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diff --git a/c-user/semaphore/directives.rst b/c-user/semaphore/directives.rst new file mode 100644 index 0000000..460947d --- /dev/null +++ b/c-user/semaphore/directives.rst @@ -0,0 +1,650 @@ +.. SPDX-License-Identifier: CC-BY-SA-4.0 + +.. Copyright (C) 1988, 2008 On-Line Applications Research Corporation (OAR) + +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. + +.. raw:: latex + + \clearpage + +.. index:: create a semaphore +.. index:: rtems_semaphore_create + +.. _rtems_semaphore_create: + +SEMAPHORE_CREATE - Create a semaphore +------------------------------------- + + +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 + ); + +DIRECTIVE STATUS CODES: + .. list-table:: + :class: rtems-table + + * - ``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_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 may cause the calling task to be preempted due to an + obtain and release of the object allocator mutex. + + 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: + + .. 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, 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*, 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 in SMP configurations + in this case. This prevents lock order reversal problems with the + allocator mutex. + +.. raw:: latex + + \clearpage + +.. index:: get ID of a semaphore +.. index:: obtain ID of a semaphore +.. index:: rtems_semaphore_ident + +.. _rtems_semaphore_ident: + +SEMAPHORE_IDENT - Get ID of a semaphore +--------------------------------------- + +CALLING SEQUENCE: + .. code-block:: c + + rtems_status_code rtems_semaphore_ident( + rtems_name name, + uint32_t node, + rtems_id *id + ); + +DIRECTIVE STATUS CODES: + .. list-table:: + :class: rtems-table + + * - ``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. + +.. raw:: latex + + \clearpage + +.. index:: delete a semaphore +.. index:: rtems_semaphore_delete + +.. _rtems_semaphore_delete: + +SEMAPHORE_DELETE - Delete a semaphore +------------------------------------- + +CALLING SEQUENCE: + .. code-block:: c + + rtems_status_code rtems_semaphore_delete( + rtems_id id + ); + +DIRECTIVE STATUS CODES: + .. list-table:: + :class: rtems-table + + * - ``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: + This directive may cause the calling task to be preempted due to an + obtain and release of the object allocator mutex. + + 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. + +.. raw:: latex + + \clearpage + +.. index:: obtain a semaphore +.. index:: lock a semaphore +.. index:: rtems_semaphore_obtain + +.. _rtems_semaphore_obtain: + +SEMAPHORE_OBTAIN - Acquire a semaphore +-------------------------------------- + +CALLING SEQUENCE: + .. code-block:: c + + rtems_status_code rtems_semaphore_obtain( + rtems_id id, + rtems_option option_set, + rtems_interval timeout + ); + +DIRECTIVE STATUS CODES: + .. list-table:: + :class: rtems-table + + * - ``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. + + In case a semaphore is not available, then ``RTEMS_UNSATISFIED`` will be + returned. This happens immediately in case ``RTEMS_NO_WAIT`` is specified, + or as a result of another task invoking the ``rtems_semaphore_flush`` + directive in case ``RTEMS_WAIT`` is specified. + + Deadlock situations are detected for MrsP semaphores and the + ``RTEMS_UNSATISFIED`` status code will be returned in SMP configurations in + this case. + +NOTES: + The following semaphore acquisition option constants are defined by RTEMS: + + .. list-table:: + :class: rtems-table + + * - ``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 in SMP configurations in this case. + +.. raw:: latex + + \clearpage + +.. index:: release a semaphore +.. index:: unlock a semaphore +.. index:: rtems_semaphore_release + +.. _rtems_semaphore_release: + +SEMAPHORE_RELEASE - Release a semaphore +--------------------------------------- + +CALLING SEQUENCE: + .. code-block:: c + + rtems_status_code rtems_semaphore_release( + rtems_id id + ); + +DIRECTIVE STATUS CODES: + .. list-table:: + :class: rtems-table + + * - ``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 in SMP + configurations in this case. + +.. raw:: latex + + \clearpage + +.. index:: flush a semaphore +.. index:: unblock all tasks waiting on a semaphore +.. index:: rtems_semaphore_flush + +.. _rtems_semaphore_flush: + +SEMAPHORE_FLUSH - Unblock all tasks waiting on a semaphore +---------------------------------------------------------- + +CALLING SEQUENCE: + .. code-block:: c + + rtems_status_code rtems_semaphore_flush( + rtems_id id + ); + +DIRECTIVE STATUS CODES: + .. list-table:: + :class: rtems-table + + * - ``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 in SMP configurations in this case. + + Using the ``rtems_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 + + #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 ); + } + + For barrier synchronization, the :ref:`barrier_manager` offers a cleaner + alternative to using the `rtems_semaphore_flush` directive. Unlike POSIX + barriers, they have a manual release option. + +.. raw:: latex + + \clearpage + +.. index:: set priority by scheduler for a semaphore +.. index:: rtems_semaphore_set_priority + +.. _rtems_semaphore_set_priority: + +SEMAPHORE_SET_PRIORITY - Set priority by scheduler for a semaphore +------------------------------------------------------------------ + +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 + ); + +DIRECTIVE STATUS CODES: + .. list-table:: + :class: rtems-table + + * - ``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 ofthe 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-block:: c + :linenos: + + #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_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> |