Clearing O_NONBLOCK from a pipe may lose data

Andrey Repin
Fri Feb 20 02:34:00 GMT 2015

Greetings, Thomas Wolff!

> Am 19.02.2015 um 10:51 schrieb Corinna Vinschen:
>> On Feb 18 22:08, Lasse Collin wrote:
>>> (Please Cc me when replying, I'm not subscribed to the list.)
>>> Hi!
>>> I suspect that there is a bug in Cygwin:
>>> 1. Create a pipe with both ends in blocking mode (O_NONBLOCK
>>>     is not set).
>>> 2. The writer sets its end to non-blocking mode.
>>> 3. The writer writes to the pipe.
>>> 4. The writer restores its end of the pipe to blocking mode
>>>     before the reader has read anything from the pipe.
>>> 5. The writer closes its end of the pipe.
>>> 6. The reader reads from the pipe in blocking mode. The last
>>>     bytes written by the writer never appear at the reader,
>>>     thus data is silently lost.
>>> Omitting the step 4 above makes the problem go away.
>> I can imagine.  A few years back, when changing the pipe code to
>> using overlapped IO, we stumbled over a problem in Windows.  When
>> closing an overlapped pipe while I/O is still ongoing, Windows
>> simply destroys the pipe buffers without flushing the data to the
>> reader.  This is not much of a problem for blocking IO, but it
>> obviously is for non-blocking.
>> The workaround for this behaviour is this:  If the pipe is closed, and
>> this is the writing side of a nonblocking pipe, a background thread gets
>> started which keeps the overlapped structure open and continues to wait
>> for IO completion (i.e. the data has been sent to the reader).
>> However, if you switch back to blocking before closing the pipe, the
>> aforementioned mechanism does not kick in.
> Could not "switching back to blocking" simply be handled like closing as 
> far as the waiting is concerned,
> thus effectively flushing the pipe buffer?

You can't "just flush" it, if the receiving end isn't reading from it.

Andrey Repin ( 20.02.2015, <02:46>

Sorry for my terrible english...

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