How to send to virtual serial port, and read from another program?

Question

I am trying to write some data to a serial port, that I will read from forexample screen in the same computer. I have tried looking at this answer how-to-send-data-to-a-serial-port-and-see-any-answer
I have added my user to group dialout and ran chmod o+rw /dev/ttyS1
Then in python I do like this, and get following error:

>>> import serial
>>> ser = serial.Serial("/dev/ttyS1",4800)  

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/el3/Envs/serial/lib/python2.7/site-packages/serial/serialutil.py", line 236, in __init__
    self.open()
  File "/home/el3/Envs/serial/lib/python2.7/site-packages/serial/serialposix.py", line 272, in open
    self._reconfigure_port(force_update=True)
  File "/home/el3/Envs/serial/lib/python2.7/site-packages/serial/serialposix.py", line 315, in _reconfigure_port
    raise SerialException("Could not configure port: {}".format(msg))
serial.serialutil.SerialException: Could not configure port: (5, 'Input/output error')

So the question is, what do I do wrong here?

Or should I look for a different approach.

The issue is, that I got some udp messages that I can receive in for example python, that I need to send to a software, that only sees serial ports.
Maybe there is a different approach?
In that case, what is a good way to do that?

Answer 1

If you need to connect two programs, one of which insisting that it uses a tty-device (of which a serial port is an example), you can instead connect them using a pseudoterminal pair.

You can either do it from Python with the standard pty module, or you use a small C program that just creates the pair, and then access it from the shell, a Python program or whatever just like a normal file.

Your suggestion from the comment uses a kernel module, which is overkill and not as portable.

Here's an example C program I have used in a similar situation, it's not very polished and needs some way to communicate the names used.

/* ptycat (ptypipe? ptypair?)
 *
 * create a pair of pseudo-terminal slaves connected to each other
 *
 * Link with -lutil
 */

#include <stdlib.h>
#include <stdio.h>
#include <stdint.h>

#include <errno.h>
#include <termios.h>
#include <unistd.h>
#include <pty.h>

#undef max
#define max(x,y) ((x) > (y) ? (x) : (y))

/*
  (void)ioctl(STDIN_FILENO, TIOCGWINSZ, &win);

*/

/* TODO: make symlinks, unlink on atexit */

static uint8_t buf[BUFSIZ]; /* BUFSIZ from stdio.h, at least 256 */
static char *log_dir = NULL;

void logdata (char *dir, uint8_t *data, int n) {
  if (dir != log_dir) fprintf (stdout, "\n%s", dir);
  log_dir = dir;
  for (; n > 0; n--, data++) fprintf (stdout, " %02x", *data);
  fflush (stdout);
}

int main (int argc, char* argv[])
{
  char name[256]; /* max namelen = 255 for most fs. */
  fd_set rfd;

  struct termios tt;
  struct winsize ws;
  int master[2], slave[2];
  int n, nfds, cc;

  if (tcgetattr (STDIN_FILENO, &tt) < 0)
  {
    perror("Cannot get terminal attributes of stdin");
    exit(1);
  }
  cfmakeraw (&tt);
  for (int i = 0; i < 2; i++)
  {
    if (openpty (&master[i], &slave[i], name, &tt, NULL /*ws*/) < 0)
    {
      perror("Cannot open pty");
      exit(1);
    }
    puts(name);
  }

  for (;;) {
    FD_ZERO(&rfd);  
    FD_SET(master[0], &rfd);
    FD_SET(master[1], &rfd);
    nfds = max(master[0], master[1]) + 1;
    n = select(nfds, &rfd, 0, 0, NULL);
    if (n > 0 || errno == EINTR)
    {
      if (FD_ISSET(master[0], &rfd))
      {
    if ((cc = read(master[0], buf, sizeof(buf))) > 0)
    {
      (void) write(master[1], buf, cc);
      logdata (">>>", buf, cc);
    }
      }

      if (FD_ISSET(master[1], &rfd))
      {
    if ((cc = read(master[1], buf, sizeof(buf))) > 0)
    {
      (void) write(master[0], buf, cc);
      logdata ("<<<", buf, cc);
    }
      }
    }
  }
  /*    This never reached */
  return 0; 
}