#include <stdio.h>
#include <stdlib.h>
#include <termios.h>
#include <sys/ioctl.h>
#include <linux/serial.h>

/* ... */

int fd;
unsigned int status, oldstate, newstate, changed, mask;

/* Assuming fd is an open file descriptor for (one of) the
 * serial port(s) (/dev/ttyS[0-9]).
 */

/* To wait for *any* of the 4 control lines to change state,
 * use this instead of the line below:
 * mask = TIOCM_CAR | TIOCM_DSR | TIOCM_CTS | TIOCM_RNG;
 */
mask = TIOCM_RNG;

/* Get current status of the control lines in mask */
if (ioctl(fd, TIOCMGET, &status) == -1) {
    perror("ioctl(TIOCMGET)");
    exit(1);
}
oldstate = status & mask; /* Save current status of control lines */


/* The ioctl() call below sleeps, and does not return until one 
 * of the  serial control lines specified in mask changes state,
 * or it was interrupted by some signal.
 */
rewait:  /* Goto-label for when we get a signal. */
if (ioctl(fd, TIOCMIWAIT, mask) == -1) {
    if (errno == EINTR) {
        /* Signal received. Handle this case. Here we're
         * just restarting the ioctl-wait call.
         */
        goto rewait;
    } else {
        /* Error condition */
        perror("ioctl(TIOCMIWAIT)");
        exit(1);
    }
}


/* Get new status of the control lines in mask */
if (ioctl(fd, TIOCMGET, &status) == -1) {
    perror("ioctl(TIOCMGET)");
    exit(1);
}
newstate = status & mask;

/* XOR old with new status to find the one(s) that changed */
changed  = newstate ^ oldstate;

/* Do something with the result of al this */
if (changed & TIOCM_RNG) {   /* RNG line has changed */
    if (newstate  & TIOCM_RNG) {
        printf("RNG went from: 0 --> 1\n");
    } else {
        printf("RNG went from: 1 --> 0\n");
    }
}