From: "Åke Rehnman" <ake.rehnman@gmail.com>
To: cygwin@cygwin.com
Subject: Re: Setting termios VMIN > 0 and VTIME > 0 on non blocking file
Date: Sat, 14 Mar 2020 11:36:39 +0100 [thread overview]
Message-ID: <8e0973e2-9554-3e69-fa88-ed51fad5b191@gmail.com> (raw)
In-Reply-To: <2156fb6d-cba9-b70e-f8c0-7043ff52ffc3@gmail.com>
On 2020-03-14 11:23, Åke Rehnman wrote:
>
> On 2020-03-13 11:12, Corinna Vinschen wrote:
>> On Mar 12 18:04, Åke Rehnman via Cygwin wrote:
>>> On 2020-03-12 16:08, Corinna Vinschen wrote:
>>>> On Mar 12 15:44, Corinna Vinschen wrote:
>>>>> On Mar 12 15:20, Åke Rehnman via Cygwin wrote:
>>>>>> I think the problem is if the number of bytes requested are more
>>>>>> than what
>>>>> To clarify: number of bytes == VMIN?
>>> no number of bytes requested from ReadFile(). As far as I know Win32
>>> has no
>>> concept of VMIN.
>> Right, just this weird TimeoutMultiplier, but nevertheless I wasn't sure
>> what you meant.
>>
>>>>>> is in the buffer it is going to overlap the read function
>>>>>> (because of VTIME)
>>>>>> and immediately after that CancelIO is called. Contrary to what
>>>>>> is mentioned
>>>>>> in the source code I think CancelIO is actually discarding data...
>>>>> So far we didn't have that experience. CancelIO is usually safe
>>>>> in this regard.
>>> The data is MIA somehow...
>> Yes, but we're calling CancelIo in other circumstances in Cygwin and
>> there were no reports of missing data. CancelIo is just supposed to
>> terminate the currently running overlapped IO, not to discard any
>> in-flight data. If that's different for serial IO, there would be no
>> way to terminate serial overlapped IO gracefully. Well, yeah, it's
>> Windows, but still...
> I have scrutinized the microsoft serial driver reference
> implementation
> (https://github.com/microsoft/Windows-driver-samples/blob/master/serial/serial/read.c)
> and it looks to me as CancelIo does not purge any data. However since
> the driver is vendor dependent in my case FTDI it is impossible to
> know exactly what is going on since it is closed source.
>>>>>> - if ((vmin_ > 0) && (vtime_ == 0))
>>>>>> + if (is_nonblocking())
>>>>>> + {
>>>>>> + to.ReadIntervalTimeout = MAXDWORD;
>>>>>> + }
>>>>>> + else if ((vmin_ > 0) && (vtime_ == 0))
>>>>> What if you switch to !O_NONBLOCK after calling tcsetattr? The
>>>>> setting of ReadIntervalTimeout would be lost then.
>>>>>
>>>>> Either we have to repeat calling SetCommTimeouts every time
>>>>> we switch mode, or we have to do the above setting temporary
>>>>> every time we call ReadFile in non blocking mode.
>>> True.
>>>> What about this:
>>>>
>>>> diff --git a/winsup/cygwin/fhandler_serial.cc
>>>> b/winsup/cygwin/fhandler_serial.cc
>>>> --- a/winsup/cygwin/fhandler_serial.cc
>>>> +++ b/winsup/cygwin/fhandler_serial.cc
>>>> @@ -68,6 +68,16 @@ fhandler_serial::raw_read (void *ptr, size_t& ulen)
>>>> goto err;
>>>> else if (ev)
>>>> termios_printf ("error detected %x", ev);
>>>> + else if (is_nonblocking ())
>>>> + {
>>>> + if (!st.cbInQue)
>>>> + {
>>>> + tot = -1;
>>>> + set_errno (EAGAIN);
>>>> + goto out;
>>>> + }
>>>> + inq = st.cbInQue;
>>>> + }
>>>> else if (st.cbInQue && !vtime_)
>>>> inq = st.cbInQue;
>>>> else if (!is_nonblocking () && !overlapped_armed)
>>> Looks promising. I will try it.
> Your patch works (for my test case and screen). Question is if we have
> to consider the case where ulen==0 ...
>>>
>>> BTW there is a gremlin in the "else if (ev)" line....
>> A gremlin? Would you mind to explain? Btw., if you find a bug
>> in the code, we do take patches :) https://cygwin.com/contrib.html
> If we have an error event in ev it will make a blocking read even if
> VTIME==0.
>
I forgot, also any CancelIo should be terminated with a blocking
GetOverlappedResult() see this excellent blog post
https://devblogs.microsoft.com/oldnewthing/20110202-00/?p=11613
// took longer than 1 second - cancel it and give up
CancelIo(h);
WaitForSingleObject(o.hEvent, INFINITE); // added // Alternatively:
GetOverlappedResult(h, &o, TRUE);
return WAIT_TIMEOUT;
/Ake
next prev parent reply other threads:[~2020-03-14 10:36 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <mailman.1473.1583981776.15804.cygwin@cygwin.com>
2020-03-11 20:48 ` Åke Rehnman
2020-03-11 21:55 ` Brian Inglis
2020-03-12 1:01 ` Norton Allen
2020-03-12 1:04 ` Åke Rehnman
2020-03-12 1:08 ` Norton Allen
2020-03-12 1:39 ` Åke Rehnman
2020-03-12 8:05 ` Thomas Dickey
2020-03-12 11:01 ` Åke Rehnman
2020-03-12 19:48 ` Achim Gratz
2020-03-12 11:40 ` Corinna Vinschen
2020-03-12 13:32 ` Åke Rehnman
2020-03-12 14:13 ` Corinna Vinschen
2020-03-12 16:42 ` Åke Rehnman
2020-03-13 10:02 ` Corinna Vinschen
2020-03-12 14:20 ` Åke Rehnman
2020-03-12 14:44 ` Corinna Vinschen
2020-03-12 15:08 ` Corinna Vinschen
2020-03-12 17:04 ` Åke Rehnman
2020-03-13 10:12 ` Corinna Vinschen
2020-03-14 10:23 ` Åke Rehnman
2020-03-14 10:36 ` Åke Rehnman [this message]
2020-03-16 9:35 ` Corinna Vinschen
2020-03-17 12:16 ` Corinna Vinschen
2020-03-17 15:54 ` Åke Rehnman
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