synchronize between reader and writer sockets

// contruct controlling context of UDP peers
CTRL_CTX *c_ctx = malloc(sizeof(CTRL_CTX));
c_ctx->in_serving = 0;
c_ctx->peerFirst = c_ctx->peerLast = NULL;

int ctrl_sockfd = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);
int data_sockfd = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);
char buffer[BUFFER_SIZE];
ssize_t recsize;
socklen_t udp_clilen;

struct sockaddr_in ctrl_saddr, data_saddr, udp_caddr;
udp_clilen = sizeof(udp_caddr);

// construct and bind the controlling socket
memset(&ctrl_saddr, 0, sizeof(ctrl_saddr));
ctrl_saddr.sin_family = AF_INET;
ctrl_saddr.sin_addr.s_addr = htonl(INADDR_ANY);
ctrl_saddr.sin_port = htons(7654);

if (bind(ctrl_sockfd,(struct sockaddr *) &ctrl_saddr, sizeof(ctrl_saddr)) == -1)
{
    perror("error: udp bind failed");
    close(ctrl_sockfd);
    exit(EXIT_FAILURE);
}

// construct and bind the data tranmitting socket
memset(&data_saddr, 0, sizeof(data_saddr));
data_saddr.sin_family = AF_INET;
data_saddr.sin_addr.s_addr = htonl(INADDR_ANY);
data_saddr.sin_port = htons(7655);
if (bind(data_sockfd,(struct sockaddr *) &data_saddr, sizeof(data_saddr)) == -1)
{
    perror("error: udp bind failed");
    close(data_sockfd);
    exit(EXIT_FAILURE);
}

/*
 * initialize the file descriptor set
 * to monitor the controlling socket use select()
 */
int rc;
struct pollfd ctrl_pfd;
ctrl_pfd.fd = ctrl_sockfd;
ctrl_pfd.events = POLLRDNORM;
int npfd = 1;

for (;;)
{
    rc = poll(&ctrl_pfd, npfd, -1);
    if (rc < 0)
    {
    perror("error: poll failed");
        exit(EXIT_FAILURE);
    }

    if (ctrl_pfd.revents & POLLRDNORM)
    {
    printf("controlling socket is available to read.\n");
    recsize = recvfrom(ctrl_sockfd, (void *)buffer, BUFFER_SIZE, 0, (struct sockaddr *) &udp_caddr, &udp_clilen);
    if (recsize < 0)
    {
            fprintf(stderr, "%s\n", strerror(errno));
            exit(EXIT_FAILURE);
        }
    else if (strncmp(buffer, "REQ", 3) == 0)
    {
            // a new client, record its information
        register_new_peer();
    }
    else if (strncmp(buffer, "STP", 3) == 0)
    {
        // a client finished, ask to stop
            de_register_a_peer();
    }
    else
        printf("no information of of the client is found.\n");
    }
    /* the data socket is used to send packets for clients
     * that have a record in the control context
     */
    PEER_CTX *peer = c_ctx->peerFirst;
    while (peer->peerNext != NULL)
    {
    // traverse the peers list, and send a packet
    send_ncpkt(data_sockfd, (struct sockaddr *) &(peer->cli_addr), peer->cli_salen, s_ctx);
    peer = peer->peerNext;
    }
}