/dev/windows and select() [was Re: Slow response to keypresses in xorg-server-1.8.0-1]

[Jon TURNEY <jon.turney@dronecode.org.uk>]

[-- Attachment #1: Type: text/plain, Size: 1046 bytes --]

On 08/08/2010 12:04, Andy Koppe wrote:
> On 7 August 2010 23:07, Jon TURNEY wrote:
>> Hmmm, looking again at the implementation of select(), I don't immediately
>> see that when waiting on /dev/windows, it checks that the message queue has
>> old messages on it before waiting.  The MSDN documentation for
>> MsgWaitForMultipleObjects() seems to says that messages which had arrived
>> before the last PeekMessage() etc. aren't considered new and so don't end
>> the wait?
>
> I think you're right, a call to PeekMessage is needed for proper
> select() semantics: it shouldn't block if data is available for
> reading.

Attached is a small test-case which seems to demonstrate this problem.

Run ./dev-windows-select-test and observe select() blocks for the full 
timeout, despite the fact that the /dev/windows fd is ready for reading (and 
it reported as such as the end of the timeout)

If you run './dev-windows-select-test -skip' to skip the PeekMessage(), 
select() returns immediately, indicating the /dev/windows fd is ready for reading.

[-- Attachment #2: dev-windows-select-test.c --]
[-- Type: text/plain, Size: 818 bytes --]


#include <stdio.h>
#include <fcntl.h>
#include <sys/select.h>
#include <errno.h>
#include <windows.h>

// gcc -o dev-windows-select-test.exe dev-windows-select-test.c -Wall -mwindows

int main(int argc, char *argv[])
{
  int fd = open("/dev/windows", O_RDONLY);

  if (PostMessage(NULL, WM_USER, 0, 0) != 0)
    printf("PostMessage succeeded\n");
  else
    printf("PostMessage failed\n");

  if (argc <= 1)
    {
      MSG msg;
      if (PeekMessage(&msg, NULL, 0, 0, PM_NOREMOVE))
        printf("PeekMessage reports a message available\n");
    }

  struct timeval timeout;
  timeout.tv_sec = 5;
  timeout.tv_usec = 0;

  fd_set readfds;
  FD_ZERO(&readfds);
  FD_SET(fd, &readfds);

  int rc = select(fd+1, &readfds, NULL, NULL, &timeout);
  printf("select returned %d %s\n", rc, strerror(errno));

  return 0;
}

[-- Attachment #3: Type: text/plain, Size: 223 bytes --]

--
Unsubscribe info:      http://cygwin.com/ml/#unsubscribe-simple
Problem reports:       http://cygwin.com/problems.html
Documentation:         http://x.cygwin.com/docs/
FAQ:                   http://x.cygwin.com/docs/faq/