#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <fcntl.h>


#ifndef ECHO2TTY_DEBUG
#  define ECHO2TTY_DEBUG 0
#endif


int main (int argc, char *argv[])
{
 int fd;

 if (argc != 2)
   {
    fprintf(stderr, "Usage: echo 'XYZ' | echo2tty /dev/ttyXXX\n");
    exit(1);
   }

 fd = open(argv[1], O_RDWR);
 if (fd < 0)
   {
    perror("open DEVICE");
    exit(1);
   }

 {
  char c;
  char* cp = &c;
#if ECHO2TTY_DEBUG
  int i = 0;
#endif

  while (read(0, cp, 1) == 1)
    {
     int r;

     /* NB!
      *    The iosctl(fd, TIOCSTI, ...) works for our own terminal fd,
      *    but FAILS for any other tty fd with "Operation not permitted"
      *    or "Bad address", even for ttys owned by the same user
      *    ("echo 'xxx' > '/dev/pts/2'" sends the data quite successfully
      *     although shell on that tty ignores the received data).
      */
     /* They say ioctl(TIOCSTI) required root privileges
      * (or CAP_SYS_ADMIN to be more specific,
      *  but that's usually the same in practice)
      * BTW, the capabilities(7) man page does NOT mention this fact!
      */
     r = ioctl(fd, TIOCSTI, (const char*)cp);
#if ECHO2TTY_DEBUG
     fprintf(stderr, "ioctl(0x%02x): %i\n", c, r);
     i++;
#endif
     if (r < 0) perror("ioctl");
    }

#if ECHO2TTY_DEBUG
  printf("%s: %i\n", argv[0], i);
#endif
 }
 close(fd);
}