About c++ : Serial-Port-Communication-using-cpp-in-linux-RS232-with-a-Mass-Flow-controller

I am trying to etablish a comunication with a mass flow controler using linux and cpp , i write the code but i am not getting any response from my MFC.
I try many think but isnt working.Did I miss somethink or i need to make some modfication ?

The MFC that i am using is an Alicat MFC, communicate via Serial Port using RS232 protocol and ASCII data.

Thats My code :

#include <stdio.h>
#include <string.h>

// Linux headers
#include <fcntl.h> // Contains file controls like O_RDWR
#include <errno.h> // Error integer and strerror() function
#include <termios.h> // Contains POSIX terminal control definitions
#include <unistd.h> // write(), read(), close()
#include <iostream>

struct termios tty;

using namespace std ;
int main(){

    cout << "step_1\n";
    int serial_port = open("/dev/ttyUSB0", O_RDWR| O_NOCTTY);

    // Check for errors
    if (serial_port < 0)
    {
        printf("Error %i from open: %s\n", errno, strerror(errno));
        exit(-2);
    }
    if(tcgetattr(serial_port, &tty) != 0)
    {
        cout << ("Error %i from tcgetattr: %s\n", errno, strerror(errno));
        exit(-3);
    }
    cout << "step_2\n";
    tty.c_cflag &= ~PARENB;
    tty.c_cflag &=  ~CSTOP;
    tty.c_cflag &=  ~CSIZE;
    tty.c_cflag |=     CS8;
    tty.c_cflag |= CRTSCTS;
    tty.c_cflag |= CREAD | CLOCAL;
    tty.c_lflag &= ~ICANON;
    tty.c_lflag &= ~ECHO; // Disable echo
    tty.c_lflag &= ~ECHOE; // Disable erasure
    tty.c_lflag &= ~ECHONL; // Disable new-line echo
    tty.c_lflag &= ~ISIG;
    tty.c_iflag &= ~(IXON | IXOFF | IXANY); // Turn off s/w flow ctrl./
    tty.c_iflag &= ~(IGNBRK|BRKINT|PARMRK|ISTRIP|INLCR|IGNCR|ICRNL);
    tty.c_oflag &= ~OPOST;
    tty.c_oflag &= ~ONLCR;


    tty.c_cc[VTIME] = 1;
    tty.c_cc[VMIN] = 5;

    cfsetispeed(&tty, B19200);
    cfsetospeed(&tty, B19200);

    tcflush( serial_port, TCIFLUSH );
    cout << "step_3\n";

    if (tcsetattr(serial_port, TCSANOW, &tty) != 0)
    {
        printf("Error %i from tcsetattr: %s\n", errno, strerror(errno));
    }
   cout << "step_4\n";
    tcflush( serial_port , TCIFLUSH );

    cout<< "step_5\n";

    unsigned char cmd[] = "A\r";
    int n_written = 0,
        spot = 0;

    do {
        n_written = write(serial_port, &cmd[spot], sizeof(cmd) - 1 );
        spot += n_written;
    } while (cmd[spot-1] != '\r' && n_written > 0);



    int n = 0;
     spot = 0;
    char buf = '\0';

    /* Whole response*/
    char response[1024];
    memset(response, '\0', sizeof response);

    do {
        n = read( serial_port, &buf, sizeof(buf));
        sprintf( &response[spot], "%c", buf );
        spot += n;
    } while( buf != '\r' && n > 0);

    if (n < 0) {
        std::cout << "Error reading: " << strerror(errno) << std::endl;
    }
    else if (n == 0) {
        std::cout << "Read nothing!" << std::endl;
    }
    else {
        std::cout << "Response: " << response << std::endl;
    }

    return -1 ; 
}