VT100 Escape codes and TTY interaction: Why `open` a TTY device as opposed to using stdin/stdout

Question

I had a look into escape codes and found some C code that reads the current cursor position. And I guess I have now stumbled over a topic of UNIX/POSIX programming I until now had not much exposure to.

The below code opens dev/tty in read-write mode and writes the appropriate escape code to the file-descriptor, then receives the answer by reading from the file-descriptor. so programmatically I understand whats going on.

Conceptionally I wonder

1. Why is the TTY device opened (as opposed to stdin/stdout usage)
2. If I open a TTY device, wouldn't I have to first find out the current tty (by opening a pipe to /usr/bin/tty for example, then opening this device).

#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int main(void) {
  int tty_fd = open("/dev/tty", O_RDWR);
  if (tty_fd < 0) {
    printf("Cannot open /devv/tty: errno = %d, %s\r\n", errno, strerror(errno));
    exit(EXIT_FAILURE);
  }

  write(tty_fd, "\x1B[6n\n", 5);

  unsigned char res[16] = {0};
  size_t j = 0;
  for (j = 0; j < sizeof(res) - 1 && read(tty_fd, res + j, 1) == 1; j++) {
    printf("%d\n", res[j]);
    if (res[j] == 'R') {
      break;
      res[j] = '\0';
    }
  }

  printf("Answerback = %s", res + 1);

  return 0;
}

Answer 1

1. Opening and using /dev/tty ensures that the process has a controlling terminal, and that that’s where the escape codes actually go. Writing to standard output and reading from standard input is subject to whatever redirections are in place, if any.

   In fact, running the example program with its output redirected to a file will result in the output going to the file, but the cursor position being read from the terminal (if there is one).

2. /dev/tty is always the current process’ controlling terminal, if it has one. See what relations are between my current controlling terminal and `/dev/tty`? for details. There’s no need for a level of indirection here, /dev/tty provides that itself.