ttyS1 loopback test

ttyS1 loopback test

While trying to do a loop back test on ttyS1, I am able to read the data
from ttyS1 without a loop back connector connected to my ttyS1 port.
Please suggest how I can clear ttyS1 buffer so that data is only read when
I have the loop back connector connected.
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <termios.h>
#define UART_SPEED B115200
char buf[512];
void init_serial (int fd)
{
struct termios termios;
int res;
res = tcgetattr (fd, &termios);
if (res < 0) {
fprintf (stderr, "Termios get error: %s\n", strerror (errno));
exit (-1);
}
cfsetispeed (&termios, UART_SPEED);
cfsetospeed (&termios, UART_SPEED);
termios.c_iflag &= ~(IGNPAR | IXON | IXOFF);
termios.c_iflag |= IGNPAR;
termios.c_cflag &= ~(CSIZE | PARENB | CSTOPB | CREAD | CLOCAL);
termios.c_cflag |= CS8;
termios.c_cflag |= CREAD;
termios.c_cflag |= CLOCAL;
termios.c_lflag &= ~(ICANON | ECHO);
termios.c_cc[VMIN] = 1;
termios.c_cc[VTIME] = 0;
res = tcsetattr (fd, TCSANOW, &termios);
if (res < 0) {
fprintf (stderr, "Termios set error: %s\n", strerror (errno));
exit (-1);
}
}
int main (int argc, char **argv)
{
int fd,fi;
int res;
int i;
if (argc < 2) {
fprintf (stderr, "Please enter device name\n");
return -1;
}
fd = open (argv[1], O_RDWR | O_NOCTTY);
if (fd < 0) {
fprintf (stderr, "Cannot open %s: %s\n", argv[1], strerror(errno));
return -1;
}
init_serial (fd);
tcflush(fd, TCIOFLUSH);
res = write (fd, "P=21\r\n", 7);
if (res < 0) {
fprintf (stderr, "Write error: %s\n", strerror(errno));
return -1;
}
tcdrain (fd);
res = read (fd, buf, 512);
fi=close(fd);
printf("%s\n","close retuns");
printf ("%d\n", fi);
if (res < 0) {
fprintf (stderr, "Read error: %s\n", strerror(errno));
return -1;
}
for (i=0; i<res; i++) {
printf ("%c", buf[i]);
}
for (i=0; i<512; i++) {
buf[i]=0;
}
return 0;
}