#include <stdio.h>   /* Standard input/output definitions */
#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 definitions */

int main()
{
      int fd; /* File descriptor for the port */

      // num of read characters
      int n = 0;
    
      const char* serial_device = "/dev/ttyS0";
    
     fd = open( serial_device , O_WRONLY | O_NOCTTY | O_NDELAY);


      if (fd == -1)
      {
               /*
        * Could not open the port.
        */
          printf("open_port: Unable to open %s \n" , serial_device);
      }
   


       const char* sentance = "aa102222233333444445555566666777778888899999aaaaabbbbbcccccddddd";

    printf("About to write %s \n to UART on %s \n" , sentance ,  serial_device);

        n = write(fd, sentance , strlen(sentance));
    
        if (n < 0)
    {
          fputs("write() byte failed!\n", stderr);
    }
    else
    {
        
        printf("writting %d bytes to UART on %s \n" , n , serial_device);
    }


    close(fd);
    return 0;
}




Process read:
---------------

#include <stdio.h>   /* Standard input/output definitions */
#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 definitions */
#include <sys/ioctl.h>
#include <sys/stat.h>
#include <stdlib.h>
#include "signal.h"
#include <assert.h>

#define N 1000

int main()
{
      int fd; /* File descriptor for the port */
    
        
      char array[N] = {'\n'};

      // num of read characters
      int n = 0;
    
    
      const char* serial_device = "/dev/ttyS0";
    

      
      fd = open(serial_device, O_RDONLY | O_NOCTTY );

      if (fd == -1)   
      {
           /*
          * Could not open the port.
         */
          printf("open_port: Unable to open %s \n" , serial_device);
          
      }
      else
      {
          fcntl(fd, F_SETFL, 0);
      }

    
      printf("Reading process form UART on %s \n" , serial_device);

    
    while (1)
    {
        printf("Begin to read\n");
        fflush(stdout);


        n = read(fd, array , 500 );

        if (n >= 0)
        {
            array[n]='\n';
        }
        
        if (n < 0)
        {
              fputs("read() failed!\n", stderr);
        }
        else
        {
        
            printf("The content of the Serial port is: %s len is %d \n" , array , n);
            fflush(stdout);

            n = 0;
    
    
        }

    }


    close(fd);
    return 0;
}