Linux C++ Serial Programming Not Working Why?

Question

   #include <fcntl.h>  
   #include <termios.h> 
   #include <unistd.h>  
   #include <errno.h>  
   #include <cerrno>
   #include <iostream>
   #include <stdio.h>
   #include <stdlib.h>
   #include <string.h>
   #include <fstream>
   #include <sys/types.h>
   #include <sys/socket.h>
   #include <netinet/in.h>
   #include <cstdlib>
   #include <ctime>


   using namespace std;
   typedef struct termios ComSet;

   int main()
   {
   int acm=-1;                                   
   acm=open("/dev/ttyACM0",O_RDWR | O_NOCTTY);
   if(acm == -1)
    {
     cout<<"Error Opening ttyACM0"<<endl;
     exit(1);
    }
    else
    {
      cout<<"Preparing ttyACM0..."<<endl;
      ComSet SerialSettings;
      tcgetattr(acm, &SerialSettings);
      cfsetispeed(&SerialSettings,B115200);
      cfsetospeed(&SerialSettings,B115200);

    /* 8N1 Mode */
    SerialSettings.c_cflag &= ~PARENB;  
    SerialSettings.c_cflag &= ~CSTOPB;   
    SerialSettings.c_cflag &= ~CSIZE;    
    SerialSettings.c_cflag |=  CS8;     

    SerialSettings.c_cflag &= ~CRTSCTS;       
    SerialSettings.c_cflag |= CREAD | CLOCAL; 


    SerialSettings.c_iflag &= ~(IXON | IXOFF | IXANY);          
    SerialSettings.c_iflag &=ICANON;
    SerialSettings.c_iflag &= ~(ECHO | ECHOE | ISIG); 
    SerialSettings.c_oflag &= ~OPOST;                           

 /* Setting Time outs */
    SerialSettings.c_cc[VMIN]  = 1;     
    SerialSettings.c_cc[VTIME] = 0;     

    if((tcsetattr(acm,TCSANOW,&SerialSettings)) != 0)   
    {
        cout<< " ERROR ! in Setting attributes"<<endl;

    }
    else
    {
        cout<< "=======ttyACM0 Setting====="<<endl;
        cout<<"BaudRate = 115200 StopBits = 1 Parity = none"<<endl;
        cout<<"Reading ttyACM0... "<<endl;
        char read_buffer[1024];                                
        bzero(read_buffer,1024);
        int read_bytes=0;                                      
        while(1)
         {

                write(acm,"Hello World !",13);                                              
                read_bytes=read(acm,&read_buffer,1024);                                    
                if(read_bytes>0)
                {
                    cout<<read_buffer;
                    bzero(read_buffer,1024);
                }
                else
                {
                    cout<<"No-Data"<<endl;
                }

                tcflush(acm, TCOFLUSH);
                tcflush(acm, TCIFLUSH);
         }
    }



  close(acm); /* Close the serial port */

}

return 0;
}

Why is this code not working ? i m trying to receive same message from the arduino but it will not work.it completely ignores write.i have also found that it works if i use cin.get() before write.

it works if i want to read from arduino only. Here is my arduino code

  String Input;
  boolean done=false;

  void setup() 
  {
     Serial.begin(115200);
  }

  void loop() 
  {
     Serial.println(GetSerial());
      Input="";
  }

  String GetSerial()
  {  
      done=false;
      Input="";
      while(!done)
  {  
      while(Serial.available()>0)
      {
         Input=Serial.readStringUntil('\n');  //Block Reading
         done=true; 
      }
   } 
   return Input;
  }

i want to receive whatever is sent on serial port as it is with as much less delay as possible