output.txt

> cc test.c
> ./a.out 
tty_fd=3
result=0
TIOCGWINSZ=1074295912
O_EVTONLY | O_NONBLOCK=32772
147 x 46

> crystal test.cr
tty_fd=8
result=0
TIOCGWINSZ=1074295912
O_EVTONLY | O_NONBLOCK=32772
73 x 73

> crystal test.cr
tty_fd=8
result=0
TIOCGWINSZ=1074295912
O_EVTONLY | O_NONBLOCK=32772
1193 x 1193

> ./a.out        
tty_fd=3
result=0
TIOCGWINSZ=1074295912
O_EVTONLY | O_NONBLOCK=32772
147 x 46

test.c

#include <stdio.h>
#include <string.h>    // strerror
#include <errno.h>     // errno
#include <fcntl.h>     // open(), O_EVTONLY, O_NONBLOCK
#include <unistd.h>    // close()
#include <sys/ioctl.h> // ioctl()

int main() {
  int tty_fd = open("/dev/tty", O_EVTONLY | O_NONBLOCK);
  if (tty_fd == -1) {
    fprintf(stderr, "Opening `/dev/tty` failed (%d): %s\n", errno, strerror(errno));
    return 1;
  }

  struct winsize ws;
  int result = ioctl(tty_fd, TIOCGWINSZ, &ws);
  close(tty_fd);

  if (result == -1) {
    fprintf(stderr, "Getting the size failed (%d): %s\n", errno, strerror(errno));
    return 1;
  }

  fprintf(stdout, "tty_fd=%d\nresult=%d\nTIOCGWINSZ=%ld\nO_EVTONLY | O_NONBLOCK=%d\n%d x %d\n", tty_fd, result, TIOCGWINSZ, O_EVTONLY | O_NONBLOCK, ws.ws_col, ws.ws_row);
  return 0;
}

test.cr

lib LibC
  TIOCGWINSZ = 0x40087468
  O_EVTONLY  =     0x8000

  struct Winsize
    ws_row : LibC::UShort
    ws_col : LibC::UShort
    ws_xpixel : LibC::UShort
    ws_ypixel : LibC::UShort
  end

  fun ioctl(fd : LibC::Int, request : LibC::ULong, winsize : LibC::Winsize*) : LibC::Int
end

tty_fd = LibC.open("/dev/tty", LibC::O_EVTONLY | LibC::O_NONBLOCK)
result = LibC.ioctl(tty_fd, LibC::TIOCGWINSZ, out screen_size)

puts "tty_fd=#{tty_fd}\nresult=#{result}\nTIOCGWINSZ=#{LibC::TIOCGWINSZ}\nO_EVTONLY | O_NONBLOCK=#{LibC::O_EVTONLY | LibC::O_NONBLOCK}\n#{screen_size.ws_col} x #{screen_size.ws_col}\n"