posix_openpt —
open a pseudo-terminal device
Standard C Library (libc, -lc)
The posix_openpt() function searches for an unused
  master pseudo-terminal device, opens it, and returns a file descriptor
  associated with the now used pseudo-terminal device. The
  oflag argument has the same meaning as in the
  open(2) call.
If successful, posix_openpt() returns a non-negative
  integer, which corresponds to a file descriptor pointing to the master
  pseudo-terminal device. Otherwise, a value of -1 is returned and
  errno is set to indicate the error.
The standards committee did not want to directly expose the cloning device, thus
  decided to wrap the functionality in this function. The equivalent code would
  be:
	int
	posix_openpt(int oflag) {
		return open("/dev/ptmx", oflag);
	}
 
The posix_openpt() function conforms to
  IEEE Std 1003.1-2001 (“POSIX.1”).