source: rtems-docs/c_user/task_manager.rst @ b8d3f6b

.. COMMENT: COPYRIGHT (c) 1988-2008.
.. COMMENT: On-Line Applications Research Corporation (OAR).
.. COMMENT: All rights reserved.

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.

.. _rtems_task_create:

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``
  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 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.

.. _rtems_task_ident:

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.

.. _rtems_task_self:

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.

.. _rtems_task_start:

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.

.. _rtems_task_restart:

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.

.. _rtems_task_delete:

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.

.. _rtems_task_suspend:

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.

.. _rtems_task_resume:

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.

.. _rtems_task_is_suspended:

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.

.. _rtems_task_set_priority:

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.

.. _rtems_task_mode:

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

.. _rtems_task_wake_after:

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.

.. _rtems_task_wake_when:

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.

.. _rtems_iterate_over_all_threads:

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.

.. _rtems_task_variable_add:

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

.. warning::

  This directive is deprecated and task variables will be removed.

**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:**

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.

.. _rtems_task_variable_get:

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

.. warning::

  This directive is deprecated and task variables will be removed.

**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:**

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.

.. _rtems_task_variable_delete:

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

.. warning::

  This directive is deprecated and task variables will be removed.

**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:**

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