On Feb 19 23:17, Thomas Wolff wrote: > 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?
Flushing the pipe buffer (using FlushFileBuffers) can block the process indefinitely if the reader doesn't read for some reason. This is a problem in the Windows implementation of pipes. For some reason there doesn't seem to be a kernel buffer which can be used to decouple writer and reader a bit. Corinna -- Corinna Vinschen Please, send mails regarding Cygwin to Cygwin Maintainer cygwin AT cygwin DOT com Red Hat
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