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SPDX-License-Identifier: CC-BY-SA-4.0 OR BSD-2-Clause
brief: |
Flushes the semaphore.
copyrights:
- Copyright (C) 2021 embedded brains GmbH (http://www.embedded-brains.de)
- Copyright (C) 1988, 2008 On-Line Applications Research Corporation (OAR)
definition:
default:
attributes: null
body: null
params:
- ${../../type/if/id:/name} ${.:/params[0]/name}
return: ${../../status/if/code:/name}
variants: []
description: |
This directive unblocks all tasks waiting on the semaphore specified by
${.:/params[0]/name}. The semaphore's count is not changed by this
directive. Tasks which are unblocked as the result of this directive will
return from the ${obtain:/name} directive with a status code of
${../../status/if/unsatisfied:/name} to indicate that the semaphore was not
obtained.
enabled-by: true
index-entries:
- flush a semaphore
- unblock all tasks waiting on a semaphore
interface-type: function
links:
- role: interface-placement
uid: header
- role: interface-ingroup
uid: group
- role: constraint
uid: ../constraint/release-isr
- role: constraint
uid: ../constraint/release-devinit
- role: constraint
uid: /constraint/directive-ctx-task
- role: constraint
uid: /constraint/unblock-may-preempt
- role: constraint
uid: /constraint/directive-remote
name: rtems_semaphore_flush
notes: |
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 local, binary semaphore using the MrsP locking
protocol and any attempt to do this will just return the
${../../status/if/not-defined:/name} status code. This error can only happen
in SMP configurations.
For barrier synchronization, the ${../../barrier/if/group:/name} offers a
cleaner alternative to using the semaphore flush directive. Unlike POSIX
barriers, they have a manual release option.
Using the 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
:linenos:
#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 );
}
params:
- description: |
is the semaphore identifier.
dir: null
name: id
return:
return: null
return-values:
- description: |
The requested operation was successful.
value: ${../../status/if/successful:/name}
- description: |
There was no semaphore associated with the identifier specified by
${.:/params[0]/name}.
value: ${../../status/if/invalid-id:/name}
- description: |
The semaphore resided on a remote node.
value: ${../../status/if/illegal-on-remote-object:/name}
- description: |
Flushing a semaphore using the MrsP locking protocol is undefined
behaviour.
value: ${../../status/if/not-defined:/name}
type: interface
|
