C.1. Sniffer by Gerd Knorr

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <fcntl.h>
#include <termios.h>
#include <ctype.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/ioctl.h>

#define BUFSIZE 1024

int
read_and_print(int fd, int sec, int usec)
{
    int         rc,l,i;
    char        buf[BUFSIZE+1];
    fd_set      set;
    struct timeval  tv;
    
    if (sec || usec) {
    FD_ZERO(&set);
    FD_SET(fd,&set);
    tv.tv_sec  = sec;

    tv.tv_usec = usec;
    if (0 == select(fd+1,&set,NULL,NULL,&tv))
        return -1;
    }
    
    switch (rc = read(fd,buf,BUFSIZE)) {
    case 0:
    printf("EOF");
    exit(0);
    break;
    case -1:
    perror("read");
    exit(1);
    default:
    for (l = 0; l < rc; l+= 16) {
        printf("%04x  ",l);
        for (i = l; i < l+16; i++) {
        if (i < rc)
            printf("%02x ",buf[i]);
        else
            printf("-- ");
        if ((i%4) == 3)
            printf(" ");
        }
        for (i = l; i < l+16; i++) {

        if (i < rc)
            printf("%c",isalnum(buf[i]) ? buf[i] : '.');
        }
        printf("\n");
    }
    break;
    }
    return rc;
}

void
setlines(int fd, int rts, int dtr)
{
    int lines = 0;

    if (rts) lines |= TIOCM_RTS;
    if (dtr) lines |= TIOCM_DTR;
    
    ioctl(fd,TIOCMSET,&lines);
}

int main(int argc, char *argv[])
{
    int         ser,i;
    struct termios      saved_attributes,tattr;
    struct winsize      win;
    char        buf[16];
    
    if (-1 == (ser = open("/dev/ttyS1",O_RDWR))) {
    perror("open /dev/ttyS1");
    exit(1);
    }
    
    /* Set the terminal mode */
    tcgetattr (ser, &tattr);
    cfmakeraw (&tattr);
    cfsetospeed (&tattr,B9600);
    cfsetispeed (&tattr,B9600);
    tcsetattr (ser, 0, &tattr);
    
    setlines(ser,0,0);
#if 0
    tcsendbreak(ser,0);
#endif

    /* main loop */
    fprintf(stderr,"setup done\n");
    while (-1 != read_and_print(ser,30,0)) {
    usleep(100000);
    }
    
    return 0;
}