[PATCH 01/16] c-user: Split up semaphore manager
Sebastian Huber
sebastian.huber at embedded-brains.de
Thu Aug 20 10:18:50 UTC 2020
This makes it easier to automatically generate parts of the manager
documentation in the future.
Update #3993.
---
c-user/index.rst | 2 +-
c-user/sem/background.rst | 180 ++++++++++
.../directives.rst} | 309 ------------------
c-user/sem/index.rst | 18 +
c-user/sem/introduction.rst | 25 ++
c-user/sem/operations.rst | 105 ++++++
6 files changed, 329 insertions(+), 310 deletions(-)
create mode 100644 c-user/sem/background.rst
rename c-user/{semaphore_manager.rst => sem/directives.rst} (64%)
create mode 100644 c-user/sem/index.rst
create mode 100644 c-user/sem/introduction.rst
create mode 100644 c-user/sem/operations.rst
diff --git a/c-user/index.rst b/c-user/index.rst
index e958481..64eac08 100644
--- a/c-user/index.rst
+++ b/c-user/index.rst
@@ -35,7 +35,7 @@ RTEMS Classic API Guide (|version|).
clock_manager
timer_manager
rate_monotonic_manager
- semaphore_manager
+ sem/index
barrier_manager
message_manager
event_manager
diff --git a/c-user/sem/background.rst b/c-user/sem/background.rst
new file mode 100644
index 0000000..9d9b151
--- /dev/null
+++ b/c-user/sem/background.rst
@@ -0,0 +1,180 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+.. Copyright (C) 1988, 2008 On-Line Applications Research Corporation (OAR)
+
+Background
+==========
+
+A semaphore can be viewed as a protected variable whose value can be modified
+only with the ``rtems_semaphore_create``, ``rtems_semaphore_obtain``, and
+``rtems_semaphore_release`` directives. RTEMS supports both binary and
+counting semaphores. A binary semaphore is restricted to values of zero or one,
+while a counting semaphore can assume any non-negative integer value.
+
+A binary semaphore (not a simple binary semaphore) can be used to control
+access to a single resource. In particular, it can be used to enforce mutual
+exclusion for a critical section in user code (mutex). In this instance, the
+semaphore would be created with an initial count of one to indicate that no
+task is executing the critical section of code. Upon entry to the critical
+section, a task must issue the ``rtems_semaphore_obtain`` directive to prevent
+other tasks from entering the critical section. Upon exit from the critical
+section, the task that obtained the binary semaphore must issue the
+``rtems_semaphore_release`` directive to allow another task to execute the
+critical section. A binary semaphore must be released by the task that
+obtained it.
+
+A counting semaphore can be used to control access to a pool of two or more
+resources. For example, access to three printers could be administered by a
+semaphore created with an initial count of three. When a task requires access
+to one of the printers, it issues the ``rtems_semaphore_obtain`` directive to
+obtain access to a printer. If a printer is not currently available, the task
+can wait for a printer to become available or return immediately. When the
+task has completed printing, it should issue the ``rtems_semaphore_release``
+directive to allow other tasks access to the printer.
+
+Task synchronization may be achieved by creating a semaphore with an initial
+count of zero. One task waits for the arrival of another task by issuing a
+``rtems_semaphore_obtain`` directive when it reaches a synchronization point.
+The other task performs a corresponding ``rtems_semaphore_release`` operation
+when it reaches its synchronization point, thus unblocking the pending task.
+
+.. _Nested Resource Access:
+
+Nested Resource Access
+----------------------
+
+Deadlock occurs when a task owning a binary semaphore attempts to acquire that
+same semaphore and blocks as result. Since the semaphore is allocated to a
+task, it cannot be deleted. Therefore, the task that currently holds the
+semaphore and is also blocked waiting for that semaphore will never execute
+again.
+
+RTEMS addresses this problem by allowing the task holding the binary semaphore
+to obtain the same binary semaphore multiple times in a nested manner. Each
+``rtems_semaphore_obtain`` must be accompanied with a
+``rtems_semaphore_release``. The semaphore will only be made available for
+acquisition by other tasks when the outermost ``rtems_semaphore_obtain`` is
+matched with a ``rtems_semaphore_release``.
+
+Simple binary semaphores do not allow nested access and so can be used for task
+synchronization.
+
+.. _Priority Inheritance:
+
+Priority Inheritance
+--------------------
+
+RTEMS supports :ref:`priority inheritance <PriorityInheritance>` for local,
+binary semaphores that use the priority task wait queue blocking discipline.
+In SMP configurations, the :ref:`OMIP` is used instead.
+
+.. _Priority Ceiling:
+
+Priority Ceiling
+----------------
+
+RTEMS supports :ref:`priority ceiling <PriorityCeiling>` for local, binary
+semaphores that use the priority task wait queue blocking discipline.
+
+.. _Multiprocessor Resource Sharing Protocol:
+
+Multiprocessor Resource Sharing Protocol
+----------------------------------------
+
+RTEMS supports the :ref:`MrsP` for local, binary semaphores that use the
+priority task wait queue blocking discipline. In uniprocessor configurations,
+the :ref:`PriorityCeiling` is used instead.
+
+.. _Building a Semaphore Attribute Set:
+
+Building a Semaphore Attribute Set
+----------------------------------
+
+In general, an attribute set is built by a bitwise OR of the desired attribute
+components. The following table lists the set of valid semaphore attributes:
+
+.. list-table::
+ :class: rtems-table
+
+ * - ``RTEMS_FIFO``
+ - tasks wait by FIFO (default)
+ * - ``RTEMS_PRIORITY``
+ - tasks wait by priority
+ * - ``RTEMS_BINARY_SEMAPHORE``
+ - restrict values to 0 and 1
+ * - ``RTEMS_COUNTING_SEMAPHORE``
+ - no restriction on values (default)
+ * - ``RTEMS_SIMPLE_BINARY_SEMAPHORE``
+ - restrict values to 0 and 1, do not allow nested access, allow deletion of
+ locked semaphore.
+ * - ``RTEMS_NO_INHERIT_PRIORITY``
+ - do not use priority inheritance (default)
+ * - ``RTEMS_INHERIT_PRIORITY``
+ - use priority inheritance
+ * - ``RTEMS_NO_PRIORITY_CEILING``
+ - do not use priority ceiling (default)
+ * - ``RTEMS_PRIORITY_CEILING``
+ - use priority ceiling
+ * - ``RTEMS_NO_MULTIPROCESSOR_RESOURCE_SHARING``
+ - do not use Multiprocessor Resource Sharing Protocol (default)
+ * - ``RTEMS_MULTIPROCESSOR_RESOURCE_SHARING``
+ - use Multiprocessor Resource Sharing Protocol
+ * - ``RTEMS_LOCAL``
+ - local semaphore (default)
+ * - ``RTEMS_GLOBAL``
+ - global semaphore
+
+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. An attribute listed as a default
+is not required to appear in the attribute list, although it is a good
+programming practice to specify default attributes. If all defaults are
+desired, the attribute ``RTEMS_DEFAULT_ATTRIBUTES`` should be specified on this
+call.
+
+This example demonstrates the attribute_set parameter needed to create a local
+semaphore with the task priority waiting queue discipline. The attribute_set
+parameter passed to the ``rtems_semaphore_create`` directive could be either
+``RTEMS_PRIORITY`` or ``RTEMS_LOCAL | RTEMS_PRIORITY``. The attribute_set
+parameter can be set to ``RTEMS_PRIORITY`` because ``RTEMS_LOCAL`` is the
+default for all created tasks. If a similar semaphore were to be known
+globally, then the attribute_set parameter would be ``RTEMS_GLOBAL |
+RTEMS_PRIORITY``.
+
+Some combinatinos of these attributes are invalid. For example, priority
+ordered blocking discipline must be applied to a binary semaphore in order to
+use either the priority inheritance or priority ceiling functionality. The
+following tree figure illustrates the valid combinations.
+
+.. figure:: ../../images/c_user/semaphore_attributes.png
+ :width: 90%
+ :align: center
+ :alt: Semaphore Attributes
+
+.. _Building a SEMAPHORE_OBTAIN Option Set:
+
+Building a SEMAPHORE_OBTAIN Option Set
+--------------------------------------
+
+In general, an option is built by a bitwise OR of the desired option
+components. The set of valid options for the ``rtems_semaphore_obtain``
+directive are listed in the following table:
+
+.. list-table::
+ :class: rtems-table
+
+ * - ``RTEMS_WAIT``
+ - task will wait for semaphore (default)
+ * - ``RTEMS_NO_WAIT``
+ - task should not wait
+
+Option 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. An option listed as a default is
+not required to appear in the list, although it is a good programming practice
+to specify default options. If all defaults are desired, the option
+``RTEMS_DEFAULT_OPTIONS`` should be specified on this call.
+
+This example demonstrates the option parameter needed to poll for a semaphore.
+The option parameter passed to the ``rtems_semaphore_obtain`` directive should
+be ``RTEMS_NO_WAIT``.
diff --git a/c-user/semaphore_manager.rst b/c-user/sem/directives.rst
similarity index 64%
rename from c-user/semaphore_manager.rst
rename to c-user/sem/directives.rst
index 31a3c79..460947d 100644
--- a/c-user/semaphore_manager.rst
+++ b/c-user/sem/directives.rst
@@ -2,315 +2,6 @@
.. Copyright (C) 1988, 2008 On-Line Applications Research Corporation (OAR)
-.. index:: semaphores
-.. index:: binary semaphores
-.. index:: counting semaphores
-.. index:: mutual exclusion
-
-Semaphore Manager
-*****************
-
-Introduction
-============
-
-The semaphore manager utilizes standard Dijkstra
-counting semaphores to provide synchronization and mutual
-exclusion capabilities. The directives provided by the
-semaphore manager are:
-
-- rtems_semaphore_create_ - Create a semaphore
-
-- rtems_semaphore_ident_ - Get ID of a semaphore
-
-- rtems_semaphore_delete_ - Delete a semaphore
-
-- rtems_semaphore_obtain_ - Acquire a semaphore
-
-- rtems_semaphore_release_ - Release a semaphore
-
-- rtems_semaphore_flush_ - Unblock all tasks waiting on a semaphore
-
-- rtems_semaphore_set_priority_ - Set priority by scheduler for a semaphore
-
-Background
-==========
-
-A semaphore can be viewed as a protected variable whose value can be modified
-only with the ``rtems_semaphore_create``, ``rtems_semaphore_obtain``, and
-``rtems_semaphore_release`` directives. RTEMS supports both binary and
-counting semaphores. A binary semaphore is restricted to values of zero or one,
-while a counting semaphore can assume any non-negative integer value.
-
-A binary semaphore (not a simple binary semaphore) can be used to control
-access to a single resource. In particular, it can be used to enforce mutual
-exclusion for a critical section in user code (mutex). In this instance, the
-semaphore would be created with an initial count of one to indicate that no
-task is executing the critical section of code. Upon entry to the critical
-section, a task must issue the ``rtems_semaphore_obtain`` directive to prevent
-other tasks from entering the critical section. Upon exit from the critical
-section, the task that obtained the binary semaphore must issue the
-``rtems_semaphore_release`` directive to allow another task to execute the
-critical section. A binary semaphore must be released by the task that
-obtained it.
-
-A counting semaphore can be used to control access to a pool of two or more
-resources. For example, access to three printers could be administered by a
-semaphore created with an initial count of three. When a task requires access
-to one of the printers, it issues the ``rtems_semaphore_obtain`` directive to
-obtain access to a printer. If a printer is not currently available, the task
-can wait for a printer to become available or return immediately. When the
-task has completed printing, it should issue the ``rtems_semaphore_release``
-directive to allow other tasks access to the printer.
-
-Task synchronization may be achieved by creating a semaphore with an initial
-count of zero. One task waits for the arrival of another task by issuing a
-``rtems_semaphore_obtain`` directive when it reaches a synchronization point.
-The other task performs a corresponding ``rtems_semaphore_release`` operation
-when it reaches its synchronization point, thus unblocking the pending task.
-
-.. _Nested Resource Access:
-
-Nested Resource Access
-----------------------
-
-Deadlock occurs when a task owning a binary semaphore attempts to acquire that
-same semaphore and blocks as result. Since the semaphore is allocated to a
-task, it cannot be deleted. Therefore, the task that currently holds the
-semaphore and is also blocked waiting for that semaphore will never execute
-again.
-
-RTEMS addresses this problem by allowing the task holding the binary semaphore
-to obtain the same binary semaphore multiple times in a nested manner. Each
-``rtems_semaphore_obtain`` must be accompanied with a
-``rtems_semaphore_release``. The semaphore will only be made available for
-acquisition by other tasks when the outermost ``rtems_semaphore_obtain`` is
-matched with a ``rtems_semaphore_release``.
-
-Simple binary semaphores do not allow nested access and so can be used for task
-synchronization.
-
-.. _Priority Inheritance:
-
-Priority Inheritance
---------------------
-
-RTEMS supports :ref:`priority inheritance <PriorityInheritance>` for local,
-binary semaphores that use the priority task wait queue blocking discipline.
-In SMP configurations, the :ref:`OMIP` is used instead.
-
-.. _Priority Ceiling:
-
-Priority Ceiling
-----------------
-
-RTEMS supports :ref:`priority ceiling <PriorityCeiling>` for local, binary
-semaphores that use the priority task wait queue blocking discipline.
-
-.. _Multiprocessor Resource Sharing Protocol:
-
-Multiprocessor Resource Sharing Protocol
-----------------------------------------
-
-RTEMS supports the :ref:`MrsP` for local, binary semaphores that use the
-priority task wait queue blocking discipline. In uniprocessor configurations,
-the :ref:`PriorityCeiling` is used instead.
-
-.. _Building a Semaphore Attribute Set:
-
-Building a Semaphore Attribute Set
-----------------------------------
-
-In general, an attribute set is built by a bitwise OR of the desired attribute
-components. The following table lists the set of valid semaphore attributes:
-
-.. list-table::
- :class: rtems-table
-
- * - ``RTEMS_FIFO``
- - tasks wait by FIFO (default)
- * - ``RTEMS_PRIORITY``
- - tasks wait by priority
- * - ``RTEMS_BINARY_SEMAPHORE``
- - restrict values to 0 and 1
- * - ``RTEMS_COUNTING_SEMAPHORE``
- - no restriction on values (default)
- * - ``RTEMS_SIMPLE_BINARY_SEMAPHORE``
- - restrict values to 0 and 1, do not allow nested access, allow deletion of
- locked semaphore.
- * - ``RTEMS_NO_INHERIT_PRIORITY``
- - do not use priority inheritance (default)
- * - ``RTEMS_INHERIT_PRIORITY``
- - use priority inheritance
- * - ``RTEMS_NO_PRIORITY_CEILING``
- - do not use priority ceiling (default)
- * - ``RTEMS_PRIORITY_CEILING``
- - use priority ceiling
- * - ``RTEMS_NO_MULTIPROCESSOR_RESOURCE_SHARING``
- - do not use Multiprocessor Resource Sharing Protocol (default)
- * - ``RTEMS_MULTIPROCESSOR_RESOURCE_SHARING``
- - use Multiprocessor Resource Sharing Protocol
- * - ``RTEMS_LOCAL``
- - local semaphore (default)
- * - ``RTEMS_GLOBAL``
- - global semaphore
-
-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. An attribute listed as a default
-is not required to appear in the attribute list, although it is a good
-programming practice to specify default attributes. If all defaults are
-desired, the attribute ``RTEMS_DEFAULT_ATTRIBUTES`` should be specified on this
-call.
-
-This example demonstrates the attribute_set parameter needed to create a local
-semaphore with the task priority waiting queue discipline. The attribute_set
-parameter passed to the ``rtems_semaphore_create`` directive could be either
-``RTEMS_PRIORITY`` or ``RTEMS_LOCAL | RTEMS_PRIORITY``. The attribute_set
-parameter can be set to ``RTEMS_PRIORITY`` because ``RTEMS_LOCAL`` is the
-default for all created tasks. If a similar semaphore were to be known
-globally, then the attribute_set parameter would be ``RTEMS_GLOBAL |
-RTEMS_PRIORITY``.
-
-Some combinatinos of these attributes are invalid. For example, priority
-ordered blocking discipline must be applied to a binary semaphore in order to
-use either the priority inheritance or priority ceiling functionality. The
-following tree figure illustrates the valid combinations.
-
-.. figure:: ../images/c_user/semaphore_attributes.png
- :width: 90%
- :align: center
- :alt: Semaphore Attributes
-
-.. _Building a SEMAPHORE_OBTAIN Option Set:
-
-Building a SEMAPHORE_OBTAIN Option Set
---------------------------------------
-
-In general, an option is built by a bitwise OR of the desired option
-components. The set of valid options for the ``rtems_semaphore_obtain``
-directive are listed in the following table:
-
-.. list-table::
- :class: rtems-table
-
- * - ``RTEMS_WAIT``
- - task will wait for semaphore (default)
- * - ``RTEMS_NO_WAIT``
- - task should not wait
-
-Option 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. An option listed as a default is
-not required to appear in the list, although it is a good programming practice
-to specify default options. If all defaults are desired, the option
-``RTEMS_DEFAULT_OPTIONS`` should be specified on this call.
-
-This example demonstrates the option parameter needed to poll for a semaphore.
-The option parameter passed to the ``rtems_semaphore_obtain`` directive should
-be ``RTEMS_NO_WAIT``.
-
-Operations
-==========
-
-.. _Creating a Semaphore:
-
-Creating a Semaphore
---------------------
-
-The ``rtems_semaphore_create`` directive creates a binary or counting semaphore
-with a user-specified name as well as an initial count. If a binary semaphore
-is created with a count of zero (0) to indicate that it has been allocated,
-then the task creating the semaphore is considered the current holder of the
-semaphore. At create time the method for ordering waiting tasks in the
-semaphore's task wait queue (by FIFO or task priority) is specified.
-Additionally, the priority inheritance or priority ceiling algorithm may be
-selected for local, binary semaphores that use the priority task wait queue
-blocking discipline. If the priority ceiling algorithm is selected, then the
-highest priority of any task which will attempt to obtain this semaphore must
-be specified. RTEMS allocates a Semaphore Control Block (SMCB) from the SMCB
-free list. This data structure is used by RTEMS to manage the newly created
-semaphore. Also, a unique semaphore ID is generated and returned to the
-calling task.
-
-.. _Obtaining Semaphore IDs:
-
-Obtaining Semaphore IDs
------------------------
-
-When a semaphore is created, RTEMS generates a unique semaphore ID and assigns
-it to the created semaphore until it is deleted. The semaphore ID may be
-obtained by either of two methods. First, as the result of an invocation of
-the ``rtems_semaphore_create`` directive, the semaphore ID is stored in a user
-provided location. Second, the semaphore ID may be obtained later using the
-``rtems_semaphore_ident`` directive. The semaphore ID is used by other
-semaphore manager directives to access this semaphore.
-
-.. _Acquiring a Semaphore:
-
-Acquiring a Semaphore
----------------------
-
-The ``rtems_semaphore_obtain`` directive is used to acquire the
-specified semaphore. A simplified version of the ``rtems_semaphore_obtain``
-directive can be described as follows:
-
- If the semaphore's count is greater than zero then decrement the
- semaphore's count else wait for release of semaphore then return
- SUCCESSFUL.
-
-When the semaphore cannot be immediately acquired, one of the following
-situations applies:
-
-- By default, the calling task will wait forever to acquire the semaphore.
-
-- Specifying ``RTEMS_NO_WAIT`` forces an immediate return with an error status
- code.
-
-- Specifying a timeout limits the interval the task will wait before returning
- with an error status code.
-
-If the task waits to acquire the semaphore, then it is placed in the
-semaphore's task wait queue in either FIFO or task priority order. If the task
-blocked waiting for a binary semaphore using priority inheritance and the
-task's priority is greater than that of the task currently holding the
-semaphore, then the holding task will inherit the priority of the blocking
-task. All tasks waiting on a semaphore are returned an error code when the
-semaphore is deleted.
-
-When a task successfully obtains a semaphore using priority ceiling and the
-priority ceiling for this semaphore is greater than that of the holder, then
-the holder's priority will be elevated.
-
-.. _Releasing a Semaphore:
-
-Releasing a Semaphore
----------------------
-
-The ``rtems_semaphore_release`` directive is used to release the specified
-semaphore. A simplified version of the ``rtems_semaphore_release`` directive
-can be described as follows:
-
- If there are no tasks are waiting on this semaphore then increment the
- semaphore's count else assign semaphore to a waiting task and return
- SUCCESSFUL.
-
-If this is the outermost release of a binary semaphore that uses priority
-inheritance or priority ceiling and the task does not currently hold any other
-binary semaphores, then the task performing the ``rtems_semaphore_release``
-will have its priority restored to its normal value.
-
-.. _Deleting a Semaphore:
-
-Deleting a Semaphore
---------------------
-
-The ``rtems_semaphore_delete`` directive removes a semaphore from the system
-and frees its control block. A semaphore can be deleted by any local task that
-knows the semaphore's ID. As a result of this directive, all tasks blocked
-waiting to acquire the semaphore will be readied and returned a status code
-which indicates that the semaphore was deleted. Any subsequent references to
-the semaphore's name and ID are invalid.
-
Directives
==========
diff --git a/c-user/sem/index.rst b/c-user/sem/index.rst
new file mode 100644
index 0000000..2110744
--- /dev/null
+++ b/c-user/sem/index.rst
@@ -0,0 +1,18 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+.. Copyright (C) 2020 embedded brains GmbH (http://www.embedded-brains.de)
+
+.. index:: semaphores
+.. index:: binary semaphores
+.. index:: counting semaphores
+.. index:: mutual exclusion
+
+Semaphore Manager
+*****************
+
+.. toctree::
+
+ introduction
+ background
+ operations
+ directives
diff --git a/c-user/sem/introduction.rst b/c-user/sem/introduction.rst
new file mode 100644
index 0000000..dee298b
--- /dev/null
+++ b/c-user/sem/introduction.rst
@@ -0,0 +1,25 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+.. Copyright (C) 1988, 2008 On-Line Applications Research Corporation (OAR)
+
+Introduction
+============
+
+The semaphore manager utilizes standard Dijkstra
+counting semaphores to provide synchronization and mutual
+exclusion capabilities. The directives provided by the
+semaphore manager are:
+
+- :ref:`rtems_semaphore_create`
+
+- :ref:`rtems_semaphore_ident`
+
+- :ref:`rtems_semaphore_delete`
+
+- :ref:`rtems_semaphore_obtain`
+
+- :ref:`rtems_semaphore_release`
+
+- :ref:`rtems_semaphore_flush`
+
+- :ref:`rtems_semaphore_set_priority`
diff --git a/c-user/sem/operations.rst b/c-user/sem/operations.rst
new file mode 100644
index 0000000..19c4be2
--- /dev/null
+++ b/c-user/sem/operations.rst
@@ -0,0 +1,105 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+.. Copyright (C) 1988, 2008 On-Line Applications Research Corporation (OAR)
+
+Operations
+==========
+
+.. _Creating a Semaphore:
+
+Creating a Semaphore
+--------------------
+
+The ``rtems_semaphore_create`` directive creates a binary or counting semaphore
+with a user-specified name as well as an initial count. If a binary semaphore
+is created with a count of zero (0) to indicate that it has been allocated,
+then the task creating the semaphore is considered the current holder of the
+semaphore. At create time the method for ordering waiting tasks in the
+semaphore's task wait queue (by FIFO or task priority) is specified.
+Additionally, the priority inheritance or priority ceiling algorithm may be
+selected for local, binary semaphores that use the priority task wait queue
+blocking discipline. If the priority ceiling algorithm is selected, then the
+highest priority of any task which will attempt to obtain this semaphore must
+be specified. RTEMS allocates a Semaphore Control Block (SMCB) from the SMCB
+free list. This data structure is used by RTEMS to manage the newly created
+semaphore. Also, a unique semaphore ID is generated and returned to the
+calling task.
+
+.. _Obtaining Semaphore IDs:
+
+Obtaining Semaphore IDs
+-----------------------
+
+When a semaphore is created, RTEMS generates a unique semaphore ID and assigns
+it to the created semaphore until it is deleted. The semaphore ID may be
+obtained by either of two methods. First, as the result of an invocation of
+the ``rtems_semaphore_create`` directive, the semaphore ID is stored in a user
+provided location. Second, the semaphore ID may be obtained later using the
+``rtems_semaphore_ident`` directive. The semaphore ID is used by other
+semaphore manager directives to access this semaphore.
+
+.. _Acquiring a Semaphore:
+
+Acquiring a Semaphore
+---------------------
+
+The ``rtems_semaphore_obtain`` directive is used to acquire the
+specified semaphore. A simplified version of the ``rtems_semaphore_obtain``
+directive can be described as follows:
+
+ If the semaphore's count is greater than zero then decrement the
+ semaphore's count else wait for release of semaphore then return
+ SUCCESSFUL.
+
+When the semaphore cannot be immediately acquired, one of the following
+situations applies:
+
+- By default, the calling task will wait forever to acquire the semaphore.
+
+- Specifying ``RTEMS_NO_WAIT`` forces an immediate return with an error status
+ code.
+
+- Specifying a timeout limits the interval the task will wait before returning
+ with an error status code.
+
+If the task waits to acquire the semaphore, then it is placed in the
+semaphore's task wait queue in either FIFO or task priority order. If the task
+blocked waiting for a binary semaphore using priority inheritance and the
+task's priority is greater than that of the task currently holding the
+semaphore, then the holding task will inherit the priority of the blocking
+task. All tasks waiting on a semaphore are returned an error code when the
+semaphore is deleted.
+
+When a task successfully obtains a semaphore using priority ceiling and the
+priority ceiling for this semaphore is greater than that of the holder, then
+the holder's priority will be elevated.
+
+.. _Releasing a Semaphore:
+
+Releasing a Semaphore
+---------------------
+
+The ``rtems_semaphore_release`` directive is used to release the specified
+semaphore. A simplified version of the ``rtems_semaphore_release`` directive
+can be described as follows:
+
+ If there are no tasks are waiting on this semaphore then increment the
+ semaphore's count else assign semaphore to a waiting task and return
+ SUCCESSFUL.
+
+If this is the outermost release of a binary semaphore that uses priority
+inheritance or priority ceiling and the task does not currently hold any other
+binary semaphores, then the task performing the ``rtems_semaphore_release``
+will have its priority restored to its normal value.
+
+.. _Deleting a Semaphore:
+
+Deleting a Semaphore
+--------------------
+
+The ``rtems_semaphore_delete`` directive removes a semaphore from the system
+and frees its control block. A semaphore can be deleted by any local task that
+knows the semaphore's ID. As a result of this directive, all tasks blocked
+waiting to acquire the semaphore will be readied and returned a status code
+which indicates that the semaphore was deleted. Any subsequent references to
+the semaphore's name and ID are invalid.
--
2.26.2
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