Reading arduino serial data from C++ in Linux(ubuntu)

Question

My setup is as follows :- OS : LINUX (ubuntu),

Arduino nano + HC-SR04 sensor. The code for which is:-

const int trigPin = 5;
const int echoPin =6;
// defines variables
long duration;
int distance;
void setup() {
pinMode(trigPin, OUTPUT); // Sets the trigPin as an Output
pinMode(echoPin, INPUT); // Sets the echoPin as an Input
Serial.begin(9600); // Starts the serial communication
}
void loop() {
// Clears the trigPin
digitalWrite(trigPin, LOW);
delayMicroseconds(2);
// Sets the trigPin on HIGH state for 10 micro seconds
digitalWrite(trigPin, HIGH);
delayMicroseconds(10);
digitalWrite(trigPin, LOW);
// Reads the echoPin, returns the sound wave travel time in microseconds
duration = pulseIn(echoPin, HIGH);
// Calculating the distance
distance= duration*0.034/2;

if (distance <50) {
Serial.print("Chko Distance: ");
Serial.println(distance); }
}

Now I want to read the serial output from C++ , the code for which is :-

// C library headers
#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()

int main() {
  // Open the serial port. Change device path as needed (currently set to an standard FTDI USB-UART cable type device)
  int serial_port = open("/dev/ttyUSB0", O_RDWR);

  // Create new termios struc, we call it 'tty' for convention
  struct termios tty;

  // Read in existing settings, and handle any error
  if(tcgetattr(serial_port, &tty) != 0) {
      printf("Error %i from tcgetattr: %s\n", errno, strerror(errno));
      return 1;
  }

  tty.c_cflag &= ~PARENB; // Clear parity bit, disabling parity (most common)
  tty.c_cflag &= ~CSTOPB; // Clear stop field, only one stop bit used in communication (most common)
  tty.c_cflag &= ~CSIZE; // Clear all bits that set the data size 
  tty.c_cflag |= CS8; // 8 bits per byte (most common)
  tty.c_cflag &= ~CRTSCTS; // Disable RTS/CTS hardware flow control (most common)
  tty.c_cflag |= CREAD | CLOCAL; // Turn on READ & ignore ctrl lines (CLOCAL = 1)

  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; // Disable interpretation of INTR, QUIT and SUSP
  tty.c_iflag &= ~(IXON | IXOFF | IXANY); // Turn off s/w flow ctrl
  tty.c_iflag &= ~(IGNBRK|BRKINT|PARMRK|ISTRIP|INLCR|IGNCR|ICRNL); // Disable any special handling of received bytes

  tty.c_oflag &= ~OPOST; // Prevent special interpretation of output bytes (e.g. newline chars)
  tty.c_oflag &= ~ONLCR; // Prevent conversion of newline to carriage return/line feed
  // tty.c_oflag &= ~OXTABS; // Prevent conversion of tabs to spaces (NOT PRESENT ON LINUX)
  // tty.c_oflag &= ~ONOEOT; // Prevent removal of C-d chars (0x004) in output (NOT PRESENT ON LINUX)

  tty.c_cc[VTIME] = 10;    // Wait for up to 1s (10 deciseconds), returning as soon as any data is received.
  tty.c_cc[VMIN] = 0;

  // Set in/out baud rate to be 9600
  cfsetispeed(&tty, B9600);
  cfsetospeed(&tty, B9600);

  // Save tty settings, also checking for error
  if (tcsetattr(serial_port, TCSANOW, &tty) != 0) {
      printf("Error %i from tcsetattr: %s\n", errno, strerror(errno));
      return 1;
  }
  
  // Allocate memory for read buffer, set size according to your needs
  char read_buf [256];

  // Normally you wouldn't do this memset() call, but since we will just receive
  // ASCII data for this example, we'll set everything to 0 so we can
  // call printf() easily.
  memset(&read_buf, '\0', sizeof(read_buf));

  // Read bytes. The behaviour of read() (e.g. does it block?,
  // how long does it block for?) depends on the configuration
  // settings above, specifically VMIN and VTIME
  int num_bytes = read(serial_port, &read_buf, sizeof(read_buf));

  // n is the number of bytes read. n may be 0 if no bytes were received, and can also be -1 to signal an error.
  if (num_bytes < 0) {
      printf("Error reading: %s", strerror(errno));
      return 1;
  }

  // Here we assume we received ASCII data, but you might be sending raw bytes (in that case, don't try and
  // print it to the screen like this!)
  printf("Read %i bytes. Received message: %s", num_bytes, read_buf);

  close(serial_port);
  return 0; // success
}

Code is working fine and all, but its printing either one or several lines from sensor output. Whereas I want just one line to be printed from sensor, which to my limited knowledge seems like the connectivity between arduino code and C++ code isnt proper.

Distance printed twice
┌─[monisha@monisha-asus]─[~/cpp/serial] $./a.out
Read 36 bytes. Received message: Chko Distance: 0
Chko Distance: 0

Distance printed only once
┌─[monisha@monisha-asus]─[~/cpp/serial]$ ./a.out
Read 18 bytes. Received message: Chko Distance: 0

Distance printed several times 
┌─[monisha@monisha-asus]─[~/cpp/serial]$ ./a.out
Read 256 bytes. Received message: Chko Distance: 0
Chko Distance: 0
Chko Distance: 0
Chko Distance: 0
Chko Distance: 0
Chko Distance: 0

Please someone help me, thanks Monisha