source: rtems-docs/c-user/semaphore/directives.rst @ 241cd06

.. SPDX-License-Identifier: CC-BY-SA-4.0

.. Copyright (C) 2020, 2021 embedded brains GmbH (http://www.embedded-brains.de)
.. Copyright (C) 1988, 2008 On-Line Applications Research Corporation (OAR)

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.. documentation at:
..
.. https://docs.rtems.org

.. _SemaphoreManagerDirectives:

Directives
==========

This section details the directives of the Semaphore Manager. A subsection is
dedicated to each of this manager's directives and lists the calling sequence,
parameters, description, return values, and notes of the directive.

.. Generated from spec:/rtems/sem/if/create

.. raw:: latex

    \clearpage

.. index:: rtems_semaphore_create()
.. index:: create a semaphore

.. _InterfaceRtemsSemaphoreCreate:

rtems_semaphore_create()
------------------------

Creates a semaphore.

.. rubric:: CALLING SEQUENCE:

.. code-block:: c

    rtems_status_code rtems_semaphore_create(
      rtems_name          name,
      uint32_t            count,
      rtems_attribute     attribute_set,
      rtems_task_priority priority_ceiling,
      rtems_id           *id
    );

.. rubric:: PARAMETERS:

``name``
    This parameter is the object name of the semaphore.

``count``
    This parameter is the initial count of the semaphore.  If the semaphore is
    a binary semaphore, then a count of 0 will make the calling task the owner
    of the binary semaphore and a count of 1 will create a binary semaphore
    without an owner.

``attribute_set``
    This parameter is the attribute set of the semaphore.

``priority_ceiling``
    This parameter is the priority ceiling if the semaphore is a binary
    semaphore with the priority ceiling or MrsP locking protocol as defined by
    the attribute set.

``id``
    This parameter is the pointer to an object identifier variable.  When the
    directive call is successful, the identifier of the created semaphore will
    be stored in this variable.

.. rubric:: DESCRIPTION:

This directive creates a semaphore which resides on the local node.  The
semaphore has the user-defined object name specified in ``name`` and the
initial count specified in ``count``.  The assigned object identifier is
returned in ``id``.  This identifier is used to access the semaphore with other
semaphore related directives.

The **attribute set** specified in ``attribute_set`` is built through a
*bitwise or* of the attribute constants described below.  Not all combinations
of attributes are allowed.  Some attributes are mutually exclusive.  If
mutually exclusive attributes are combined, the behaviour is undefined.
Attributes not mentioned below are not evaluated by this directive and have no
effect.  Default attributes can be selected by using the
:c:macro:`RTEMS_DEFAULT_ATTRIBUTES` constant.  The attribute set defines

* the scope of the semaphore: :c:macro:`RTEMS_LOCAL` (default) or
  :c:macro:`RTEMS_GLOBAL`,

* the task wait queue discipline used by the semaphore: :c:macro:`RTEMS_FIFO`
  (default) or :c:macro:`RTEMS_PRIORITY`,

* the class of the semaphore: :c:macro:`RTEMS_COUNTING_SEMAPHORE` (default),
  :c:macro:`RTEMS_BINARY_SEMAPHORE`, or
  :c:macro:`RTEMS_SIMPLE_BINARY_SEMAPHORE`, and

* the locking protocol of a binary semaphore: no locking protocol (default),
  :c:macro:`RTEMS_INHERIT_PRIORITY`, :c:macro:`RTEMS_PRIORITY_CEILING`, or
  :c:macro:`RTEMS_MULTIPROCESSOR_RESOURCE_SHARING`.

The semaphore has a local or global **scope** in a multiprocessing network
(this attribute does not refer to SMP systems).  The scope is selected by the
mutually exclusive :c:macro:`RTEMS_LOCAL` and :c:macro:`RTEMS_GLOBAL`
attributes.

* A **local scope** is the default and can be emphasized through the use of the
  :c:macro:`RTEMS_LOCAL` attribute.  A local semaphore can be only used by the
  node which created it.

* A **global scope** is established if the :c:macro:`RTEMS_GLOBAL` attribute is
  set.  Setting the global attribute in a single node system has no effect.

The **task wait queue discipline** is selected by the mutually exclusive
:c:macro:`RTEMS_FIFO` and :c:macro:`RTEMS_PRIORITY` attributes.

* The **FIFO discipline** is the default and can be emphasized through use of
  the :c:macro:`RTEMS_FIFO` attribute.

* The **priority discipline** is selected by the :c:macro:`RTEMS_PRIORITY`
  attribute.  The locking protocols require the priority discipline.

The **semaphore class** is selected by the mutually exclusive
:c:macro:`RTEMS_COUNTING_SEMAPHORE`, :c:macro:`RTEMS_BINARY_SEMAPHORE`, and
:c:macro:`RTEMS_SIMPLE_BINARY_SEMAPHORE` attributes.

* The **counting semaphore class** is the default and can be emphasized through
  use of the :c:macro:`RTEMS_COUNTING_SEMAPHORE` attribute.

* The **binary semaphore class** is selected by the
  :c:macro:`RTEMS_BINARY_SEMAPHORE` attribute.  Binary semaphores are mutual
  exclusion (mutex) synchronization primitives which may have an owner.  The
  count of a binary semaphore is restricted to 0 and 1 values.

* The **simple binary semaphore class** is selected by the
  :c:macro:`RTEMS_SIMPLE_BINARY_SEMAPHORE` attribute.  Simple binary semaphores
  have no owner.  They may be used for task and interrupt synchronization.  The
  count of a simple binary semaphore is restricted to 0 and 1 values.

Binary semaphores may use a **locking protocol**.  If a locking protocol is
selected, then the scope shall be local and the priority task wait queue
discipline shall be selected.  The locking protocol is selected by the mutually
exclusive :c:macro:`RTEMS_INHERIT_PRIORITY`, :c:macro:`RTEMS_PRIORITY_CEILING`,
and :c:macro:`RTEMS_MULTIPROCESSOR_RESOURCE_SHARING` attributes.

* The default is **no locking protocol**.  This can be emphasized through use
  of the :c:macro:`RTEMS_NO_INHERIT_PRIORITY`,
  :c:macro:`RTEMS_NO_MULTIPROCESSOR_RESOURCE_SHARING`, and
  :c:macro:`RTEMS_NO_PRIORITY_CEILING` attributes.

* The **priority inheritance locking protocol** is selected by the
  :c:macro:`RTEMS_INHERIT_PRIORITY` attribute.

* The **priority ceiling locking protocol** is selected by the
  :c:macro:`RTEMS_PRIORITY_CEILING` attribute.  For this locking protocol a
  priority ceiling shall be specified in ``priority_ceiling``.

* The **MrsP locking protocol** is selected by the
  :c:macro:`RTEMS_MULTIPROCESSOR_RESOURCE_SHARING` attribute in SMP
  configurations, otherwise this attribute selects the **priority ceiling
  locking protocol**.  For these locking protocols a priority ceiling shall be
  specified in ``priority_ceiling``.  This priority is used to set the priority
  ceiling for all schedulers.  This can be changed later with the
  :ref:`InterfaceRtemsSemaphoreSetPriority` directive using the returned object
  identifier.

.. rubric:: RETURN VALUES:

:c:macro:`RTEMS_SUCCESSFUL`
    The requested operation was successful.

:c:macro:`RTEMS_INVALID_NAME`
    The ``name`` parameter was invalid.

:c:macro:`RTEMS_INVALID_ADDRESS`
    The ``id`` parameter was `NULL
    <https://en.cppreference.com/w/c/types/NULL>`_.

:c:macro:`RTEMS_INVALID_NUMBER`
    The ``count`` parameter was invalid.

:c:macro:`RTEMS_NOT_DEFINED`
    The ``attribute_set`` parameter was invalid.

:c:macro:`RTEMS_TOO_MANY`
    There was no inactive object available to create a semaphore.  The number
    of semaphores available to the application is configured through the
    :ref:`CONFIGURE_MAXIMUM_SEMAPHORES` application configuration option.

:c:macro:`RTEMS_TOO_MANY`
    In multiprocessing configurations, there was no inactive global object
    available to create a global semaphore.  The number of global objects
    available to the application is configured through the
    :ref:`CONFIGURE_MP_MAXIMUM_GLOBAL_OBJECTS` application configuration
    option.

:c:macro:`RTEMS_INVALID_PRIORITY`
    The ``priority_ceiling`` parameter was invalid.

.. rubric:: NOTES:

For control and maintenance of the semaphore, RTEMS allocates a :term:`SMCB`
from the local SMCB free pool and initializes it.

The SMCB for a global semaphore is allocated on the local node.  Semaphores
should not be made global unless remote tasks must interact with the semaphore.
This is to avoid the system overhead incurred by the creation of a global
semaphore.  When a global semaphore is created, the semaphore's name and
identifier must be transmitted to every node in the system for insertion in the
local copy of the global object table.

.. rubric:: CONSTRAINTS:

The following constraints apply to this directive:

* The directive may be called from within device driver initialization context.

* The directive may be called from within task context.

* The directive may obtain and release the object allocator mutex.  This may
  cause the calling task to be preempted.

* When the directive operates on a global object, the directive sends a message
  to remote nodes.  This may preempt the calling task.

* The number of semaphores available to the application is configured through
  the :ref:`CONFIGURE_MAXIMUM_SEMAPHORES` application configuration option.

* Where the object class corresponding to the directive is configured to use
  unlimited objects, the directive may allocate memory from the RTEMS
  Workspace.

* The number of global objects available to the application is configured
  through the :ref:`CONFIGURE_MP_MAXIMUM_GLOBAL_OBJECTS` application
  configuration option.

.. Generated from spec:/rtems/sem/if/ident

.. raw:: latex

    \clearpage

.. index:: rtems_semaphore_ident()

.. _InterfaceRtemsSemaphoreIdent:

rtems_semaphore_ident()
-----------------------

Identifies a semaphore by the object name.

.. rubric:: CALLING SEQUENCE:

.. code-block:: c

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

.. rubric:: PARAMETERS:

``name``
    This parameter is the object name to look up.

``node``
    This parameter is the node or node set to search for a matching object.

``id``
    This parameter is the pointer to an object identifier variable.  When the
    directive call is successful, the object identifier of an object with the
    specified name will be stored in this variable.

.. rubric:: DESCRIPTION:

This directive obtains a semaphore identifier associated with the semaphore
name specified in ``name``.

The node to search is specified in ``node``.  It shall be

* a valid node number,

* the constant :c:macro:`RTEMS_SEARCH_ALL_NODES` to search in all nodes,

* the constant :c:macro:`RTEMS_SEARCH_LOCAL_NODE` to search in the local node
  only, or

* the constant :c:macro:`RTEMS_SEARCH_OTHER_NODES` to search in all nodes
  except the local node.

.. rubric:: RETURN VALUES:

:c:macro:`RTEMS_SUCCESSFUL`
    The requested operation was successful.

:c:macro:`RTEMS_INVALID_ADDRESS`
    The ``id`` parameter was `NULL
    <https://en.cppreference.com/w/c/types/NULL>`_.

:c:macro:`RTEMS_INVALID_NAME`
    The ``name`` parameter was 0.

:c:macro:`RTEMS_INVALID_NAME`
    There was no object with the specified name on the specified nodes.

:c:macro:`RTEMS_INVALID_NODE`
    In multiprocessing configurations, the specified node was invalid.

.. rubric:: NOTES:

If the semaphore name is not unique, then the semaphore identifier will match
the first semaphore with that name in the search order.  However, this
semaphore identifier is not guaranteed to correspond to the desired semaphore.

The objects are searched from lowest to the highest index.  If ``node`` is
:c:macro:`RTEMS_SEARCH_ALL_NODES`, all nodes are searched with the local node
being searched first.  All other nodes are searched from lowest to the highest
node number.

If node is a valid node number which does not represent the local node, then
only the semaphores 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.

The semaphore identifier is used with other semaphore related directives to
access the semaphore.

.. rubric:: CONSTRAINTS:

The following constraints apply to this directive:

* The directive may be called from within any runtime context.

* The directive will not cause the calling task to be preempted.

.. Generated from spec:/rtems/sem/if/delete

.. raw:: latex

    \clearpage

.. index:: rtems_semaphore_delete()
.. index:: delete a semaphore

.. _InterfaceRtemsSemaphoreDelete:

rtems_semaphore_delete()
------------------------

Deletes the semaphore.

.. rubric:: CALLING SEQUENCE:

.. code-block:: c

    rtems_status_code rtems_semaphore_delete( rtems_id id );

.. rubric:: PARAMETERS:

``id``
    This parameter is the semaphore identifier.

.. rubric:: DESCRIPTION:

This directive deletes the semaphore specified by ``id``.

.. rubric:: RETURN VALUES:

:c:macro:`RTEMS_SUCCESSFUL`
    The requested operation was successful.

:c:macro:`RTEMS_INVALID_ID`
    There was no semaphore associated with the identifier specified by ``id``.

:c:macro:`RTEMS_ILLEGAL_ON_REMOTE_OBJECT`
    The semaphore resided on a remote node.

:c:macro:`RTEMS_RESOURCE_IN_USE`
    The binary semaphore had an owner.

.. rubric:: NOTES:

Binary semaphores with an owner cannot be deleted.

When a semaphore is deleted, all tasks blocked waiting to obtain the semaphore
will be readied and returned a status code which indicates that the semaphore
was deleted.

The :term:`SMCB` for the deleted semaphore is reclaimed by RTEMS.

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

The semaphore must reside on the local node, even if the semaphore was created
with the :c:macro:`RTEMS_GLOBAL` attribute.

Proxies, used to represent remote tasks, are reclaimed when the semaphore is
deleted.

.. rubric:: CONSTRAINTS:

The following constraints apply to this directive:

* The directive may be called from within device driver initialization context.

* The directive may be called from within task context.

* The directive may obtain and release the object allocator mutex.  This may
  cause the calling task to be preempted.

* When the directive operates on a global object, the directive sends a message
  to remote nodes.  This may preempt the calling task.

* The calling task does not have to be the task that created the object.  Any
  local task that knows the object identifier can delete the object.

* Where the object class corresponding to the directive is configured to use
  unlimited objects, the directive may free memory to the RTEMS Workspace.

.. Generated from spec:/rtems/sem/if/obtain

.. raw:: latex

    \clearpage

.. index:: rtems_semaphore_obtain()
.. index:: obtain a semaphore
.. index:: lock a semaphore

.. _InterfaceRtemsSemaphoreObtain:

rtems_semaphore_obtain()
------------------------

Obtains the semaphore.

.. rubric:: CALLING SEQUENCE:

.. code-block:: c

    rtems_status_code rtems_semaphore_obtain(
      rtems_id       id,
      rtems_option   option_set,
      rtems_interval timeout
    );

.. rubric:: PARAMETERS:

``id``
    This parameter is the semaphore identifier.

``option_set``
    This parameter is the option set.

``timeout``
    This parameter is the timeout in clock ticks if the :c:macro:`RTEMS_WAIT`
    option is set.  Use :c:macro:`RTEMS_NO_TIMEOUT` to wait potentially
    forever.

.. rubric:: DESCRIPTION:

This directive obtains the semaphore specified by ``id``.

The **option set** specified in ``option_set`` is built through a *bitwise or*
of the option constants described below.  Not all combinations of options are
allowed.  Some options are mutually exclusive.  If mutually exclusive options
are combined, the behaviour is undefined.  Options not mentioned below are not
evaluated by this directive and have no effect. Default options can be selected
by using the :c:macro:`RTEMS_DEFAULT_OPTIONS` constant.

The calling task can **wait** or **try to obtain** the semaphore according to
the mutually exclusive :c:macro:`RTEMS_WAIT` and :c:macro:`RTEMS_NO_WAIT`
options.

* **Waiting to obtain** the semaphore is the default and can be emphasized
  through the use of the :c:macro:`RTEMS_WAIT` option.  The ``timeout``
  parameter defines how long the calling task is willing to wait.  Use
  :c:macro:`RTEMS_NO_TIMEOUT` to wait potentially forever, otherwise set a
  timeout interval in clock ticks.

* **Trying to obtain** the semaphore is selected by the
  :c:macro:`RTEMS_NO_WAIT` option.  If this option is defined, then the
  ``timeout`` parameter is ignored.  When the semaphore cannot be immediately
  obtained, then the :c:macro:`RTEMS_UNSATISFIED` status is returned.

With either :c:macro:`RTEMS_WAIT` or :c:macro:`RTEMS_NO_WAIT` if the current
semaphore count is positive, then it is decremented by one and the semaphore is
successfully obtained by returning immediately with the
:c:macro:`RTEMS_SUCCESSFUL` status code.

If the calling task chooses to return immediately and the current semaphore
count is zero, then the :c:macro:`RTEMS_UNSATISFIED` status code is returned
indicating that the semaphore is not available.

If the calling task chooses to wait for a semaphore and the current semaphore
count is zero, then the calling task is placed on the semaphore's wait queue
and blocked.  If a local, binary semaphore was created with the
:c:macro:`RTEMS_INHERIT_PRIORITY` attribute, then the priority of the task
currently holding the binary semaphore will inherit the current priority set of
the blocking task.  The priority inheritance is carried out recursively.  This
means, that if the task currently holding the binary semaphore is blocked on
another local, binary semaphore using the priority inheritance locking
protocol, then the owner of this semaphore will inherit the current priority
sets of both tasks, and so on.  A task has a current priority for each
scheduler.

.. rubric:: RETURN VALUES:

:c:macro:`RTEMS_SUCCESSFUL`
    The requested operation was successful.

:c:macro:`RTEMS_INVALID_ID`
    There was no semaphore associated with the identifier specified by ``id``.

:c:macro:`RTEMS_UNSATISFIED`
    The semaphore could not be obtained immediately.

:c:macro:`RTEMS_INCORRECT_STATE`
    Acquiring of the local, binary semaphore by the calling task would have
    cased a deadlock.

:c:macro:`RTEMS_INCORRECT_STATE`
    The calling task attempted to recursively obtain a local, binary semaphore
    using the MrsP locking protocol.

:c:macro:`RTEMS_UNSATISFIED`
    The semaphore was flushed while the calling task was waiting to obtain the
    semaphore.

:c:macro:`RTEMS_TIMEOUT`
    The timeout happened while the calling task was waiting to obtain the
    semaphore.

:c:macro:`RTEMS_OBJECT_WAS_DELETED`
    The semaphore was deleted while the calling task was waiting to obtain the
    semaphore.

.. rubric:: NOTES:

If a local, binary semaphore was created with the
:c:macro:`RTEMS_PRIORITY_CEILING` or
:c:macro:`RTEMS_MULTIPROCESSOR_RESOURCE_SHARING` attribute, a task successfully
obtains the semaphore, and the priority of that task is greater than the
ceiling priority for this semaphore, then the priority of the task acquiring
the semaphore is elevated to that of the ceiling.

Deadlock situations are detected for local, binary semaphores.  If a deadlock
is detected, then the directive immediately returns the
:c:macro:`RTEMS_INCORRECT_STATE` status code.

It is not allowed to recursively obtain (nested access) a local, binary
semaphore using the MrsP locking protocol and any attempt to do this will just
return the :c:macro:`RTEMS_INCORRECT_STATE` status code.  This error can only
happen in SMP configurations.

If the semaphore was created with the :c:macro:`RTEMS_PRIORITY` attribute, then
the calling task is inserted into the wait queue according to its priority.
However, if the semaphore was created with the :c:macro:`RTEMS_FIFO` attribute,
then the calling task is placed at the rear of the wait queue.

Attempting to obtain a global semaphore which does not reside on the local node
will generate a request to the remote node to access the semaphore.  If the
semaphore is not available and :c:macro:`RTEMS_NO_WAIT` was not specified, then
the task must be blocked until the semaphore is released.  A proxy is allocated
on the remote node to represent the task until the semaphore is released.

.. rubric:: CONSTRAINTS:

The following constraints apply to this directive:

* When a local, counting semaphore or a local, simple binary semaphore is
  accessed and the :c:macro:`RTEMS_NO_WAIT` option is set, the directive may be
  called from within interrupt context.

* When a local semaphore is accessed and the request can be immediately
  satisfied, the directive may be called from within device driver
  initialization context.

* The directive may be called from within task context.

* When the request cannot be immediately satisfied and the
  :c:macro:`RTEMS_WAIT` option is set, the calling task blocks at some point
  during the directive call.

* The timeout functionality of the directive requires a :term:`clock tick`.

* When the directive operates on a remote object, the directive sends a message
  to the remote node and waits for a reply.  This will preempt the calling
  task.

.. Generated from spec:/rtems/sem/if/release

.. raw:: latex

    \clearpage

.. index:: rtems_semaphore_release()
.. index:: release a semaphore
.. index:: unlock a semaphore

.. _InterfaceRtemsSemaphoreRelease:

rtems_semaphore_release()
-------------------------

Releases the semaphore.

.. rubric:: CALLING SEQUENCE:

.. code-block:: c

    rtems_status_code rtems_semaphore_release( rtems_id id );

.. rubric:: PARAMETERS:

``id``
    This parameter is the semaphore identifier.

.. rubric:: DESCRIPTION:

This directive releases the semaphore specified by ``id``.  If the semaphore's
wait queue is not empty, then

* the first task on the wait queue is removed and unblocked, the semaphore's
  count is not changed, otherwise

* the semaphore's count is incremented by one for counting semaphores and set
  to one for binary and simple binary semaphores.

.. rubric:: RETURN VALUES:

:c:macro:`RTEMS_SUCCESSFUL`
    The requested operation was successful.

:c:macro:`RTEMS_INVALID_ID`
    There was no semaphore associated with the identifier specified by ``id``.

:c:macro:`RTEMS_NOT_OWNER_OF_RESOURCE`
    The calling task was not the owner of the semaphore.

:c:macro:`RTEMS_UNSATISFIED`
    The semaphore's count already had the maximum value of `UINT32_MAX
    <https://en.cppreference.com/w/c/types/integer>`_.

.. rubric:: NOTES:

The calling task may be preempted if it causes a higher priority task to be
made ready for execution.

The outermost release of a local, binary semaphore using the priority
inheritance, priority ceiling, or MrsP locking protocol may result in the
calling task having its priority lowered.  This will occur if the highest
priority of the calling task was available due to the ownership of the released
semaphore.  If a task was on the semaphore's wait queue, then the priority
associated with the semaphore will be transferred to the new owner.

Releasing a global semaphore which does not reside on the local node will
generate a request telling the remote node to release the semaphore.

If the task to be unblocked resides on a different node from the semaphore,
then the semaphore allocation is forwarded to the appropriate node, the waiting
task is unblocked, and the proxy used to represent the task is reclaimed.

.. rubric:: CONSTRAINTS:

The following constraints apply to this directive:

* When a local, counting semaphore or a local, simple binary semaphore is
  accessed, the directive may be called from within interrupt context.

* When a local semaphore is accessed, the directive may be called from within
  device driver initialization context.

* The directive may be called from within task context.

* The directive may unblock another task which may preempt the calling task.

* When the directive operates on a remote object, the directive sends a message
  to the remote node and waits for a reply.  This will preempt the calling
  task.

.. Generated from spec:/rtems/sem/if/flush

.. raw:: latex

    \clearpage

.. index:: rtems_semaphore_flush()
.. index:: flush a semaphore
.. index:: unblock all tasks waiting on a semaphore

.. _InterfaceRtemsSemaphoreFlush:

rtems_semaphore_flush()
-----------------------

Flushes the semaphore.

.. rubric:: CALLING SEQUENCE:

.. code-block:: c

    rtems_status_code rtems_semaphore_flush( rtems_id id );

.. rubric:: PARAMETERS:

``id``
    This parameter is the semaphore identifier.

.. rubric:: DESCRIPTION:

This directive unblocks all tasks waiting on the semaphore specified by ``id``.
The semaphore's count is not changed by this directive.  Tasks which are
unblocked as the result of this directive will return from the
:ref:`InterfaceRtemsSemaphoreObtain` directive with a status code of
:c:macro:`RTEMS_UNSATISFIED` to indicate that the semaphore was not obtained.

.. rubric:: RETURN VALUES:

:c:macro:`RTEMS_SUCCESSFUL`
    The requested operation was successful.

:c:macro:`RTEMS_INVALID_ID`
    There was no semaphore associated with the identifier specified by ``id``.

:c:macro:`RTEMS_ILLEGAL_ON_REMOTE_OBJECT`
    The semaphore resided on a remote node.

:c:macro:`RTEMS_NOT_DEFINED`
    Flushing a semaphore using the MrsP locking protocol is undefined
    behaviour.

.. rubric:: NOTES:

If the task to be unblocked resides on a different node from the semaphore,
then the waiting task is unblocked, and the proxy used to represent the task is
reclaimed.

It is not allowed to flush a local, binary semaphore using the MrsP locking
protocol and any attempt to do this will just return the
:c:macro:`RTEMS_NOT_DEFINED` status code.  This error can only happen in SMP
configurations.

For barrier synchronization, the :ref:`RTEMSAPIClassicBarrier` offers a cleaner
alternative to using the semaphore flush directive.  Unlike POSIX barriers,
they have a manual release option.

Using the semaphore flush directive for condition synchronization in concert
with another semaphore may be subject to the lost wake-up problem.  The
following attempt to implement a condition variable is broken.

.. code-block:: c
    :linenos:

    #include <rtems.h>
    #include <assert.h>

    void cnd_wait( rtems_id cnd, rtems_id mtx )
    {
      rtems_status_code sc;

      sc = rtems_semaphore_release( mtx );
      assert( sc == RTEMS_SUCCESSFUL );

      // Here, a higher priority task may run and satisfy the condition.
      // We may never wake up from the next semaphore obtain.

      sc = rtems_semaphore_obtain( cnd, RTEMS_WAIT, RTEMS_NO_TIMEOUT );
      assert( sc == RTEMS_UNSATISFIED );

      sc = rtems_semaphore_obtain( mtx, RTEMS_WAIT, RTEMS_NO_TIMEOUT );
      assert( sc == RTEMS_SUCCESSFUL );
    }

    void cnd_broadcast( rtems_id cnd )
    {
      rtems_status_code sc;

      sc = rtems_semaphore_flush( cnd );
      assert( sc == RTEMS_SUCCESSFUL );
    }

.. rubric:: CONSTRAINTS:

The following constraints apply to this directive:

* When a local, counting semaphore or a local, simple binary semaphore is
  accessed, the directive may be called from within interrupt context.

* When a local semaphore is accessed, the directive may be called from within
  device driver initialization context.

* The directive may be called from within task context.

* The directive may unblock another task which may preempt the calling task.

* When the directive operates on a remote object, the directive sends a message
  to the remote node and waits for a reply.  This will preempt the calling
  task.

.. Generated from spec:/rtems/sem/if/set-priority

.. raw:: latex

    \clearpage

.. index:: rtems_semaphore_set_priority()
.. index:: set priority by scheduler for a semaphore

.. _InterfaceRtemsSemaphoreSetPriority:

rtems_semaphore_set_priority()
------------------------------

Sets the priority by scheduler for the semaphore.

.. rubric:: CALLING SEQUENCE:

.. code-block:: c

    rtems_status_code rtems_semaphore_set_priority(
      rtems_id             semaphore_id,
      rtems_id             scheduler_id,
      rtems_task_priority  new_priority,
      rtems_task_priority *old_priority
    );

.. rubric:: PARAMETERS:

``semaphore_id``
    This parameter is the semaphore identifier.

``scheduler_id``
    This parameter is the identifier of the scheduler corresponding to the new
    priority.

``new_priority``
    This parameter is the new priority corresponding to the specified
    scheduler.

``old_priority``
    This parameter is the pointer to a task priority variable.  When the
    directive call is successful, the old priority of the semaphore
    corresponding to the specified scheduler will be stored in this variable.

.. rubric:: DESCRIPTION:

This directive sets the priority of the semaphore specified by
``semaphore_id``.  The priority corresponds to the scheduler specified by
``scheduler_id``.

The special priority value :c:macro:`RTEMS_CURRENT_PRIORITY` can be used to get
the current priority without changing it.

The availability and use of a priority depends on the class and locking
protocol of the semaphore:

* For local, binary semaphores using the MrsP locking protocol, the ceiling
  priority for each scheduler can be set by this directive.

* For local, binary semaphores using the priority ceiling protocol, the ceiling
  priority can be set by this directive.

* For other semaphore classes and locking protocols, setting a priority is
  undefined behaviour.

.. rubric:: RETURN VALUES:

:c:macro:`RTEMS_SUCCESSFUL`
    The requested operation was successful.

:c:macro:`RTEMS_INVALID_ADDRESS`
    The ``old_priority`` parameter was `NULL
    <https://en.cppreference.com/w/c/types/NULL>`_.

:c:macro:`RTEMS_INVALID_ID`
    There was no scheduler associated with the identifier specified by
    ``scheduler_id``.

:c:macro:`RTEMS_INVALID_ID`
    There was no semaphore associated with the identifier specified by
    ``semaphore_id``.

:c:macro:`RTEMS_ILLEGAL_ON_REMOTE_OBJECT`
    The semaphore resided on a remote node.

:c:macro:`RTEMS_INVALID_PRIORITY`
    The ``new_priority`` parameter was invalid.

:c:macro:`RTEMS_NOT_DEFINED`
    Setting a priority for the class or locking protocol of the semaphore is
    undefined behaviour.

.. rubric:: NOTES:

Please have a look at the following example:

.. code-block:: c
    :linenos:

    #include <assert.h>
    #include <rtems.h>

    #define SCHED_A rtems_build_name( ' ', ' ', ' ', 'A' )
    #define SCHED_B rtems_build_name( ' ', ' ', ' ', 'B' )

    static void Init( rtems_task_argument arg )
    {
      rtems_status_code   sc;
      rtems_id            semaphore_id;
      rtems_id            scheduler_a_id;
      rtems_id            scheduler_b_id;
      rtems_task_priority prio;

      (void) arg;

      // Get the scheduler identifiers
      sc = rtems_scheduler_ident( SCHED_A, &scheduler_a_id );
      assert( sc == RTEMS_SUCCESSFUL );
      sc = rtems_scheduler_ident( SCHED_B, &scheduler_b_id );
      assert( sc == RTEMS_SUCCESSFUL );

      // Create a local, binary semaphore using the MrsP locking protocol
      sc = rtems_semaphore_create(
        rtems_build_name( 'M', 'R', 'S', 'P' ),
        1,
        RTEMS_BINARY_SEMAPHORE | RTEMS_PRIORITY |
          RTEMS_MULTIPROCESSOR_RESOURCE_SHARING,
        1,
        &semaphore_id
      );
      assert( sc == RTEMS_SUCCESSFUL );

      // The ceiling priority for each scheduler is equal to the priority
      // specified for the semaphore creation.
      prio = RTEMS_CURRENT_PRIORITY;
      sc = rtems_semaphore_set_priority( semaphore_id, scheduler_a_id, prio, &prio );
      assert( sc == RTEMS_SUCCESSFUL );
      assert( prio == 1 );

      // Check the old value and set a new ceiling priority for scheduler B
      prio = 2;
      sc = rtems_semaphore_set_priority( semaphore_id, scheduler_b_id, prio, &prio );
      assert( sc == RTEMS_SUCCESSFUL );
      assert( prio == 1 );

      // Check the ceiling priority values
      prio = RTEMS_CURRENT_PRIORITY;
      sc = rtems_semaphore_set_priority( semaphore_id, scheduler_a_id, prio, &prio );
      assert( sc == RTEMS_SUCCESSFUL );
      assert( prio == 1 );
      prio = RTEMS_CURRENT_PRIORITY;
      sc = rtems_semaphore_set_priority( semaphore_id, scheduler_b_id, prio, &prio );
      assert( sc == RTEMS_SUCCESSFUL );
      assert( prio == 2 );

      sc = rtems_semaphore_delete( semaphore_id );
      assert( sc == RTEMS_SUCCESSFUL );

      rtems_shutdown_executive( 0 );
    }

    #define CONFIGURE_APPLICATION_NEEDS_CLOCK_DRIVER
    #define CONFIGURE_APPLICATION_NEEDS_CONSOLE_DRIVER
    #define CONFIGURE_MAXIMUM_TASKS 1
    #define CONFIGURE_MAXIMUM_SEMAPHORES 1
    #define CONFIGURE_MAXIMUM_PROCESSORS 2

    #define CONFIGURE_SCHEDULER_SIMPLE_SMP

    #include <rtems/scheduler.h>

    RTEMS_SCHEDULER_CONTEXT_SIMPLE_SMP( a );
    RTEMS_SCHEDULER_CONTEXT_SIMPLE_SMP( b );

    #define CONFIGURE_SCHEDULER_TABLE_ENTRIES \
        RTEMS_SCHEDULER_TABLE_SIMPLE_SMP( a, SCHED_A ), \
        RTEMS_SCHEDULER_TABLE_SIMPLE_SMP( b, SCHED_B )

    #define CONFIGURE_SCHEDULER_ASSIGNMENTS \
        RTEMS_SCHEDULER_ASSIGN( 0, RTEMS_SCHEDULER_ASSIGN_PROCESSOR_MANDATORY ), \
        RTEMS_SCHEDULER_ASSIGN( 1, RTEMS_SCHEDULER_ASSIGN_PROCESSOR_MANDATORY )

    #define CONFIGURE_RTEMS_INIT_TASKS_TABLE
    #define CONFIGURE_INIT

    #include <rtems/confdefs.h>

.. rubric:: CONSTRAINTS:

The following constraints apply to this directive:

* The directive may be called from within interrupt context.

* The directive may be called from within device driver initialization context.

* The directive may be called from within task context.

* The directive may change the priority of another task which may preempt the
  calling task.