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W2K BUG: timeout of serial driver uses absolute time

, 28 Sep 2000
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The serial driver uses an absolute date/time instead of a relative time for its timeout, causing problems in situations like daylight savings.
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During testing we uncovered a bug in the standard serial driver of Windows 2000.

If you do a ReadFile() with a certain timeout value and you set the date/time backwards while this timeout has not yet occurred then you can wait a until the specified date/time is reached again before the call returns.

This means that the serial driver uses an absolute date/time instead of a relative time for its timeout. This is a serious problem for everyone doing serial communication especially in daylight saving areas.

This problem was found in Windows 2000 / SP1 with the standard serial port driver.

Just run the following code and change the date/time backwards to see the problem:

#include <windows.h>
#include <stdio.h>
#include <conio.h>

int main(void)
   COMMTIMEOUTS timeouts;
   HANDLE handle;
   DCB dcb;
   char buffer[1000] = "abcdefghijklmnopqrstuvwxyz";
   DWORD dwLen;

   handle = CreateFile("\\\\.\\COM1", GENERIC_READ | GENERIC_WRITE,

   if (handle == INVALID_HANDLE_VALUE)
      printf("CreateFile() Failed\n");
      return 1;

   timeouts.ReadIntervalTimeout        = 300;
   timeouts.ReadTotalTimeoutMultiplier = 0;
   timeouts.ReadTotalTimeoutConstant   = 0;
   timeouts.WriteTotalTimeoutMultiplier= 0;
   timeouts.WriteTotalTimeoutConstant  = 0;
   if (!SetCommTimeouts(handle, &timeouts))
      printf("SetCommTimeouts() Failed\n");
      return 2;

   ZeroMemory(&dcb, sizeof(dcb));

   dcb.DCBlength           = sizeof(dcb);
   dcb.BaudRate            = 19200; 
   dcb.fParity             = TRUE;
   dcb.fOutxCtsFlow        = false;
   dcb.fOutxDsrFlow        = false;
   dcb.fDtrControl         = DTR_CONTROL_ENABLE;
   dcb.fDsrSensitivity     = FALSE;
   dcb.fTXContinueOnXoff   = FALSE;
   dcb.fOutX               = false;
   dcb.fInX                = false;
   dcb.fNull               = FALSE;
   dcb.fRtsControl         = RTS_CONTROL_ENABLE;
   dcb.fAbortOnError       = false;
   dcb.ByteSize            = 8;
   dcb.Parity              = ODDPARITY;
   dcb.StopBits            = ONESTOPBIT;

   if (!SetCommState(handle, &dcb))
      printf("SetCommState() Failed\n");
      return 1;

   while (! _kbhit() )

      if (!WriteFile(handle, buffer, 40, &dwLen, 0))
         printf("\rWriteFile() returned %d\n", GetLastError());


      if ( !ReadFile(handle, buffer, 100, &dwLen, 0) )
         printf("\rReadFile() returned %d\n", GetLastError());



   return 0;


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About the Author

Ben Kokx
United States United States
No Biography provided

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Comments and Discussions

QuestionHelp-me ? Pin
Alexandte1-Nov-02 7:37
sussAlexandte1-Nov-02 7:37 
Generali have the problem on one machine ! Pin
Anonymous24-Apr-01 3:13
memberAnonymous24-Apr-01 3:13 
GeneralNo way !! Pin
Riccardo Modesti28-Mar-01 3:02
memberRiccardo Modesti28-Mar-01 3:02 
GeneralOK under W2K/No SP Pin
Serge Wautier2-Oct-00 22:34
sussSerge Wautier2-Oct-00 22:34 

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