Clearing O_NONBLOCK from a pipe may lose data

Thomas Wolff
Fri Feb 20 02:13:00 GMT 2015

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?

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