setting EXTPROC on the master side works on Linux

.gitignore

/extproc

extproc.c

/** from @Hans Lub's answer
https://stackoverflow.com/questions/21641754/when-pty-pseudo-terminal-slave-fd-settings-are-changed-by-tcsetattr-how-ca
 Demo program for managing a pty in packet mode with the slave's
** EXTPROC bit set, where the master gets notified of changes in the
** slaves terminal attributes
**
** save as extproc.c, compile with gcc -o extproc extproc.c -lutil
*/



#include <stdio.h>
#include <pty.h>
#include <termios.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <sys/select.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <stdlib.h>

#define BUFSIZE 512

void main() {
  int master;      // fd of master side
  pid_t pid;

  if((pid = forkpty(&master, NULL, NULL, NULL))) { // we're parent
    fd_set rfds, xfds;
    int retval, nread, status = 0, nonzero = 1;
    char buf[BUFSIZE];

    ioctl(master, TIOCPKT, &nonzero); // initiate packet mode - necessary to get notified of
                                      // ioctl() on the slave side
    struct termios tio;
    // First set the EXTPROC bit in the XXX *master* end termios structure
    tcgetattr(master, &tio);
    tio.c_lflag |= EXTPROC;
    tcsetattr(master, TCSANOW, &tio);

    while(1) {
      // prepare the file descriptor sets
      FD_ZERO(&rfds);
      FD_SET(master, &rfds);

      FD_ZERO(&xfds);
      FD_SET(master, &xfds);

      // now wait until status of master changes
      printf("---- waiting for something to happen -----\n");
      select(1 + master, &rfds, NULL, &xfds, NULL);

      char *r_text = (FD_ISSET(master, &rfds) ? "master ready for reading" : "- ");
      char *x_text = (FD_ISSET(master, &xfds) ? "exception on master" : "- ");

      printf("rfds: %s, xfds: %s\n", r_text, x_text);
      if ((nread = read(master, buf, BUFSIZE-1)) < 0)
        perror("read error");
      else {
        buf[nread] = '\0';
        // In packet mode *buf will be the status byte , and buf + 1 the "payload"
        char *pkt_txt = (*buf &  TIOCPKT_IOCTL ? " (TIOCPKT_IOCTL)" : "");
        printf("read %d bytes: status byte %x%s, payload <%s>\n", nread, *buf, pkt_txt, buf + 1);
      }
      if (waitpid(pid, &status, WNOHANG) && WIFEXITED(status)) {
        printf("child exited with status %x\n", status);
        exit(EXIT_SUCCESS);
      }
    }
  } else { // child


    // Wait a bit and do an ordinary write()
    sleep(1);
    write(STDOUT_FILENO,"blah", 4);

    // Wait a bit and change the pty terminal attributes. This will be picked up by the master end
    sleep(1);
    struct termios tio;
    tcgetattr(STDIN_FILENO, &tio);
    tio.c_cc[VINTR] = 0x07;
    tcsetattr(STDIN_FILENO, TCSANOW, &tio);

    // Wait a bit and exit
    sleep(1);
  }
}

Makefile

NAME := $(patsubst %.c,%,$(wildcard *.c))
LDLIBS= -lutil

run: $(NAME)
	./$(NAME)

Awesome, I wander how you knew how to do this, I haven't find any information on this.
Can pty be used if we have serial in userspace and want to treat it as a regular linux device in /dev ? (which can be opened and set baudread, and read/write to) , thanks,.