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c-users: Clarify semaphore obtain/flush

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Sebastian Huber 2017-11-16 10:29:43 +01:00
parent c6066d4532
commit d9ecff105d
1 changed files with 43 additions and 1 deletions
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44
c-user/semaphore_manager.rst
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@ -614,6 +614,11 @@ DESCRIPTION:
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.
@ -734,7 +739,7 @@ DIRECTIVE STATUS CODES:
* - ``RTEMS_INVALID_ID``
- invalid semaphore id
* - ``RTEMS_NOT_DEFINED``
- operation not defined for the protocol ofthe semaphore
- operation not defined for the protocol of the semaphore
* - ``RTEMS_ILLEGAL_ON_REMOTE_OBJECT``
- not supported for remote semaphores
@ -762,6 +767,43 @@ NOTES:
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 );
}
.. raw:: latex
\clearpage