Hi Tycho,
Ping on the question below!
Thanks,
Michael
On 10/1/20 9:45 AM, Michael Kerrisk (man-pages) wrote:
> On 10/1/20 1:03 AM, Tycho Andersen wrote:
>> On Wed, Sep 30, 2020 at 10:34:51PM +0200, Michael Kerrisk (man-pages) wrote:
>>> Hi Tycho,
>>>
>>> Thanks for taking time to look at the page!
>>>
>>> On 9/30/20 5:03 PM, Tycho Andersen wrote:
>>>> On Wed, Sep 30, 2020 at 01:07:38PM +0200, Michael Kerrisk (man-pages) wrote:
>
> [...]
>
>>>>> ┌─────────────────────────────────────────────────────┐
>>>>> │FIXME │
>>>>> ├─────────────────────────────────────────────────────┤
>>>>> │Interestingly, after the event had been received, │
>>>>> │the file descriptor indicates as writable (verified │
>>>>> │from the source code and by experiment). How is this │
>>>>> │useful? │
>>>>
>>>> You're saying it should just do EPOLLOUT and not EPOLLWRNORM? Seems
>>>> reasonable.
>>>
>>> No, I'm saying something more fundamental: why is the FD indicating as
>>> writable? Can you write something to it? If yes, what? If not, then
>>> why do these APIs want to say that the FD is writable?
>>
>> You can't via read(2) or write(2), but conceptually NOTIFY_RECV and
>> NOTIFY_SEND are reading and writing events from the fd. I don't know
>> that much about the poll interface though -- is it possible to
>> indicate "here's a pseudo-read event"? It didn't look like it, so I
>> just (ab-)used POLLIN and POLLOUT, but probably that's wrong.
>
> I think the POLLIN thing is fine.
>
> So, I think maybe I now understand what you intended with setting
> POLLOUT: the notification has been received ("read") and now the
> FD can be used to NOTIFY_SEND ("write") a response. Right?
>
> If that's correct, I don't have a problem with it. I just wonder:
> is it useful? IOW: are there situations where the process doing the
> NOTIFY_SEND might want to test for POLLOUT because the it doesn't
> know whether a NOTIFY_RECV has occurred?
>
> Thanks,
>
> Michael
>
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
