Не работает драйвер для работы приема/передачи по ком порту

#include <stdio.h> // standard input / output functions
#include <string.h> // string function definitions
#include <unistd.h> // UNIX standard function definitions
#include <fcntl.h> // File control definitions
#include <errno.h> // Error number definitions
#include <termios.h> // POSIX terminal control definitionss
#include <time.h>   // time calls
#include <sys/ioctl.h>
#include <linux/serial.h>           // Здесь описана serial_icount_struct
 
int fd;
int open_port(void)
{
int iret;
int fl_flags;
 
     
    fd = open("/dev/ttyS0", O_NONBLOCK | O_RDWR);
 
    if(fd == -1) // if open is unsucessful
    {
        
        printf("open_port: Unable to open /dev/ttyS0. \n");
        return -1;
    }
    else
    {
        fl_flags = fcntl(fd,F_GETFL);
        iret = fcntl(fd, F_SETFL, FNDELAY);
//      iret = fcntl(fd, F_SETFL, fl_flags & ~O_NONBLOCK);
        if (iret)
            printf("Error fcntl %s\n", strerror(errno));
        printf("port is open.\n");
 
    // Переведем устройство в exclusive mode
    iret = ioctl(fd, TIOCEXCL, 0);
    if (iret < 0) {
        return -1;
    };
 
        return 0;
    }
 
}
 
int configure_port()
{
int iret;
    struct termios port_settings;      // structure to store the port settings in
 
    iret = tcgetattr(fd, &port_settings);
    if (iret) 
        printf("Error tcgetattr %s\n", strerror(errno));
    cfsetispeed(&port_settings, B110);   // set baud rates
    cfsetospeed(&port_settings, B110);
 
/*  cfsetispeed(&port_settings, B9600);    // set baud rates
    cfsetospeed(&port_settings, B9600);
 
        port_settings.c_cflag     &= ~(CSIZE | CSTOPB | PARENB | CLOCAL);
        port_settings.c_cflag     |= CS8 | CREAD | HUPCL;
 
        port_settings.c_iflag      = IGNBRK | IGNPAR;
        port_settings.c_oflag      = 0;
        port_settings.c_lflag      = 0;
        port_settings.c_cc[VMIN]   = 1;
        port_settings.c_cc[VTIME]  = 10;
*/
    port_settings.c_iflag      = IGNBRK | IGNPAR;
    port_settings.c_oflag      = 0;
    port_settings.c_lflag      = 0;
    port_settings.c_cc[VMIN]   = 1;
    port_settings.c_cc[VTIME]  = 10;
 
 
    port_settings.c_cflag &= ~PARENB;    // set no parity, stop bits, data bits
 
    port_settings.c_cflag &= ~CSTOPB;
 
    port_settings.c_cflag &= ~CSIZE;
 
    port_settings.c_cflag |= CS8;
    port_settings.c_cflag |= (CLOCAL | CREAD); 
 
    iret = tcsetattr(fd, TCSANOW, &port_settings);    // apply the settings to the port
 
    return 0;
}
 
int read_write()
{
 
 
    unsigned char send_bytes[] = {'h','e','l','l','o'};
    char buffer[5];
 
    int n_write = write(fd, send_bytes, 5);  //Send data
    printf("Wrote the bytes. \n");
    usleep(10000000);
    int n_read=read(fd,buffer,1);
    if (n_read == -1)
        printf("Error read %s\n", strerror(errno));
 
    printf("read %d bytes. \n", n_read);
//  bufptr=buffer;
//  while((n_read=read(fd, bufptr, buffer+sizeof(buffer)-bufptr-1))>0)  
//{
//  bufptr+=n_read;
//} 
printf(buffer); 
 
    return 0;
 
 
 
}
 
int main(void)
{
    open_port();
 
    configure_port();
 
    read_write();
 
    close(fd);
 
    return(0);
 
 
 
}

struct pollfd pfd;
pfd.fd = fd;
pfd.events = POLLIN;
if (poll(&pfd, 1, -1) == -1 || pfd.revents & (POLLERR|POLLHUP|POLLNVAL))
  /* ошибка */
/* пора читать */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif
 
#include <stdio.h>
#include <stdlib.h>
#include <termios.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/signal.h>
#include <sys/types.h>
#include <sys/poll.h>
#include <errno.h>
 
int fd;
 
int main(void)
{
    unsigned char send_bytes[] = {'H','e','l','l','o'};
    char buffer[255];
    int ret;
    int k;
 
        struct termios oldtio,newtio;
 
        fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY| O_NONBLOCK);
        if (fd <0) { printf("error opening port"); return(-1);}
 
        fcntl(fd, F_SETOWN, getpid());
    fcntl(fd, F_SETFL, 0);
 
 
        tcgetattr(fd,&oldtio); /* save current port settings */
 
    newtio.c_ispeed = B9600;
    newtio.c_ospeed = B9600;
        newtio.c_cflag = (CS8 | CLOCAL | CREAD | CRTSCTS);
        newtio.c_iflag = (IGNPAR | ICRNL);
        newtio.c_oflag = OPOST;
        newtio.c_lflag = (ICANON|ECHO|ECHOE|ISIG);
        newtio.c_cc[VMIN]=1;
        newtio.c_cc[VTIME]=10;
        tcflush(fd, TCIFLUSH);
        tcsetattr(fd,TCSANOW,&newtio);
 
    struct pollfd poll_fd[1];
    k = poll(poll_fd, 1, 100);
    poll_fd[0].fd = fd;
    poll_fd[0].events = POLLIN;
    poll_fd[0].revents = POLLIN;
    int n_write = write(fd, send_bytes, sizeof(send_bytes));  //Send data
    printf("Wrote the bytes. \n");
    sleep(5);
    if((poll_fd[0].revents & POLLIN) !=0)
{
    int n_read=read(fd,buffer,sizeof(buffer));
    if (n_read == -1)
        printf("Error read %s\n", strerror(errno));
 
    printf("read %d bytes. \n", n_read);
}
 
printf(buffer); 
 
    return 0;
 
}

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif
 
#include <stdio.h>
#include <stdlib.h>
#include <termios.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/signal.h>
#include <sys/types.h>
#include <sys/poll.h>
#include <errno.h>
 
int fd;
 
int main(void)
{
    unsigned char send_bytes[] = {'H','e','l','l','o'};
    char buffer[255];
    int ret;
    int k;
 
        struct termios oldtio,newtio;
 
        fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_NONBLOCK);
        if (fd <0) { printf("error opening port"); return(-1);}
 
 
    fcntl(fd, F_SETFL, O_NONBLOCK);
 
 
        tcgetattr(fd,&newtio); /* save current port settings */
 
    cfsetispeed(&newtio, B9600);    // set baud rates
    cfsetospeed(&newtio, B9600);
        newtio.c_cflag = (CS8 | CLOCAL | CREAD);
        newtio.c_iflag = (IGNPAR | ICRNL);
        newtio.c_oflag &= ~OPOST;
        newtio.c_lflag &= ~(ICANON|ECHO|ECHOE|ISIG);
        newtio.c_cc[VMIN]=1;
        newtio.c_cc[VTIME]=10;
 
        tcsetattr(fd,TCSANOW,&newtio);
 
    int n_write = write(fd, send_bytes, sizeof(send_bytes));  //Send data
    if (n_write < 0)
{
    return -1;
}
    if (n_write > 0)
{
    printf("Wrote the bytes. \n");
}
    struct pollfd poll_fd[1];
    k = poll(poll_fd, 1, 100);
    poll_fd[0].fd = fd;
    poll_fd[0].events = POLLIN;
    poll_fd[0].revents = POLLIN;
    if((poll_fd[0].revents & POLLIN) !=0)
{
    int n_read=read(fd,buffer,sizeof(buffer));
    if (n_read < 0)
        printf("Error read %s\n", strerror(errno));
 
    printf("read %d bytes. \n", n_read);
}
 
printf(buffer); 
 
    return 0;
 
}