On 4/27/2012 3:38 PM, James Johnston wrote: > So I think for sure, Cygwin's use of message pipes is breaking a lot > of Windows software, because of the null writes. And ALSO additionally > perhaps because of this: while reading MSDN today, I came across an > interesting snippet that probably indicates a second source of problems > I have not explicitly tested for, but would probably find: > > "If the specified number of bytes to read is less than the size of > the > next message, the function reads as much of the message as possible > before returning zero (the GetLastError function returns > ERROR_MORE_DATA). The remainder of the message can be read using another > read operation."
Confusingly, a PIPE_TYPE_MESSAGE pipe's read end can be in either PIPE_READMODE_MESSAGE or PIPE_READMODE_BYTE mode. You're describing the behavior of PIPE_READMODE_MESSAGE; Cygwin's pipes are in PIPE_READMODE_BYTE, so they don't have this particular problem --- although the null-write problem remains. > 1. Why on earth did Cygwin start using message pipes? "adopted to > fix > a problem" doesn't help... Googling site:Cygwin.com "message pipe" - > didn't yield anything either. I do know older Cygwin versions worked > fine, and I had no problem with piping data to/from both Cygwin and > non-Cygwin programs. I am sure it was changed for a good reason, but > would someone be willing to offer an explanation for why message pipes > are now used? Do they have problems reading from a byte pipe? Writing to > a byte pipe? Both? Under what conditions? IIRC, it had something to do with preserving message boundaries at the TTY later. Again IIRC (and I only faintly recall this part) the PTY layer has to deliver one line at a time to readers. If a client calls read(2) with a giant buffer, he's still supposed to get back only a single line at a time. A message-mode pipe seems like a good way to provide these semantics because it preserves write boundaries --- so if you write only a line at a time, the reader reads only a line at a time. It's a shame that this approach causes problems. Why can't only pseudoterminal pipes be created as PIPE_TYPE_MESSAGE? Cygwin already has special logic to tell whether a handle it's opening is a pty and to treat it specially in that case. -- Problem reports: http://cygwin.com/problems.html FAQ: http://cygwin.com/faq/ Documentation: http://cygwin.com/docs.html Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple