C Programming - read from TTY serial port followed by write response loses data

Question

I have a simple test program which waits for a set of 3 characters to be sent to the USB port (0x10, 0x04 and 0x02) and then should respond with "Hello World". For some reason, the first character is missed off the response so I only see "ello World".

I am using the USB-OTG port with modprobe g_serial for communication if that makes any difference.

What am I missing?

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <termios.h>
#include <unistd.h>
#include <sys/poll.h>
#include <sys/ioctl.h>
#include <linux/usb/g_printer.h>
#include <string.h>
#include <dirent.h>
#include <sys/stat.h>

#define ODROID_SERIAL_GADGET        "/dev/ttyGS0"
#define BUF_SIZE    1024

int main()
{
    struct pollfd   fd[0];
    struct termios tty;
    int timeover=1000;
    static char buf[BUF_SIZE];
    char obuff[] = ".";

    int ox10Found, ox04SeqeunceFound, i;    
    int bytes_read, retval;
    int max_read_time = 2; /* units of 250ms after which incoming data times out - 1 causes larger files to be split */

    fd[0].fd = open(ODROID_SERIAL_GADGET, O_RDWR);
    if (fd[0].fd < 0) {
        printf("Error %d opening %s\n", fd[0].fd, ODROID_SERIAL_GADGET);
        close(fd[0].fd);
        return(-1);
    }

    tcgetattr(fd[0].fd,&tty); /* save current serial port settings */
    tty.c_iflag &= ~(IGNBRK | BRKINT | PARMRK | ISTRIP | INLCR | IGNCR | ICRNL | IXON);
    tty.c_lflag &= ~(ECHO | ECHONL | ICANON | ISIG | IEXTEN); // Enable canonical input - we need to read contorol characters
    tty.c_oflag = 0; // no output processing
    tty.c_cflag = (tty.c_cflag & ~CSIZE) | CS8;     // 8-bit chars
    tty.c_cc[VMIN]  = 0;            // read doesn't block
    tty.c_cc[VTIME] = 5;            // 0.5 seconds read timeout

    tty.c_iflag &= ~(IXON | IXOFF | IXANY); // shut off xon/xoff ctrl

    tty.c_cflag |= (CLOCAL | CREAD);// ignore modem controls,
                                    // enable reading
    tty.c_cflag &= ~(PARENB | PARODD);      // shut off parity
    tty.c_cflag |= 0;
    tty.c_cflag &= ~CSTOPB;
    tty.c_cflag &= ~CRTSCTS;

    tcsetattr(fd[0].fd,TCSANOW,&tty);

    fd[0].events = POLLIN | POLLRDNORM;

    ox10Found=0;
    ox04SeqeunceFound=0;

    while (1) {
        /* Wait for up to 1/4 second for data. */
        retval = poll(fd, 1, 250);
        timeover++;
        if (timeover>1000) {
            timeover=max_read_time+1;
        }

        if (retval) {
            bytes_read = read(fd[0].fd, buf, BUF_SIZE);

            if (bytes_read < 0) {
                /* Open device file for serial gadget. */
                printf("Error %d reading from %s\n",fd[0].fd, ODROID_SERIAL_GADGET);
                close(fd[0].fd);
                return(-1);
            }

            for (i = 0; i < bytes_read; i++) {
                if (buf[i]==0x10 && !ox10Found && !ox04SeqeunceFound) {
                    ox10Found=1;
                    continue;
                } else if (buf[i]==0x04 && ox10Found) {
                    ox04SeqeunceFound=1;
                    continue;
                } else {
                    ox10Found=0;
                }

                if (buf[i]==0x02 && ox04SeqeunceFound) {
                    write(fd[0].fd, "Hello World", 11);
                    tcdrain(fd[0].fd); // Wait for output
                }
            }
        }
    }
}