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rtems-docs/c-user/semaphore/directives.rst
Sebastian Huber 16ee8cf45b c-user: Split up semaphore manager 
This makes it easier to automatically generate parts of the manager
documentation in the future.

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.. 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>
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