On 02/02/17 16:56, Gedare Bloom wrote:
+.. c:function:: void rtems_binary_semaphore_post(rtems_binary_semaphore
>>>*semaphore)
>>>+
>>>+Posts the binary semaphore. In case at least one thread is waiting on
>>>the
>>>+binary semaphore, then the highest priority thread is woken up,
>>>otherwise the
>>>+current count is set to one.
>>>+
>>>+This function may be called from interrupt context. In case it is
>>>called from
>>>+thread context, then interrupts must be enabled.
>>>
>>What is the difference between mutex and binary semaphore?
>
>
>Yes, there is a big difference. A binary semaphore is a counting semaphore
>those values are restricted to 0 and 1. It has no owner.
>
Whichever thread passes 'wait()' is (implicitly) owning the binary
semaphore. Semantically there is very little difference to me.
Certainly based on the description here I don't know what the
difference is and why if I'm a user I should pick rtems_mutex or
rtems_binary_semaphore.
Ok, the RTEMS term for this "binary semaphore" here is simple binary
semaphore. We could name it rtems_simple_binary_semaphore(), however,
this would be a bit long. Normally, if you need something for mutual
exclusion it is called mutex or lock and not semaphore.
Do the counting/binary semaphores also use priority inheritance?
No, they have no owner. The are not useful for mutual exclusion.
If the binary semaphore is identically equal to a counting semaphore
initialized to 1, then why even include it in the API?
It stays as 1 if you post it again and again. It can be used for
task/interrupt synchronization. One example is Termios
https://git.rtems.org/rtems/tree/cpukit/libcsupport/src/termios.c#n1774
--
Sebastian Huber, embedded brains GmbH
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