backend.c

/* Author: Kris Burney & Jake Drahos
 *
 * BlueTooth socket program for passing vrpn data to quad.
 */

#define _GNU_SOURCE
#define _BSD_SOURCE

//system includes
#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/select.h>
#include <sys/stat.h>
#include <bluetooth/bluetooth.h>
#include <bluetooth/rfcomm.h>
#include <pthread.h>
#include <assert.h>
#include <errno.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <netinet/tcp.h>

//user created includes
#include "communication.h"
#include "commands.h"
#include "vrpn_tracker.hpp"
#include "type_def.h"
#include "packet.h"
#include "responseparam.h"
#include "update.h"
#include "config.h"
#include "cmHandler.h"
#include "getparam.h"
#include "setparam.h"

#define QUAD_BT_ADDR  "00:06:66:64:61:D6"
#define QUAD_BT_CHANNEL  0x01
#define CMD_MAX_LENGTH 1024
#define MAX_HASH_SIZE 50

/* Backend-internal command magics */
#define TD_MAGIC "TRACKERDATA"

// function prototypes
void readAndPrint(void);
void sendVrpnPacket(struct ucart_vrpn_TrackerData *);
void sendStartPacket(void);
void getVRPNPacket(struct ucart_vrpn_TrackerData *);
void printVrpnData(struct ucart_vrpn_TrackerData *);
int connectToZybo();
int safe_fd_set(int , fd_set* , int* );
int safe_fd_clr(int , fd_set* , int* );
static void safe_close_fd(int fd, pthread_mutex_t *mutexLock);

static void cb(struct ucart_vrpn_TrackerData *);
static int new_client(int fd);
/* Return index of client, or -1 */
static ssize_t get_client_index(int fd);
/* Returns pointer to client buffer, or -1 */
static char * get_client_buffer(int fd);
/* Return pointer to client pending responses, or -1*/ 
static int * get_client_pend_responses(int fd);
/* Return positive integer if successful, -1 otherwise */
static int clientAddPendResponses(int fd, unsigned char *packet);
/* Returns -1 on error */
static int remove_client(int fd);
/* Receive data from client */
static void client_recv(int fd);
/* Receive data from quad */
static void quad_recv();
/* Checks to see if socket has disconnected. Returns 1 on disconnect, else returns 0 */
static int wasDisconnected(int fd);
/* handle controller responses from quad to frontend */
static void handleResponseParam(struct metadata *m, uint8_t * data);


/* Thread-safe wrappers */
pthread_mutex_t quadSocketMutex;
static ssize_t writeQuad(const uint8_t * buf, size_t count);
static ssize_t readQuad(char * buf, size_t count);

/* Functions for recording Latencies */
void findTimeDiff(int respID);
int timeval_subtract (struct timeval *result, struct timeval *x, struct timeval *y);
//time stamp checking
static unsigned int currMessageID = 0;
struct timeval timeArr[MAX_HASH_SIZE];

// global variables
static volatile int keepRunning = 1;
const char *TRACKER_IP = "UAV@192.168.0.120:3883";
static int zyboSocket;
static int backendSocket;
struct ucart_vrpn_tracker * tracker = NULL;

#define MAX_CLIENTS 32
#define CLIENT_BUFFER_SIZE 1024
#define CLIENT_MAX_PENDING_RESPONSES 5
static char client_buffers[MAX_CLIENTS][CLIENT_BUFFER_SIZE];
static int client_fds[MAX_CLIENTS];
static int client_pending_responses[MAX_CLIENTS][CLIENT_MAX_PENDING_RESPONSES];
fd_set rfds_master;
int max_fd = 0;

static FILE * quadlog_file;

pthread_mutex_t quadResponseMutex, cliInputMutex ;
unsigned char *commandBuf;
int newQuadResponse = 0, newCliInput = 0;

// Structures to be used throughout
modular_structs_t structs;

// Callback to be ran whenever the tracker receives data.
// Currently doing much more than it should. It will be slimmed down 
// 		in the future.
static void cb(struct ucart_vrpn_TrackerData * td) {
	static int count = 0;

	if(!(count % 10)) {
		sendVrpnPacket(td);
	}
	count++;
}

int main(int argc, char **argv)
{
	int activity;
	FD_ZERO(&rfds_master);
	char log_file[256] = "logs/";

	/* 
	 * Create backend listening socket
	 */
	/* Determine socket path */
	char * backend_socket_path = DEFAULT_SOCKET;
	if (getenv(SOCKET_ENV)) {
		backend_socket_path = getenv(SOCKET_ENV);
	}
	/* Unlink if it exists */
	unlink(backend_socket_path);

	/* Create socket */
	mode_t old_umask = umask(0111);
	backendSocket = socket(AF_UNIX, SOCK_STREAM | SOCK_NONBLOCK, 0);
	if (backendSocket < 0) {
		err(-1, "socket");
	}

	/* Create sockaddr and bind */
	struct sockaddr_un sa;
	sa.sun_family = AF_UNIX;
	strncpy(sa.sun_path, backend_socket_path, 107);
	sa.sun_path[107] = '\0';
	if (bind(backendSocket, (struct sockaddr *) &sa, sizeof(sa))) {
		err(-1, "bind");
	}
	umask(old_umask);

	/* Listen */
	if (listen(backendSocket, 16)) {
		err(-1, "listen");
	}

	/* Add to socket set */
	safe_fd_set(backendSocket, &rfds_master, &max_fd);

	/* Initialize client buffers */
	for (int i = 0; i < MAX_CLIENTS; i++) {
		client_fds[i] = -1;
		client_buffers[i][0] = '\n';
		for(int j = 0; j < CLIENT_MAX_PENDING_RESPONSES; j++) {
			client_pending_responses[i][j] = -1;
		}
	}

	if (pthread_mutex_lock(&quadSocketMutex)) {
		err(-2, "pthrtead_mutex_lock (%s:%d):", __FILE__, __LINE__);
	}

	if ((zyboSocket = connectToZybo()) < 0)
	{
		perror("Error connecting to Quad...");
		free(commandBuf);
		exit(1);
	}

	if (pthread_mutex_unlock(&quadSocketMutex)) {
		err(-2, "pthrtead_mutex_unlock (%s:%d):", __FILE__, __LINE__);
	}
	
	// watch for input from stdin (fd 0) to see when it has input
	safe_fd_set(fileno(stdin), &rfds_master, &max_fd);

	if (!getenv(NOQUAD_ENV)) {
		// watch for input from the zybo socket
		safe_fd_set(zyboSocket, &rfds_master, &max_fd);
	}

	if(argc >= 2)
	{
		strncat(log_file, argv[1], strlen(argv[1]));
	} else {
		time_t rawtime;
		char timestr [30];
		time(&rawtime);
		sprintf(timestr,"%s",ctime(&rawtime));
		// Lets convert space to _ in
		char *p = timestr;
		size_t i = 0;
		while(i < strlen(timestr))
		{ 
		    if (*p == ' ')
		          *p = '_';
			i++;
			p++;
		}
		// timestr has a weird char at the end of it.
		// we will not include it in our file name
		strncat(log_file, timestr, strlen(timestr) -1 );
		strcat(log_file, ".txt");
	}

	printf("Creating log file '%s'...\n",log_file);
	quadlog_file = fopen(log_file, "a");

	// Tell the quad we are ready to send it vrpn data
	sendStartPacket();


	if(!getenv(NOVRPN_ENV)){
		// create vrpnTracker instance
		tracker = ucart_vrpn_tracker_createInstance(TRACKER_IP);
		// this function will be called whenever tracker receives data
		ucart_vrpn_tracker_addCallback(tracker, cb);	
	}

	struct timeval timeout = {
		.tv_sec = 1,
		.tv_usec = 0
	};

	while(keepRunning)
	{
		fd_set rfds;
		rfds = rfds_master;
		activity = select(max_fd+1, &rfds, NULL, NULL, NULL);
		if(activity == -1) {
			perror("select() ");
		} else if (activity) {
			for(int fd = 0; fd <= max_fd; ++fd) {
				if (FD_ISSET(fd, &rfds)) {
					if(wasDisconnected(fd)){
						break;
					}
					if (fd == fileno(stdin)) {
						/**
						 * Ignore stdin from the backend
						 */
					} else if (fd == zyboSocket) {
						printf("recieving from quad\n");
						quad_recv();
					} else if (fd == backendSocket) {
						int new_fd = 0;
						new_fd = accept(backendSocket, NULL, NULL);
						if (new_fd < 0) {
							warn("accept");
						} else {
							printf("Connection\n");
							if (new_client(new_fd)) {
								printf("Added client\n");
								safe_fd_set(new_fd, &rfds_master, &max_fd);
							}
						}
					} else if (get_client_index(fd) > -1) {
						/* It is a socket to a frontend */
						client_recv(fd);
					}
				}
			}
		} else {
			timeout.tv_sec = 1;
			timeout.tv_usec = 0;
		}
	}

	ucart_vrpn_tracker_freeInstance(tracker);
	safe_close_fd(zyboSocket, &quadSocketMutex);
	fclose(quadlog_file);
	return 0;
}

void sendStartPacket() {
	uint8_t packet[64];
	struct metadata m;
	m.msg_type = BEGINUPDATE_ID;
	m.data_len = 0;
	m.msg_id = currMessageID++;

	ssize_t psize;

	if ((psize = EncodePacket(packet, 64, &m, NULL)) < 0) {
		warnx("Big problems. sendStartPacket");
		return;
	}

	writeQuad(packet, psize);
	printf("Start Packet sent...\n");
}

void sendVrpnPacket(struct ucart_vrpn_TrackerData *info) {
	uint8_t packet[64];
	struct metadata m;
	uint8_t data[128];

	struct position_update u;
	u.id = currMessageID;
	u.x = info->x;
	u.y = info->y;
	u.z = info->z;
	u.pitch = info->pitch;
	u.roll = info->roll;
	u.yaw = info->yaw;

	if (EncodeUpdate(&m, data, 128, &u) < 0) {
		warnx("Big problems. sendVrpnPacket . EncodeUpdate");
		return;
	}
	m.msg_id = currMessageID++;

	ssize_t psize;
	if ((psize = EncodePacket(packet, 64, &m, data)) < 0) {
		warnx("Big problems. sendVrpnPacket. EncodePacket");
		return;
	}

	writeQuad(packet, psize);
}

void getVRPNPacket(struct ucart_vrpn_TrackerData *td) {
	int status;
	if((status = ucart_vrpn_tracker_getData(tracker, td)) < 0)
	{
		perror("Error receiving VRPN data from tracker...");
		keepRunning = 0;
	}
}

void printVrpnData(struct ucart_vrpn_TrackerData * td) {
	printf("FPS: %lf Pos (xyz): (%lf %lf %lf) Att (pry): (%lf %lf %lf)\n",
		td->fps, td->x, td->y, td->z, td->pitch, td->roll, td->yaw);
}

int connectToZybo() {
	int sock;
	int status = 0;

	if (getenv(NOQUAD_ENV)) {
		return 0;
	}

	/* Use bluetooth by default */
	if (!getenv(QUAD_WIFI_ENV)) {
		printf("Using BT Settings\n");
		struct sockaddr_rc addr;

		// allocate a socket	
		sock = socket(AF_BLUETOOTH, SOCK_STREAM, BTPROTO_RFCOMM);

		//set the connection params ie. who to connect to	
		addr.rc_family = AF_BLUETOOTH;
		addr.rc_channel = (uint8_t) QUAD_BT_CHANNEL;
		str2ba( QUAD_BT_ADDR, &addr.rc_bdaddr );
		
		printf("Attempting to connect to zybo. Please be patient...\n");
		// blocking call to connect to socket sock ie. zybo board
		status = connect(sock, (struct sockaddr *)&addr, sizeof(addr));
	} else {
		printf("Using WIFI settings\n");
		struct sockaddr_in addr;

		addr.sin_family = AF_INET;

		/* Quick and Dirty */
		if (getenv(QUAD_IP_ENV)) {
			 if (!inet_aton(getenv(QUAD_IP_ENV), &addr.sin_addr)) {
				printf("Env var %s invalid IP %s\n",
					     	QUAD_IP_ENV, getenv(QUAD_IP_ENV));
				return -1;
			 }
		} else {
			if (!inet_aton(QUAD_IP_DEFAULT, &addr.sin_addr)) {
				printf("Default IP %s is invalid\n",
					     	QUAD_IP_DEFAULT);
				return -1;
			}
		}

		if (getenv(QUAD_PORT_ENV)) {
			/* Quick 'n dirty, oh yeah! */
			addr.sin_port = htons(atoi(getenv(QUAD_PORT_ENV)));		
		} else {
			addr.sin_port = htons(QUAD_PORT_DEFAULT);
		}

		sock = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);
		if (sock < 0) {
			perror("socket");
			return -1;
		}
		printf("Connecting to Quad @ %s:%u\n", inet_ntoa(addr.sin_addr), ntohs(addr.sin_port));

		status = connect(sock, (struct sockaddr *)&addr, sizeof(addr));
	}

	// connection failed
	if(status < 0)
	{
		close(sock);
		perror("connect");
		return -1;
	}
	else
	{
		printf("connection successful!...\n");
		return sock;
	}
}

/* add a fd to fd_set, and update max_fd */
int safe_fd_set(int fd, fd_set* fds, int* max_fd) {
    assert(max_fd != NULL);

    FD_SET(fd, fds);
    if (fd > *max_fd) {
        *max_fd = fd;
    }
    return 0;
}

/* clear fd from fds, update max fd if needed */
int safe_fd_clr(int fd, fd_set* fds, int* max_fd) {
    assert(max_fd != NULL);

    FD_CLR(fd, fds);
    if (fd == *max_fd) {
        (*max_fd)--;
    }
    return 0;
}

static ssize_t writeQuad(const uint8_t * buf, size_t count) {
	ssize_t retval;
	if (getenv(NOQUAD_ENV)) {
		return count;
	}
	if (pthread_mutex_lock(&quadSocketMutex)) {
		err(-2, "pthrtead_mutex_lock (%s:%d):", __FILE__, __LINE__);
	}
	retval = write(zyboSocket, buf, count);
	if (pthread_mutex_unlock(&quadSocketMutex)) {
		err(-2, "pthrtead_mutex_unlock (%s:%d):", __FILE__, __LINE__);
	}

	return retval;
}

static ssize_t readQuad(char * buf, size_t count) {
	ssize_t retval;
	if (getenv(NOQUAD_ENV)) {
		return count;
	}
	if (pthread_mutex_lock(&quadSocketMutex)) {
		err(-2, "pthrtead_mutex_lock (%s:%d):", __FILE__, __LINE__);
	}
	retval = read(zyboSocket, buf, count);
	if (pthread_mutex_unlock(&quadSocketMutex)) {
		err(-2, "pthrtead_mutex_unlock (%s:%d):", __FILE__, __LINE__);
	}
	return retval;
}

static int new_client(int fd) {
	ssize_t new_slot = -1;
	for (ssize_t i = 0; i < MAX_CLIENTS; i++) {
		if (client_fds[i] < 0) {
			new_slot = i;
			break;
		}
	}
	if (new_slot == -1) {
		warnx("Ran out of room! Consider increasing MAX_CLIENTS!");
		return 0;
	}

	client_fds[new_slot] = fd;
	client_buffers[new_slot][0] = '\0';

	return 1;
}

static ssize_t get_client_index(int fd) {
	for (ssize_t i = 0; i < MAX_CLIENTS; i++) {
		if (client_fds[i] == fd) {
			return i;
		}
	}

	return -1;
}

static char * get_client_buffer(int fd) {
	ssize_t slot = get_client_index(fd);
	if (slot == -1) {
		return NULL;
	} else {
		return client_buffers[slot];
	}
}

static int * get_client_pend_responses(int fd) {
	ssize_t slot = get_client_index(fd);
	if (slot == -1) {
		return NULL;
	} else {
		return client_pending_responses[slot];
	}
}

static int clientAddPendResponses(int fd, unsigned char *packet) {
	int *pendingResponses = get_client_pend_responses(fd);
	int packetID = (packet[4] << 8) | (packet[3]);
	for(int i = 0; i < CLIENT_MAX_PENDING_RESPONSES; i++) {
		if(pendingResponses[i] == -1) {
			pendingResponses[i] = packetID;
			return i;
		}
	}
	return -1;
}

static int remove_client(int fd) {
	ssize_t slot = get_client_index(fd);
	if(slot == -1)
		return -1;
	char *clientBuffer = get_client_buffer(fd);
	if(clientBuffer == NULL)
		return -1;
	clientBuffer[0] = '\0';
	int *pendingResponses = get_client_pend_responses(fd);
	if(pendingResponses == NULL)
		return -1;
	for(int i = 0; i < CLIENT_MAX_PENDING_RESPONSES; i++) {
		pendingResponses[i] = -1;
	}
	client_fds[slot] = -1;
	return 0;
}

static void safe_close_fd(int fd, pthread_mutex_t *mutexLock) {
	if (pthread_mutex_lock(mutexLock)) {
		err(-2, "pthrtead_mutex_lock (%s:%d):", __FILE__, __LINE__);
	}
	close(fd);
	if (pthread_mutex_unlock(mutexLock)) {
		err(-2, "pthrtead_mutex_unlock (%s:%d):", __FILE__, __LINE__);
	}
}

static void client_recv(int fd) {
	char * buffer;
	ssize_t len_pre;
	char cmdString[64];
	char * cursor;
	ssize_t r;
	int index = 0;


    buffer = get_client_buffer(fd);
	len_pre = strlen(buffer);
	cursor = buffer + len_pre;

	r = read(fd, cursor, CLIENT_BUFFER_SIZE - len_pre - 1);
	if (r < 0) {
		warn("read (fd: %d)", fd);
	}
	buffer[len_pre + r] = '\0';

	/* Parse buffer and handle commands */
	while (1) {
		/* not using strtok because reasons */
		size_t len = strlen(buffer);
		ssize_t newline = -1;
		for (size_t i = 0; i < len; i++) {
			if (buffer[i] == '\n') {
				newline = i;
				break;
			}
		}

		/* No newline found. End parsing */
		if (newline == -1) {
			break;
		}
		buffer[newline] = '\0';
		
		printf("Client(%d) : '%s'\n",fd, buffer);

		struct controller_message cm;
		if (stringToCm(buffer, &cm) == NULL) {
			/* buffer was not a quad command, handling internally to
			 * backend instead of forwarding to quad
			 */
			if (strncmp(buffer, TD_MAGIC, strlen(TD_MAGIC)) == 0) {
				/* Request for tracker data */
				struct ucart_vrpn_TrackerData td;
				if (tracker == NULL) {
					char * dummy = TD_MAGIC " 1.0 2.0 3.0 4.0 5.0 6.0\n";
					write(fd, dummy, strlen(dummy));
				} else if (ucart_vrpn_tracker_getData(tracker, &td)) {
					write(fd, TD_MAGIC " ERROR\n", strlen(TD_MAGIC " ERROR\n"));
				} else { 
					/* more than sufficient buffer */
					char buffer[2048];
					/* Map VRPN XYZ to Height Lat Long (right now it's
					 * a guess). Format is Height Lat Long P R Y */
					if (snprintf(buffer,
							2048,
							TD_MAGIC " %lf %lf %lf %lf %lf %lf\n",
							td.z,
							td.y,
							td.x,
							td.pitch,
							td.roll,
							td.yaw) >= 2048) {

						/* Output longer than buffer */
						warnx("Increase format buffer size, output was too long!");
						write(fd, TD_MAGIC " ERROR\n", strlen(TD_MAGIC " ERROR\n"));
					}
					write(fd, buffer, strlen(buffer));
				}
			}
		} else {
			uint8_t packet[64];
			struct metadata m;
			uint8_t data[128];
			ssize_t result;

			
			if (strncmp(buffer, "set", 3) == 0) {
				result = EncodeSetparam(&m, data, 128, &cm);
			} else if (strncmp(buffer, "get", 3) == 0) {
				result = EncodeGetparam(&m, data, 128, &cm);
			} else {
				warnx("not implemented yet");
			}

			if (result < 0) {
				warnx("Big problems. client_recv. EncodeMetaData");
				return;
			}

			ssize_t psize;
			if ((psize = EncodePacket(packet, 64, &m, data)) < 0) {
				warnx("Big problems. client_recv. EncodePacket");
				return;
			}

			m.msg_id = currMessageID++;
			
			/* Only add the client to the pending responses if it was a getparam command */
			if (m.msg_type == GETPARAM_ID) {
				if (clientAddPendResponses(fd, packet) == -1) {
					warnx("Ran out of room! Consider increasing CLIENT_MAX_PENDING_RESPONSES!\n");
				}
			}
			
			writeQuad(packet, psize);

		}

		
		char * rest = &buffer[newline] + 1;
		size_t restLen = (strlen(rest) == 0) ? 1 : strlen(rest);
		/* Delete parsed data and move the rest to the left */
		memmove(buffer, rest, restLen +1);
	}
}

static void quad_recv() {
	static unsigned char respBuf[4096];
	static size_t respBufLen;

	int unfinished = 1;
	struct metadata m;
	uint8_t data[4096];
	size_t respLen;
	ssize_t datalen;
	size_t packetlen;

	respLen = readQuad((char *) respBuf + respBufLen, 
			CMD_MAX_LENGTH - respBufLen);
	if (respLen <= 0) {
		perror("ERROR reading from quad...\n");
		return;
	}
	respBufLen += respLen;

	// printf("packet = '");
	// for(int i = 0; i < (int)respBufLen; ++i) {
	// 	printf(" %.2x ", respBuf[i]);
	// }
	// printf("'\n");

	while(unfinished){
		datalen = DecodePacket(&m, data, 2048, respBuf, respBufLen);
	
		if (datalen == -1) {
			warnx("No start Byte!\n");
			respBufLen = 0;
			return;
		}
		if (datalen == -5) {
			warnx("Chechsum mismatch!\n");
			respBufLen = 0;
			return;
		}
		if (datalen < 0){
			/* Not enough data yet. We need to wait for more */
			return;	
		}

		packetlen = PacketSize(&m);
		memmove(respBuf, respBuf + packetlen, respBufLen - packetlen);
		respBufLen -= packetlen;

		switch (m.msg_type) {
			case DEBUG_ID :
			case PACKETLOG_ID :
			case GETPACKETLOGS_ID :
			case UPDATE_ID :
			case BEGINUPDATE_ID :
				printf("(Backend): Command '%s' ignored\n", MessageTypes[m.msg_type].cmdText);
				break;
			case LOG_ID:
				/* something like this */
				fwrite((char *) data, sizeof(char), m.data_len, quadlog_file);
				fflush(quadlog_file);
				break;
			case RESPONSE_ID:
				printf("(Backend): response id found\n");
				break;
			case SETPARAM_ID:
			case GETPARAM_ID:
				printf("(Backend): Command '%s' ignored\n", MessageTypes[m.msg_type].cmdText);
				break;
			case RESPPARAM_ID:
				printf("(Backend): response param id found\n");
				handleResponseParam(&m, data);
				break;
			default:
				printf("(Backend): message type %d unrecognized\n", m.msg_type);
		}
		if (respBufLen == 0) {
			unfinished = 0;
		}
	}
	
}

static void handleResponseParam(struct metadata *m, uint8_t * data)
{
	struct controller_message cm;
	if (DecodeResponseParam(&cm, m, data) < 0) {
		warnx("DecodeResponse error");
		return;
	}

	char buffer[128];

	const char * message = cmToString(RESPPARAM_ID, &cm);

	size_t len = snprintf(buffer, 128, "%s %f\n", message, cm.value);

	for(int fd = 0; fd <= max_fd; ++fd) {
		if (get_client_index(fd) > -1) {
			write(fd, buffer, len);
		}
	}
}

static int wasDisconnected(int fd) {
	char buff;
	if(recv(fd, &buff, 1, MSG_PEEK | MSG_DONTWAIT) == 0)
	{
		remove_client(fd);
		safe_fd_clr(fd, &rfds_master, &max_fd);
		printf("fd %d has disconnect and was removed\n", fd);
		return 1;
	}
	return 0;
}

int timeval_subtract (struct timeval *result, struct timeval *x, struct timeval *y) {
  /* Perform the carry for the later subtraction by updating y. */
  if (x->tv_usec < y->tv_usec) {
    int nsec = (y->tv_usec - x->tv_usec) / 1000000 + 1;
    y->tv_usec -= 1000000 * nsec;
    y->tv_sec += nsec;
  }
  if (x->tv_usec - y->tv_usec > 1000000) {
    int nsec = (x->tv_usec - y->tv_usec) / 1000000;
    y->tv_usec += 1000000 * nsec;
    y->tv_sec -= nsec;
  }
  /* Compute the time remaining to wait.
     tv_usec is certainly positive. */
  result->tv_sec = x->tv_sec - y->tv_sec;
  result->tv_usec = x->tv_usec - y->tv_usec;
  /* Return 1 if result is negative. */
  return x->tv_sec < y->tv_sec;
}

void findTimeDiff(int respID) {
	struct timeval result, tend;
	gettimeofday(&tend, NULL);
	timeval_subtract(&result, &tend, &timeArr[respID%MAX_HASH_SIZE]);
	printf("(BackEnd): Elapsed time = %ld ms\n", result.tv_usec/1000);
}