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diff --git a/ada_user/semaphore_manager.rst b/ada_user/semaphore_manager.rst deleted file mode 100644 index 1081be4..0000000 --- a/ada_user/semaphore_manager.rst +++ /dev/null @@ -1,908 +0,0 @@ -Semaphore Manager -################# - -.. index:: semaphores -.. index:: binary semaphores -.. index:: counting semaphores -.. index:: mutual exclusion - -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 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. 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 must issue the``rtems.semaphore_release`` directive to -allow another task to execute the critical section. - -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 ----------------------- - -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 Inversion ------------------- - -Priority inversion is a form of indefinite -postponement which is common in multitasking, preemptive -executives with shared resources. Priority inversion occurs -when a high priority tasks requests access to shared resource -which is currently allocated to low priority task. The high -priority task must block until the low priority task releases -the resource. This problem is exacerbated when the low priority -task is prevented from executing by one or more medium priority -tasks. Because the low priority task is not executing, it -cannot complete its interaction with the resource and release -that resource. The high priority task is effectively prevented -from executing by lower priority tasks. - - -Priority Inheritance --------------------- - -Priority inheritance is an algorithm that calls for -the lower priority task holding a resource to have its priority -increased to that of the highest priority task blocked waiting -for that resource. Each time a task blocks attempting to obtain -the resource, the task holding the resource may have its -priority increased. - -On SMP configurations, in case the task holding the resource and the task that -blocks attempting to obtain the resource are in different scheduler instances, -the priority of the holder is raised to the pseudo-interrupt priority (priority -boosting). The pseudo-interrupt priority is the highest priority. - -RTEMS supports priority inheritance for local, binary -semaphores that use the priority task wait queue blocking -discipline. When a task of higher priority than the task -holding the semaphore blocks, the priority of the task holding -the semaphore is increased to that of the blocking task. When -the task holding the task completely releases the binary -semaphore (i.e. not for a nested release), the holder’s priority -is restored to the value it had before any higher priority was -inherited. - -The RTEMS implementation of the priority inheritance -algorithm takes into account the scenario in which a task holds -more than one binary semaphore. The holding task will execute -at the priority of the higher of the highest ceiling priority or -at the priority of the highest priority task blocked waiting for -any of the semaphores the task holds. Only when the task -releases ALL of the binary semaphores it holds will its priority -be restored to the normal value. - -Priority Ceiling ----------------- - -Priority ceiling is an algorithm that calls for the -lower priority task holding a resource to have its priority -increased to that of the highest priority task which will EVER -block waiting for that resource. This algorithm addresses the -problem of priority inversion although it avoids the possibility -of changing the priority of the task holding the resource -multiple times. The priority ceiling algorithm will only change -the priority of the task holding the resource a maximum of one -time. The ceiling priority is set at creation time and must be -the priority of the highest priority task which will ever -attempt to acquire that semaphore. - -RTEMS supports priority ceiling for local, binary -semaphores that use the priority task wait queue blocking -discipline. When a task of lower priority than the ceiling -priority successfully obtains the semaphore, its priority is -raised to the ceiling priority. When the task holding the task -completely releases the binary semaphore (i.e. not for a nested -release), the holder’s priority is restored to the value it had -before any higher priority was put into effect. - -The need to identify the highest priority task which -will attempt to obtain a particular semaphore can be a difficult -task in a large, complicated system. Although the priority -ceiling algorithm is more efficient than the priority -inheritance algorithm with respect to the maximum number of task -priority changes which may occur while a task holds a particular -semaphore, the priority inheritance algorithm is more forgiving -in that it does not require this apriori information. - -The RTEMS implementation of the priority ceiling -algorithm takes into account the scenario in which a task holds -more than one binary semaphore. The holding task will execute -at the priority of the higher of the highest ceiling priority or -at the priority of the highest priority task blocked waiting for -any of the semaphores the task holds. Only when the task -releases ALL of the binary semaphores it holds will its priority -be restored to the normal value. - -Multiprocessor Resource Sharing Protocol ----------------------------------------- - -The Multiprocessor Resource Sharing Protocol (MrsP) is defined in *A. -Burns and A.J. Wellings, A Schedulability Compatible Multiprocessor Resource -Sharing Protocol - MrsP, Proceedings of the 25th Euromicro Conference on -Real-Time Systems (ECRTS 2013), July 2013*. It is a generalization of the -Priority Ceiling Protocol to SMP systems. Each MrsP semaphore uses a ceiling -priority per scheduler instance. These ceiling priorities can be specified -with ``rtems_semaphore_set_priority()``. A task obtaining or owning a MrsP -semaphore will execute with the ceiling priority for its scheduler instance as -specified by the MrsP semaphore object. Tasks waiting to get ownership of a -MrsP semaphore will not relinquish the processor voluntarily. In case the -owner of a MrsP semaphore gets preempted it can ask all tasks waiting for this -semaphore to help out and temporarily borrow the right to execute on one of -their assigned processors. - -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: - -- ``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 or -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 or 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. - -.. code:: c - - Not available in ASCII representation - -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: - -- ``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 --------------------- - -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 ------------------------ - -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 ---------------------- - -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: -.. code:: c - - if semaphore's count is greater than zero - then decrement semaphore's count - else wait for release of semaphore - 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 ---------------------- - -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: -.. code:: c - - if no tasks are waiting on this semaphore - then increment semaphore's count - else assign semaphore to a waiting task - 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 --------------------- - -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 -========== - -This section details the semaphore 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. - -SEMAPHORE_CREATE - Create a semaphore -------------------------------------- -.. index:: create a semaphore - -**CALLING SEQUENCE:** - -.. code:: c - - procedure Semaphore_Create ( - Name : in RTEMS.Name; - Count : in RTEMS.Unsigned32; - Attribute_Set : in RTEMS.Attribute; - Priority_Ceiling : in RTEMS.Task_Priority; - ID : out RTEMS.ID; - Result : out RTEMS.Status_Codes - ); - -**DIRECTIVE STATUS CODES:** - -``RTEMS.SUCCESSFUL`` - semaphore created successfully -``RTEMS.INVALID_NAME`` - invalid semaphore name -``RTEMS.INVALID_ADDRESS`` - ``id`` is NULL -``RTEMS.TOO_MANY`` - too many semaphores created -``RTEMS.NOT_DEFINED`` - invalid attribute set -``RTEMS.INVALID_NUMBER`` - invalid starting count for binary semaphore -``RTEMS.MP_NOT_CONFIGURED`` - multiprocessing not configured -``RTEMS.TOO_MANY`` - too many global objects - -**DESCRIPTION:** - -This directive creates a semaphore which resides on -the local node. The created semaphore has the user-defined name -specified in name and the initial count specified in count. For -control and maintenance of the semaphore, RTEMS allocates and -initializes a SMCB. The RTEMS-assigned semaphore id is returned -in id. This semaphore id is used with other semaphore related -directives to access the semaphore. - -Specifying PRIORITY in attribute_set causes tasks -waiting for a semaphore to be serviced according to task -priority. When FIFO is selected, tasks are serviced in First -In-First Out order. - -**NOTES:** - -This directive will not cause the calling task to be -preempted. - -The priority inheritance and priority ceiling -algorithms are only supported for local, binary semaphores that -use the priority task wait queue blocking discipline. - -The following semaphore attribute constants are -defined by RTEMS: - -- ``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, block on 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 - -Semaphores should not be made global unless remote -tasks must interact with the created 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 id must be transmitted to every node in the system for -insertion in the local copy of the global object table. - -Note that some combinations of attributes are not valid. See the -earlier discussion on this. - -The total number of global objects, including semaphores, is limited by -the maximum_global_objects field in the Configuration Table. - -It is not allowed to create an initially locked MrsP semaphore and the``RTEMS.INVALID_NUMBER`` status code will be returned on SMP -configurations in this case. This prevents lock order reversal problems with -the allocator mutex. - -SEMAPHORE_IDENT - Get ID of a semaphore ---------------------------------------- -.. index:: get ID of a semaphore -.. index:: obtain ID of a semaphore - -**CALLING SEQUENCE:** - -.. code:: c - - procedure Semaphore_Ident ( - Name : in RTEMS.Name; - Node : in RTEMS.Unsigned32; - ID : out RTEMS.ID; - Result : out RTEMS.Status_Codes - ); - -**DIRECTIVE STATUS CODES:** - -``RTEMS.SUCCESSFUL`` - semaphore identified successfully -``RTEMS.INVALID_NAME`` - semaphore name not found -``RTEMS.INVALID_NODE`` - invalid node id - -**DESCRIPTION:** - -This directive obtains the semaphore id associated -with the semaphore name. If the semaphore name is not unique, -then the semaphore id will match one of the semaphores with that -name. However, this semaphore id is not guaranteed to -correspond to the desired semaphore. The semaphore id is used -by other semaphore related directives to access the semaphore. - -**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 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. - -SEMAPHORE_DELETE - Delete a semaphore -------------------------------------- -.. index:: delete a semaphore - -**CALLING SEQUENCE:** - -.. code:: c - - procedure Semaphore_Delete ( - ID : in RTEMS.ID; - Result : out RTEMS.Status_Codes - ); - -**DIRECTIVE STATUS CODES:** - -``RTEMS.SUCCESSFUL`` - semaphore deleted successfully -``RTEMS.INVALID_ID`` - invalid semaphore id -``RTEMS.RESOURCE_IN_USE`` - binary semaphore is in use -``RTEMS.ILLEGAL_ON_REMOTE_OBJECT`` - cannot delete remote semaphore - -**DESCRIPTION:** - -This directive deletes the semaphore specified by ``id``. -All tasks blocked waiting to acquire the semaphore will be -readied and returned a status code which indicates that the -semaphore was deleted. The SMCB for this semaphore is reclaimed -by RTEMS. - -**NOTES:** - -The calling task will be preempted if it is enabled -by the task’s execution mode and a higher priority local task is -waiting on the deleted semaphore. The calling task will NOT be -preempted if all of the tasks that are waiting on the semaphore -are remote tasks. - -The calling task does not have to be the task that -created the semaphore. Any local task that knows the semaphore -id can delete the semaphore. - -When a global semaphore is deleted, the semaphore id -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 ``RTEMS.GLOBAL`` option. - -Proxies, used to represent remote tasks, are -reclaimed when the semaphore is deleted. - -SEMAPHORE_OBTAIN - Acquire a semaphore --------------------------------------- -.. index:: obtain a semaphore -.. index:: lock a semaphore - -**CALLING SEQUENCE:** - -.. code:: c - - procedure Semaphore_Obtain ( - ID : in RTEMS.ID; - Option_Set : in RTEMS.Option; - Timeout : in RTEMS.Interval; - Result : out RTEMS.Status_Codes - ); - -**DIRECTIVE STATUS CODES:** - -``RTEMS.SUCCESSFUL`` - semaphore obtained successfully -``RTEMS.UNSATISFIED`` - semaphore not available -``RTEMS.TIMEOUT`` - timed out waiting for semaphore -``RTEMS.OBJECT_WAS_DELETED`` - semaphore deleted while waiting -``RTEMS.INVALID_ID`` - invalid semaphore id - -**DESCRIPTION:** - -This directive acquires the semaphore specified by -id. The ``RTEMS.WAIT`` and ``RTEMS.NO_WAIT`` components of the options parameter -indicate whether the calling task wants to wait for the -semaphore to become available or return immediately if the -semaphore is not currently available. With either ``RTEMS.WAIT`` or``RTEMS.NO_WAIT``, if the current semaphore count is positive, then it is -decremented by one and the semaphore is successfully acquired by -returning immediately with a successful return code. - -If the calling task chooses to return immediately and the current -semaphore count is zero or negative, then a 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 or -negative, then it is decremented by one and the calling task is placed on -the semaphore’s wait queue and blocked. If the semaphore was created with -the ``RTEMS.PRIORITY`` attribute, then the calling task is -inserted into the queue according to its priority. However, if the -semaphore was created with the ``RTEMS.FIFO`` attribute, then -the calling task is placed at the rear of the wait queue. If the binary -semaphore was created with the ``RTEMS.INHERIT_PRIORITY`` -attribute, then the priority of the task currently holding the binary -semaphore is guaranteed to be greater than or equal to that of the -blocking task. If the binary semaphore was created with the``RTEMS.PRIORITY_CEILING`` 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 -obtaining the semaphore is elevated to that of the ceiling. - -The timeout parameter specifies the maximum interval the calling task is -willing to be blocked waiting for the semaphore. If it is set to``RTEMS.NO_TIMEOUT``, then the calling task will wait forever. -If the semaphore is available or the ``RTEMS.NO_WAIT`` option -component is set, then timeout is ignored. - -Deadlock situations are detected for MrsP semaphores and the``RTEMS.UNSATISFIED`` status code will be returned on SMP -configurations in this case. - -**NOTES:** - -The following semaphore acquisition option constants -are defined by RTEMS: - -- ``RTEMS.WAIT`` - task will wait for semaphore (default) - -- ``RTEMS.NO_WAIT`` - task should not wait - -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 ``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. - -A clock tick is required to support the timeout functionality of -this directive. - -It is not allowed to obtain a MrsP semaphore more than once by one task at a -time (nested access) and the ``RTEMS.UNSATISFIED`` status code will -be returned on SMP configurations in this case. - -SEMAPHORE_RELEASE - Release a semaphore ---------------------------------------- -.. index:: release a semaphore -.. index:: unlock a semaphore - -**CALLING SEQUENCE:** - -.. code:: c - - procedure Semaphore_Release ( - ID : in RTEMS.ID; - Result : out RTEMS.Status_Codes - ); - -**DIRECTIVE STATUS CODES:** - -``RTEMS.SUCCESSFUL`` - semaphore released successfully -``RTEMS.INVALID_ID`` - invalid semaphore id -``RTEMS.NOT_OWNER_OF_RESOURCE`` - calling task does not own semaphore -``RTEMS.INCORRECT_STATE`` - invalid unlock order - -**DESCRIPTION:** - -This directive releases the semaphore specified by -id. The semaphore count is incremented by one. If the count is -zero or negative, then the first task on this semaphore’s wait -queue is removed and unblocked. The unblocked task may preempt -the running task if the running task’s preemption mode is -enabled and the unblocked task has a higher priority than the -running task. - -**NOTES:** - -The calling task may be preempted if it causes a -higher priority task to be made ready for execution. - -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. - -The outermost release of a local, binary, priority -inheritance or priority ceiling semaphore may result in the -calling task having its priority lowered. This will occur if -the calling task holds no other binary semaphores and it has -inherited a higher priority. - -The MrsP semaphores must be released in the reversed obtain order, otherwise -the ``RTEMS.INCORRECT_STATE`` status code will be returned on SMP -configurations in this case. - -SEMAPHORE_FLUSH - Unblock all tasks waiting on a semaphore ----------------------------------------------------------- -.. index:: flush a semaphore -.. index:: unblock all tasks waiting on a semaphore - -**CALLING SEQUENCE:** - -.. code:: c - - procedure Semaphore_Flush ( - ID : in RTEMS.ID; - Result : out RTEMS.Status_Codes - ); - -**DIRECTIVE STATUS CODES:** - -``RTEMS.SUCCESSFUL`` - semaphore released successfully -``RTEMS.INVALID_ID`` - invalid semaphore id -``RTEMS.NOT_DEFINED`` - operation not defined for the protocol of -the semaphore -``RTEMS.ILLEGAL_ON_REMOTE_OBJECT`` - not supported for remote semaphores - -**DESCRIPTION:** - -This directive unblocks all tasks waiting on the semaphore specified by -id. Since there are tasks blocked on the semaphore, the semaphore’s -count is not changed by this directive and thus is zero before and -after this directive is executed. Tasks which are unblocked as the -result of this directive will return from the``rtems.semaphore_obtain`` directive with a -status code of ``RTEMS.UNSATISFIED`` to indicate -that the semaphore was not obtained. - -This directive may unblock any number of tasks. Any of the unblocked -tasks may preempt the running task if the running task’s preemption mode is -enabled and an unblocked task has a higher priority than the -running task. - -**NOTES:** - -The calling task may be preempted if it causes a -higher priority task to be made ready for execution. - -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 MrsP semaphore and the``RTEMS.NOT_DEFINED`` status code will be returned on SMP -configurations in this case. - -SEMAPHORE_SET_PRIORITY - Set priority by scheduler for a semaphore ------------------------------------------------------------------- -.. index:: set priority by scheduler for a semaphore - -**CALLING SEQUENCE:** - -**DIRECTIVE STATUS CODES:** - -``RTEMS.SUCCESSFUL`` - successful operation -``RTEMS.INVALID_ID`` - invalid semaphore or scheduler id -``RTEMS.INVALID_ADDRESS`` - ``old_priority`` is NULL -``RTEMS.INVALID_PRIORITY`` - invalid new priority value -``RTEMS.NOT_DEFINED`` - operation not defined for the protocol of -the semaphore -``RTEMS.ILLEGAL_ON_REMOTE_OBJECT`` - not supported for remote semaphores - -**DESCRIPTION:** - -This directive sets the priority value with respect to the specified scheduler -of a semaphore. - -The special priority value ``RTEMS_CURRENT_PRIORITY`` can be used to get the -current priority value without changing it. - -The interpretation of the priority value depends on the protocol of the -semaphore object. - -- The Multiprocessor Resource Sharing Protocol needs a ceiling priority per - scheduler instance. This operation can be used to specify these priority - values. - -- For the Priority Ceiling Protocol the ceiling priority is used with this - operation. - -- For other protocols this operation is not defined. - -**EXAMPLE:** - -.. code:: c - - #include <assert.h> - #include <stdlib.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; - /* 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 MrsP semaphore object \*/ - sc = rtems_semaphore_create( - rtems_build_name('M', 'R', 'S', 'P'), - 1, - RTEMS_MULTIPROCESSOR_RESOURCE_SHARING - | RTEMS_BINARY_SEMAPHORE, - 1, - &semaphore_id - ); - assert(sc == RTEMS_SUCCESSFUL); - /* - * The ceiling priority values per scheduler are equal to the value specified - * for object 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); - exit(0); - } - #define CONFIGURE_SMP_APPLICATION - #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_MRSP_SEMAPHORES 1 - #define CONFIGURE_SMP_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_CONTROLS \\ - RTEMS_SCHEDULER_CONTROL_SIMPLE_SMP(a, SCHED_A), \\ - RTEMS_SCHEDULER_CONTROL_SIMPLE_SMP(b, SCHED_B) - #define CONFIGURE_SMP_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> - -.. COMMENT: COPYRIGHT (c) 1988-2002. - -.. COMMENT: On-Line Applications Research Corporation (OAR). - -.. COMMENT: All rights reserved. - |