) selection. For most
applications, POSIX Keys should be used. These are available in all RTEMS
build configurations. It is also possible that Thread Local Storage is
an option for some use cases.

Directives
==========

This section details the task manager’s directives.  A
subsection is dedicated to each of this manager’s directives and
describes the calling sequence, related constants, usage, and
status codes.

TASK_CREATE - Create a task
---------------------------
.. index:: create a task

**CALLING SEQUENCE:**

.. index:: rtems_task_create

.. code:: c

    rtems_status_code rtems_task_create(
    rtems_name           name,
    rtems_task_priority  initial_priority,
    size_t               stack_size,
    rtems_mode           initial_modes,
    rtems_attribute      attribute_set,
    rtems_id            \*id
    );

**DIRECTIVE STATUS CODES:**

``RTEMS_SUCCESSFUL`` - task created successfully
``RTEMS_INVALID_ADDRESS`` - ``id`` is NULL
``RTEMS_INVALID_NAME`` - invalid task name
``RTEMS_INVALID_PRIORITY`` - invalid task priority
``RTEMS_MP_NOT_CONFIGURED`` - multiprocessing not configured
``RTEMS_TOO_MANY`` - too many tasks created
``RTEMS_UNSATISFIED`` - not enough memory for stack/FP context
``RTEMS_TOO_MANY`` - too many global objects

**DESCRIPTION:**

This directive creates a task which resides on the local node.
It allocates and initializes a TCB, a stack, and an optional
floating point context area.  The mode parameter contains values
which sets the task’s initial execution mode.  The``RTEMS_FLOATING_POINT`` attribute should be
specified if the created task
is to use a numeric coprocessor.  For performance reasons, it is
recommended that tasks not using the numeric coprocessor should
specify the ``RTEMS_NO_FLOATING_POINT`` attribute.
If the ``RTEMS_GLOBAL``
attribute is specified, the task can be accessed from remote
nodes.  The task id, returned in id, is used in other task
related directives to access the task.  When created, a task is
placed in the dormant state and can only be made ready to
execute using the directive ``rtems_task_start``.

**NOTES:**

This directive will not cause the calling task to be preempted.

Valid task priorities range from a high of 1 to a low of 255.

If the requested stack size is less than the configured
minimum stack size, then RTEMS will use the configured
minimum as the stack size for this task.  In addition
to being able to specify the task stack size as a integer,
there are two constants which may be specified:

- ``RTEMS_MINIMUM_STACK_SIZE``
  is the minimum stack size *RECOMMENDED* for use on this processor.
  This value is selected by the RTEMS developers conservatively to
  minimize the risk of blown stacks for most user applications.
  Using this constant when specifying the task stack size, indicates
  that the stack size will be at least``RTEMS_MINIMUM_STACK_SIZE`` bytes in size.  If the
  user configured minimum stack size is larger than the recommended
  minimum, then it will be used.

- ``RTEMS_CONFIGURED_MINIMUM_STACK_SIZE``
  indicates that this task is to be created with a stack size
  of the minimum stack size that was configured by the application.
  If not explicitly configured by the application, the default
  configured minimum stack size is the processor dependent value``RTEMS_MINIMUM_STACK_SIZE``.  Since this uses
  the configured minimum stack size value, you may get a stack
  size that is smaller or larger than the recommended minimum.  This
  can be used to provide large stacks for all tasks on complex
  applications or small stacks on applications that are trying
  to conserve memory.

Application developers should consider the stack usage of the
device drivers when calculating the stack size required for
tasks which utilize the driver.

The following task attribute constants are defined by RTEMS:

- ``RTEMS_NO_FLOATING_POINT`` - does not use coprocessor (default)

- ``RTEMS_FLOATING_POINT`` - uses numeric coprocessor

- ``RTEMS_LOCAL`` - local task (default)

- ``RTEMS_GLOBAL`` - global task

The following task mode constants are defined by RTEMS:

- ``RTEMS_PREEMPT`` - enable preemption (default)

- ``RTEMS_NO_PREEMPT`` - disable preemption

- ``RTEMS_NO_TIMESLICE`` - disable timeslicing (default)

- ``RTEMS_TIMESLICE`` - enable timeslicing

- ``RTEMS_ASR`` - enable ASR processing (default)

- ``RTEMS_NO_ASR`` - disable ASR processing

- ``RTEMS_INTERRUPT_LEVEL(0)`` - enable all interrupts (default)

- ``RTEMS_INTERRUPT_LEVEL(n)`` - execute at interrupt level n

The interrupt level portion of the task execution mode
supports a maximum of 256 interrupt levels.  These levels are
mapped onto the interrupt levels actually supported by the
target processor in a processor dependent fashion.

Tasks should not be made global unless remote tasks must
interact with them.  This avoids the system overhead incurred by
the creation of a global task.  When a global task is created,
the task’s name and id must be transmitted to every node in the
system for insertion in the local copy of the global object
table.

The total number of global objects, including tasks, is limited
by the maximum_global_objects field in the Configuration Table.

TASK_IDENT - Get ID of a task
-----------------------------
.. index:: get ID of a task

**CALLING SEQUENCE:**

.. index:: rtems_task_ident

.. code:: c

    rtems_status_code rtems_task_ident(
    rtems_name  name,
    uint32_t    node,
    rtems_id   \*id
    );

**DIRECTIVE STATUS CODES:**

``RTEMS_SUCCESSFUL`` - task identified successfully
``RTEMS_INVALID_ADDRESS`` - ``id`` is NULL
``RTEMS_INVALID_NAME`` - invalid task name
``RTEMS_INVALID_NODE`` - invalid node id

**DESCRIPTION:**

This directive obtains the task id associated with the task name
specified in name.  A task may obtain its own id by specifying``RTEMS_SELF`` or its own task name in name.  If the task name is not
unique, then the task id returned will match one of the tasks
with that name.  However, this task id is not guaranteed to
correspond to the desired task.  The task id, returned in id, is
used in other task related directives to access the task.

**NOTES:**

This directive will not cause the running task to be preempted.

If node is ``RTEMS_SEARCH_ALL_NODES``, all nodes are searched with the
local node being searched first.  All other nodes are searched
with the lowest numbered node searched first.

If node is a valid node number which does not represent the
local node, then only the tasks exported by the designated node
are searched.

This directive does not generate activity on remote nodes.  It
accesses only the local copy of the global object table.

TASK_SELF - Obtain ID of caller
-------------------------------
.. index:: obtain ID of caller

**CALLING SEQUENCE:**

.. index:: rtems_task_self

.. code:: c

    rtems_id rtems_task_self(void);

**DIRECTIVE STATUS CODES:**

Returns the object Id of the calling task.

**DESCRIPTION:**

This directive returns the Id of the calling task.

**NOTES:**

If called from an interrupt service routine, this directive
will return the Id of the interrupted task.

TASK_START - Start a task
-------------------------
.. index:: starting a task

**CALLING SEQUENCE:**

.. index:: rtems_task_start

.. code:: c

    rtems_status_code rtems_task_start(
    rtems_id            id,
    rtems_task_entry    entry_point,
    rtems_task_argument argument
    );

**DIRECTIVE STATUS CODES:**

``RTEMS_SUCCESSFUL`` - ask started successfully
``RTEMS_INVALID_ADDRESS`` - invalid task entry point
``RTEMS_INVALID_ID`` - invalid task id
``RTEMS_INCORRECT_STATE`` - task not in the dormant state
``RTEMS_ILLEGAL_ON_REMOTE_OBJECT`` - cannot start remote task

**DESCRIPTION:**

This directive readies the task, specified by ``id``, for execution
based on the priority and execution mode specified when the task
was created.  The starting address of the task is given in``entry_point``.  The task’s starting argument is contained in
argument.  This argument can be a single value or used as an index into an
array of parameter blocks.  The type of this numeric argument is an unsigned
integer type with the property that any valid pointer to void can be converted
to this type and then converted back to a pointer to void.  The result will
compare equal to the original pointer.

**NOTES:**

The calling task will be preempted if its preemption mode is
enabled and the task being started has a higher priority.

Any actions performed on a dormant task such as suspension or
change of priority are nullified when the task is initiated via
the ``rtems_task_start`` directive.

TASK_RESTART - Restart a task
-----------------------------
.. index:: restarting a task

**CALLING SEQUENCE:**

.. index:: rtems_task_restart

.. code:: c

    rtems_status_code rtems_task_restart(
    rtems_id            id,
    rtems_task_argument argument
    );

**DIRECTIVE STATUS CODES:**

``RTEMS_SUCCESSFUL`` - task restarted successfully
``RTEMS_INVALID_ID`` - task id invalid
``RTEMS_INCORRECT_STATE`` - task never started
``RTEMS_ILLEGAL_ON_REMOTE_OBJECT`` - cannot restart remote task

**DESCRIPTION:**

This directive resets the task specified by id to begin
execution at its original starting address.  The task’s priority
and execution mode are set to the original creation values.  If
the task is currently blocked, RTEMS automatically makes the
task ready.  A task can be restarted from any state, except the
dormant state.

The task’s starting argument is contained in argument.  This argument can be a
single value or an index into an array of parameter blocks.  The type of this
numeric argument is an unsigned integer type with the property that any valid
pointer to void can be converted to this type and then converted back to a
pointer to void.  The result will compare equal to the original pointer.  This
new argument may be used to distinguish
between the initial ``rtems_task_start``
of the task and any ensuing calls
to ``rtems_task_restart``
of the task.  This can be beneficial in deleting
a task.  Instead of deleting a task using
the ``rtems_task_delete``
directive, a task can delete another task by restarting that
task, and allowing that task to release resources back to RTEMS
and then delete itself.

**NOTES:**

If id is ``RTEMS_SELF``, the calling task will be restarted and will not
return from this directive.

The calling task will be preempted if its preemption mode is
enabled and the task being restarted has a higher priority.

The task must reside on the local node, even if the task was
created with the ``RTEMS_GLOBAL`` option.

TASK_DELETE - Delete a task
---------------------------
.. index:: deleting a task

**CALLING SEQUENCE:**

.. index:: rtems_task_delete

.. code:: c

    rtems_status_code rtems_task_delete(
    rtems_id id
    );

**DIRECTIVE STATUS CODES:**

``RTEMS_SUCCESSFUL`` - task deleted successfully
``RTEMS_INVALID_ID`` - task id invalid
``RTEMS_ILLEGAL_ON_REMOTE_OBJECT`` - cannot restart remote task

**DESCRIPTION:**

This directive deletes a task, either the calling task or
another task, as specified by id.  RTEMS stops the execution of
the task and reclaims the stack memory, any allocated delay or
timeout timers, the TCB, and, if the task is ``RTEMS_FLOATING_POINT``, its
floating point context area.  RTEMS does not reclaim the
following resources: region segments, partition buffers,
semaphores, timers, or rate monotonic periods.

**NOTES:**

A task is responsible for releasing its resources back to RTEMS
before deletion.  To insure proper deallocation of resources, a
task should not be deleted unless it is unable to execute or
does not hold any RTEMS resources.  If a task holds RTEMS
resources, the task should be allowed to deallocate its
resources before deletion.  A task can be directed to release
its resources and delete itself by restarting it with a special
argument or by sending it a message, an event, or a signal.

Deletion of the current task (``RTEMS_SELF``) will force RTEMS to select
another task to execute.

When a global task is deleted, the task id must be transmitted
to every node in the system for deletion from the local copy of
the global object table.

The task must reside on the local node, even if the task was
created with the ``RTEMS_GLOBAL`` option.

TASK_SUSPEND - Suspend a task
-----------------------------
.. index:: suspending a task

**CALLING SEQUENCE:**

.. index:: rtems_task_suspend

.. code:: c

    rtems_status_code rtems_task_suspend(
    rtems_id id
    );

**DIRECTIVE STATUS CODES:**

``RTEMS_SUCCESSFUL`` - task suspended successfully
``RTEMS_INVALID_ID`` - task id invalid
``RTEMS_ALREADY_SUSPENDED`` - task already suspended

**DESCRIPTION:**

This directive suspends the task specified by id from further
execution by placing it in the suspended state.  This state is
additive to any other blocked state that the task may already be
in.  The task will not execute again until another task issues
the ``rtems_task_resume``
directive for this task and any blocked state
has been removed.

**NOTES:**

The requesting task can suspend itself by specifying ``RTEMS_SELF`` as id.
In this case, the task will be suspended and a successful
return code will be returned when the task is resumed.

Suspending a global task which does not reside on the local node
will generate a request to the remote node to suspend the
specified task.

If the task specified by id is already suspended, then the``RTEMS_ALREADY_SUSPENDED`` status code is returned.

TASK_RESUME - Resume a task
---------------------------
.. index:: resuming a task

**CALLING SEQUENCE:**

.. index:: rtems_task_resume

.. code:: c

    rtems_status_code rtems_task_resume(
    rtems_id id
    );

**DIRECTIVE STATUS CODES:**

``RTEMS_SUCCESSFUL`` - task resumed successfully
``RTEMS_INVALID_ID`` - task id invalid
``RTEMS_INCORRECT_STATE`` - task not suspended

**DESCRIPTION:**

This directive removes the task specified by id from the
suspended state.  If the task is in the ready state after the
suspension is removed, then it will be scheduled to run.  If the
task is still in a blocked state after the suspension is
removed, then it will remain in that blocked state.

**NOTES:**

The running task may be preempted if its preemption mode is
enabled and the local task being resumed has a higher priority.

Resuming a global task which does not reside on the local node
will generate a request to the remote node to resume the
specified task.

If the task specified by id is not suspended, then the``RTEMS_INCORRECT_STATE`` status code is returned.

TASK_IS_SUSPENDED - Determine if a task is Suspended
----------------------------------------------------
.. index:: is task suspended

**CALLING SEQUENCE:**

.. index:: rtems_task_is_suspended

.. code:: c

    rtems_status_code rtems_task_is_suspended(
    rtems_id id
    );

**DIRECTIVE STATUS CODES:**

``RTEMS_SUCCESSFUL`` - task is NOT suspended
``RTEMS_ALREADY_SUSPENDED`` - task is currently suspended
``RTEMS_INVALID_ID`` - task id invalid
``RTEMS_ILLEGAL_ON_REMOTE_OBJECT`` - not supported on remote tasks

**DESCRIPTION:**

This directive returns a status code indicating whether or
not the specified task is currently suspended.

**NOTES:**

This operation is not currently supported on remote tasks.

TASK_SET_PRIORITY - Set task priority
-------------------------------------
.. index:: rtems_task_set_priority
.. index:: current task priority
.. index:: set task priority
.. index:: get task priority
.. index:: obtain task priority

**CALLING SEQUENCE:**

.. code:: c

    rtems_status_code rtems_task_set_priority(
    rtems_id             id,
    rtems_task_priority  new_priority,
    rtems_task_priority \*old_priority
    );

**DIRECTIVE STATUS CODES:**

``RTEMS_SUCCESSFUL`` - task priority set successfully
``RTEMS_INVALID_ID`` - invalid task id
``RTEMS_INVALID_ADDRESS`` - invalid return argument pointer
``RTEMS_INVALID_PRIORITY`` - invalid task priority

**DESCRIPTION:**

This directive manipulates the priority of the task specified by
id.  An id of ``RTEMS_SELF`` is used to indicate
the calling task.  When new_priority is not equal to``RTEMS_CURRENT_PRIORITY``, the specified
task’s previous priority is returned in old_priority.  When
new_priority is ``RTEMS_CURRENT_PRIORITY``,
the specified task’s current
priority is returned in old_priority.  Valid priorities range
from a high of 1 to a low of 255.

**NOTES:**

The calling task may be preempted if its preemption mode is
enabled and it lowers its own priority or raises another task’s
priority.

In case the new priority equals the current priority of the task, then nothing
happens.

Setting the priority of a global task which does not reside on
the local node will generate a request to the remote node to
change the priority of the specified task.

If the task specified by id is currently holding any binary
semaphores which use the priority inheritance algorithm, then
the task’s priority cannot be lowered immediately.  If the
task’s priority were lowered immediately, then priority
inversion results.  The requested lowering of the task’s
priority will occur when the task has released all priority
inheritance binary semaphores.  The task’s priority can be
increased regardless of the task’s use of priority inheritance
binary semaphores.

TASK_MODE - Change the current task mode
----------------------------------------
.. index:: current task mode
.. index:: set task mode
.. index:: get task mode
.. index:: set task preemption mode
.. index:: get task preemption mode
.. index:: obtain task mode

**CALLING SEQUENCE:**

.. index:: rtems_task_mode

.. code:: c

    rtems_status_code rtems_task_mode(
    rtems_mode  mode_set,
    rtems_mode  mask,
    rtems_mode \*previous_mode_set
    );

**DIRECTIVE STATUS CODES:**

``RTEMS_SUCCESSFUL`` - task mode set successfully
``RTEMS_INVALID_ADDRESS`` - ``previous_mode_set`` is NULL

**DESCRIPTION:**

This directive manipulates the execution mode of the calling
task.  A task’s execution mode enables and disables preemption,
timeslicing, asynchronous signal processing, as well as
specifying the current interrupt level.  To modify an execution
mode, the mode class(es) to be changed must be specified in the
mask parameter and the desired mode(s) must be specified in the
mode parameter.

**NOTES:**

The calling task will be preempted if it enables preemption and
a higher priority task is ready to run.

Enabling timeslicing has no effect if preemption is disabled.  For
a task to be timesliced, that task must have both preemption and
timeslicing enabled.

A task can obtain its current execution mode, without modifying
it, by calling this directive with a mask value of``RTEMS_CURRENT_MODE``.

To temporarily disable the processing of a valid ASR, a task
should call this directive with the ``RTEMS_NO_ASR``
indicator specified in mode.

The set of task mode constants and each mode’s corresponding
mask constant is provided in the following table:

- ``RTEMS_PREEMPT`` is masked by``RTEMS_PREEMPT_MASK`` and enables preemption

- ``RTEMS_NO_PREEMPT`` is masked by``RTEMS_PREEMPT_MASK`` and disables preemption

- ``RTEMS_NO_TIMESLICE`` is masked by``RTEMS_TIMESLICE_MASK`` and disables timeslicing

- ``RTEMS_TIMESLICE`` is masked by``RTEMS_TIMESLICE_MASK`` and enables timeslicing

- ``RTEMS_ASR`` is masked by``RTEMS_ASR_MASK`` and enables ASR processing

- ``RTEMS_NO_ASR`` is masked by``RTEMS_ASR_MASK`` and disables ASR processing

- ``RTEMS_INTERRUPT_LEVEL(0)`` is masked by``RTEMS_INTERRUPT_MASK`` and enables all interrupts

- ``RTEMS_INTERRUPT_LEVEL(n)`` is masked by``RTEMS_INTERRUPT_MASK`` and sets interrupts level n

TASK_WAKE_AFTER - Wake up after interval
----------------------------------------
.. index:: delay a task for an interval
.. index:: wake up after an interval

**CALLING SEQUENCE:**

.. index:: rtems_task_wake_after

.. code:: c

    rtems_status_code rtems_task_wake_after(
    rtems_interval ticks
    );

**DIRECTIVE STATUS CODES:**

``RTEMS_SUCCESSFUL`` - always successful

**DESCRIPTION:**

This directive blocks the calling task for the specified number
of system clock ticks.  When the requested interval has elapsed,
the task is made ready.  The ``rtems_clock_tick``
directive automatically updates the delay period.

**NOTES:**

Setting the system date and time with the``rtems_clock_set`` directive
has no effect on a ``rtems_task_wake_after`` blocked task.

A task may give up the processor and remain in the ready state
by specifying a value of ``RTEMS_YIELD_PROCESSOR`` in ticks.

The maximum timer interval that can be specified is the maximum
value which can be represented by the uint32_t type.

A clock tick is required to support the functionality of this directive.

TASK_WAKE_WHEN - Wake up when specified
---------------------------------------
.. index:: delay a task until a wall time
.. index:: wake up at a wall time

**CALLING SEQUENCE:**

.. index:: rtems_task_wake_when

.. code:: c

    rtems_status_code rtems_task_wake_when(
    rtems_time_of_day \*time_buffer
    );

**DIRECTIVE STATUS CODES:**

``RTEMS_SUCCESSFUL`` - awakened at date/time successfully
``RTEMS_INVALID_ADDRESS`` - ``time_buffer`` is NULL
``RTEMS_INVALID_TIME_OF_DAY`` - invalid time buffer
``RTEMS_NOT_DEFINED`` - system date and time is not set

**DESCRIPTION:**

This directive blocks a task until the date and time specified
in time_buffer.  At the requested date and time, the calling
task will be unblocked and made ready to execute.

**NOTES:**

The ticks portion of time_buffer structure is ignored.  The
timing granularity of this directive is a second.

A clock tick is required to support the functionality of this directive.

ITERATE_OVER_ALL_THREADS - Iterate Over Tasks
---------------------------------------------
.. index:: iterate over all threads

**CALLING SEQUENCE:**

.. index:: rtems_iterate_over_all_threads

.. code:: c

    typedef void (\*rtems_per_thread_routine)(
    Thread_Control \*the_thread
    );
    void rtems_iterate_over_all_threads(
    rtems_per_thread_routine routine
    );

**DIRECTIVE STATUS CODES: NONE**

**DESCRIPTION:**

This directive iterates over all of the existant threads in the
system and invokes ``routine`` on each of them.  The user should
be careful in accessing the contents of ``the_thread``.

This routine is intended for use in diagnostic utilities and is
not intented for routine use in an operational system.

**NOTES:**

There is NO protection while this routine is called.  Thus it is
possible that ``the_thread`` could be deleted while this is operating.
By not having protection, the user is free to invoke support routines
from the C Library which require semaphores for data structures.

TASK_VARIABLE_ADD - Associate per task variable
-----------------------------------------------
.. index:: per-task variable
.. index:: task private variable
.. index:: task private data

**CALLING SEQUENCE:**

.. index:: rtems_task_variable_add

.. code:: c

    rtems_status_code rtems_task_variable_add(
    rtems_id  tid,
    void    \**task_variable,
    void    (\*dtor)(void \*)
    );

**DIRECTIVE STATUS CODES:**

``RTEMS_SUCCESSFUL`` - per task variable added successfully
``RTEMS_INVALID_ADDRESS`` - ``task_variable`` is NULL
``RTEMS_INVALID_ID`` - invalid task id
``RTEMS_NO_MEMORY`` - invalid task id
``RTEMS_ILLEGAL_ON_REMOTE_OBJECT`` - not supported on remote tasks

**DESCRIPTION:**

This directive adds the memory location specified by the
ptr argument to the context of the given task.  The variable will
then be private to the task.  The task can access and modify the
variable, but the modifications will not appear to other tasks, and
other tasks’ modifications to that variable will not affect the value
seen by the task.  This is accomplished by saving and restoring the
variable’s value each time a task switch occurs to or from the calling task.
If the dtor argument is non-NULL it specifies the address of a ‘destructor’
function which will be called when the task is deleted.  The argument
passed to the destructor function is the task’s value of the variable.

**NOTES:**

This directive is deprecated and task variables will be removed.

Task variables increase the context switch time to and from the
tasks that own them so it is desirable to minimize the number of
task variables.  One efficient method
is to have a single task variable that is a pointer to a dynamically
allocated structure containing the task’s private ‘global’ data.
In this case the destructor function could be ‘free’.

Per-task variables are disabled in SMP configurations and this service
is not available.

TASK_VARIABLE_GET - Obtain value of a per task variable
-------------------------------------------------------
.. index:: get per-task variable
.. index:: obtain per-task variable

**CALLING SEQUENCE:**

.. index:: rtems_task_variable_get

.. code:: c

    rtems_status_code rtems_task_variable_get(
    rtems_id  tid,
    void    \**task_variable,
    void    \**task_variable_value
    );

**DIRECTIVE STATUS CODES:**

``RTEMS_SUCCESSFUL`` - per task variable obtained successfully
``RTEMS_INVALID_ADDRESS`` - ``task_variable`` is NULL
``RTEMS_INVALID_ADDRESS`` - ``task_variable_value`` is NULL
``RTEMS_INVALID_ADDRESS`` - ``task_variable`` is not found
``RTEMS_NO_MEMORY`` - invalid task id
``RTEMS_ILLEGAL_ON_REMOTE_OBJECT`` - not supported on remote tasks

**DESCRIPTION:**

This directive looks up the private value of a task variable for a
specified task and stores that value in the location pointed to by
the result argument.  The specified task is usually not the calling
task, which can get its private value by directly accessing the variable.

**NOTES:**

This directive is deprecated and task variables will be removed.

If you change memory which ``task_variable_value`` points to,
remember to declare that memory as volatile, so that the compiler
will optimize it correctly.  In this case both the pointer``task_variable_value`` and data referenced by ``task_variable_value``
should be considered volatile.

Per-task variables are disabled in SMP configurations and this service
is not available.

TASK_VARIABLE_DELETE - Remove per task variable
-----------------------------------------------
.. index:: per-task variable
.. index:: task private variable
.. index:: task private data

**CALLING SEQUENCE:**

.. index:: rtems_task_variable_delete

.. code:: c

    rtems_status_code rtems_task_variable_delete(
    rtems_id  id,
    void    \**task_variable
    );

**DIRECTIVE STATUS CODES:**

``RTEMS_SUCCESSFUL`` - per task variable deleted successfully
``RTEMS_INVALID_ID`` - invalid task id
``RTEMS_NO_MEMORY`` - invalid task id
``RTEMS_INVALID_ADDRESS`` - ``task_variable`` is NULL
``RTEMS_ILLEGAL_ON_REMOTE_OBJECT`` - not supported on remote tasks

**DESCRIPTION:**

This directive removes the given location from a task’s context.

**NOTES:**

This directive is deprecated and task variables will be removed.

Per-task variables are disabled in SMP configurations and this service
is not available.

.. COMMENT: COPYRIGHT (c) 1988-2008.

.. COMMENT: On-Line Applications Research Corporation (OAR).

.. COMMENT: All rights reserved.