Error : Resource temporarily unavailable Unable to receive serial data - c

Question

My C code is sending data to Uart perfectly but while receiving it gives error.
Here is my code
#include <stdio.h> // standard input / output functions
#include <stdlib.h>
#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>
void main()
{
int USB = open( "/dev/ttyUSB0", O_RDWR| O_NONBLOCK | O_NDELAY );
struct termios tty;
struct termios tty_old;
memset (&tty, 0, sizeof tty);
/* Error Handling */
if ( USB < 0 )
{
printf( "Error %d form tcgetattr : %s /n", errno, strerror( errno ) );
}
/* Save old tty parameters */
tty_old = tty;
/* Set Baud Rate */
cfsetospeed (&tty, (speed_t)B9600);
cfsetispeed (&tty, (speed_t)B9600);
/* Setting other Port Stuff */
tty.c_cflag &= ~PARENB; // Make 8n1
tty.c_cflag &= ~CSTOPB;
tty.c_cflag &= ~CSIZE;
tty.c_cflag |= CS8;
tty.c_cflag &= ~CRTSCTS; // no flow control
tty.c_cc[VMIN] = 1; // read doesn't block
tty.c_cc[VTIME] = 5; // 0.5 seconds read timeout
tty.c_cflag |= CREAD | CLOCAL; // turn on READ & ignore ctrl lines
/* Make raw */
cfmakeraw(&tty);
/* Flush Port, then applies attributes */
tcflush( USB, TCIFLUSH );
if ( tcsetattr ( USB, TCSANOW, &tty ) != 0)
{
printf("Error %d from tcsetattr /n",errno);
// std::cout << "Error " << errno << " from tcsetattr" << std::endl;
}
/* WRITE */
unsigned char cmd[] = "YES this program is writing \r";
int n_written = 0,spot = 0;
do {
n_written = write( USB, &cmd[spot], 1 );
spot += n_written;
} while (cmd[spot-1] != '\r' && n_written > 0);
/* READ */
int n = 0,spot_r =0;
char buf = '\0';
/* Whole response*/
char response[1024];
memset(response, '\0', sizeof response);
do {
n = read( USB, &buf, 1 );
sprintf( &response[spot_r], "%c", buf );
spot_r += n;
} while( buf != '\r' && n > 0);
if (n < 0) {
printf("Error reading %s",strerror(errno));
}
else if (n==0) {
printf("read nothing");
}
else {
printf("Response %s",response);
}
}
Error Message
Error reading Resource temporarily unavailable
It is sending data to UART perfectly.
But at reception side it gives error of resource temporary unavailable.
Should i put delay in-between?
Or any other function that could help me out.
Thank you.
Abhi