2 may
2021

Delay in reading from serial port on Windows

I run a tool on both Linux and Windows to interact with a USB device over a serial port. The tool is written in C/C++ (cygwin on Windows).

On Linux this runs smoothly, but on Windows I get a delay.

I connect like this (leaving out some error checking):

  device = open("/dev/ttyS0", O_RDWR | O_NOCTTY);

  tcflush(device, TCIOFLUSH); // flush before configuring

  termios options;
  tcgetattr(device, &options);
  options.c_cflag = B230400|CS8|CREAD|CLOCAL;
  options.c_iflag = 0;
  options.c_oflag = 0;
  options.c_lflag = 0;

  // avoid canonical mode
  options.c_cc[VMIN] = 255;  // block for up till x characters (0-255)
  options.c_cc[VTIME] = 1;  // x tenths of seconds read timeout

  cfsetospeed(&options, B230400);
  tcsetattr(device, TCSANOW, &options);

The device sends packets of over 100 bytes at 25Hz, but in Windows (not on Linux) data comes through with a delay of about 1 sec. The delays does not change over time.

I already tried:

  • read through Serial Programming Wikibook
  • open with O_NDELAY
  • set termios VMIN/VTIME to 0 (in different combinations)
  • cfmakeraw
  • the tool is multi-threaded, so I disabled all locking to avoid any waiting

Any suggestions would be highly appreciated.

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