[PATCH] c-user: Add self-contained objects chapter

[Gedare Bloom]

On Fri, Feb 3, 2017 at 3:09 AM, Sebastian Huber
<sebastian.huber at embedded-brains.de> wrote:
>
>
> 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.
>
OK continue to use rtems_binary_semaphore.

>>
>> Do the counting/binary semaphores also use priority inheritance?
>
>
> No, they have no owner. The are not useful for mutual exclusion.
>
Ah. This should be mentioned so the casual user does not trip on it.

>>
>> 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
>
Alright that makes sense. Briefly mention this in the doc as well please.

>
>
> --
> Sebastian Huber, embedded brains GmbH
>
> Address : Dornierstr. 4, D-82178 Puchheim, Germany
> Phone   : +49 89 189 47 41-16
> Fax     : +49 89 189 47 41-09
> E-Mail  : sebastian.huber at embedded-brains.de
> PGP     : Public key available on request.
>
> Diese Nachricht ist keine geschäftliche Mitteilung im Sinne des EHUG.
>