Setting termios VMIN > 0 and VTIME > 0 on non blocking file

Åke Rehnman ake.rehnman@gmail.com
Thu Mar 12 14:20:02 GMT 2020


On 2020-03-12 12:40, Corinna Vinschen wrote:
>
> I inspected the serial I/O read function and I only see a subtil
> difference in terms of VMIN/VTIME which doesn't seem to be the culprit
> at first glance.  In O_NONBLOCK mode, the underlying Windows function
> ReadFile is called unconditionally.  My current hunch is this:
>
> - If VMIN>0 && VTIME>0, the Windows equivalent of tcsetattr is
>    told that VMIN>0.
>
> - So, assuming VMIN == 2 in Windows.  If VTIME > 0, the Cygwin read
>    function sets the number of bytes_to_read to 1 (this is old code,
>    don't ask why).
Do you have a pointer to this? I can not find it...
>
> - My assumption now is that the ReadFile function fails because it's
>    supposed to return only 1 char, but VMIN is > 1.
>
> There are two issues here.
>
> - If my assumption is correct, the tcsetattr function must not
>    set the Windows VMIN equivalent to != 0 in the O_NONBLOCK case.
>
> - Also, as mention above, if VTIME > 0, Cygwin's read sets the number of
>    bytes_to_read to 1.   Weirdly, it does so even if there are more than
>    1 byte in the inbound queue.  This sounds wrong to me.
>
I think the problem is if the number of bytes requested are more than 
what 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...

   /* Use CancelIo rather than PurgeComm (PURGE_RXABORT) since
          PurgeComm apparently discards in-flight bytes while CancelIo
          only stops the overlapped IO routine. */


My suggestion is the following patch:

diff --git a/winsup/cygwin/fhandler_serial.cc 
b/winsup/cygwin/fhandler_serial.cc
index 69e5768f6..afa8871bf 100644
--- a/winsup/cygwin/fhandler_serial.cc
+++ b/winsup/cygwin/fhandler_serial.cc
@@ -898,7 +898,11 @@ fhandler_serial::tcsetattr (int action, const 
struct termios *t)
    {
      memset (&to, 0, sizeof (to));

-    if ((vmin_ > 0) && (vtime_ == 0))
+       if (is_nonblocking())
+       {
+               to.ReadIntervalTimeout = MAXDWORD;
+       }
+    else if ((vmin_ > 0) && (vtime_ == 0))
        {
         /* Returns immediately with whatever is in buffer on a ReadFile();
            or blocks if nothing found.  We will keep calling 
ReadFile(); until





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