如何使用select从服务器发送数据到客户端

我在一些教程的帮助下编写了一个代码，使用select函数与客户端连接并接受来自客户端的消息。现在我要做的是在服务器需要时将数据发送到特定的客户机。如何做到这一点??提前谢谢。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#define PORT "9999"   // port we're listening on
// get sockaddr, IPv4 or IPv6:
void *get_in_addr(struct sockaddr *sa)
{
if (sa->sa_family == AF_INET) 
{
    return &(((struct sockaddr_in*)sa)->sin_addr);
}
return &(((struct sockaddr_in6*)sa)->sin6_addr);
}
int main(void)
{
struct timeval tv;
tv.tv_sec = 2;
tv.tv_usec = 500000;
fd_set master;    // master file descriptor list
fd_set read_fds;  // temp file descriptor list for select()
fd_set write_fds;
int fdmax;        // maximum file descriptor number
int listener;     // listening socket descriptor
int newfd;        // newly accept()ed socket descriptor
struct sockaddr_storage remoteaddr; // client address
socklen_t addrlen;
char buf[256];    // buffer for client data
int nbytes;
char remoteIP[INET6_ADDRSTRLEN];
int yes=1;        // for setsockopt() SO_REUSEADDR, below
int i, j, rv;
struct addrinfo hints, *ai, *p;
FD_ZERO(&master);    // clear the master and temp sets
FD_ZERO(&read_fds);
FD_ZERO(&write_fds);
// get us a socket and bind it
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
hints.ai_flags = AI_PASSIVE;
if ((rv = getaddrinfo(NULL, PORT, &hints, &ai)) != 0) 
{
    fprintf(stderr, "selectserver: %sn", gai_strerror(rv));
    exit(1);
}
for(p = ai; p != NULL; p = p->ai_next) 
{
    listener = socket(p->ai_family, p->ai_socktype, p->ai_protocol);
    if (listener < 0) 
    { 
        continue;
    }
    // lose the pesky "address already in use" error message
    setsockopt(listener, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int));
    if (bind(listener, p->ai_addr, p->ai_addrlen) < 0) 
    {
        close(listener);
        continue;
    }
    break;
}
// if we got here, it means we didn't get bound
if (p == NULL) 
{
    fprintf(stderr, "selectserver: failed to bindn");
    exit(2);
}
freeaddrinfo(ai); // all done with this
// listen
if (listen(listener, 10) == -1) {
    perror("listen");
    exit(3);
}
// add the listener to the master set
FD_SET(listener, &master);
printf("Listener is %d n" , listener);
// keep track of the biggest file descriptor
fdmax = listener; // so far, it's this one
//accept 3 clients

// main loop
for(;;) {
    read_fds = master; // copy it
    write_fds = master;
    if (select(fdmax+1, &read_fds, &write_fds, NULL, NULL) == -1) 
    {
        perror("select");
        exit(4);
    }
    //WRITE TO CONNECTIONS IF SOME SIGNAL OCCURED
    if(signal occured)
    {
        send data to some client;
    }

    // run through the existing connections looking for data to read
    // ADD NEW CONNECTIONS READ FROM CONNECTIONS
    for(i = 0; i <= fdmax; i++) 
    {
        if (FD_ISSET(i, &read_fds)) 
        { // we got one!!
            // handle new connections
            if (i == listener) 
            {                    
                addrlen = sizeof(remoteaddr);
                newfd = accept(listener, (struct sockaddr *)&remoteaddr, &addrlen);
                //write(listener+1 , "SERVER MESSAGE :: ACCEPTED" ,26);
                if (newfd == -1) 
                {
                    perror("accept");
                } 
                else 
                {
                    FD_SET(newfd, &master); // add to master set
                    if (newfd > fdmax) 
                    {    // keep track of the max
                        fdmax = newfd;
                    }
                    printf("selectserver: new connection from %s on socket %dn", inet_ntop(remoteaddr.ss_family,
                            get_in_addr((struct sockaddr*)&remoteaddr), remoteIP, INET6_ADDRSTRLEN), newfd);
                } 
            } else 
            {
                // handle data from a client
                if ((nbytes = recv(i, buf, sizeof buf, 0)) <= 0) 
                {
                    // got error or connection closed by client
                    if (nbytes == 0) 
                    {
                        // connection closed
                        printf("selectserver: socket %d hung upn", i);
                    } else 
                    {
                        perror("recv");
                    }
                    close(i); // bye!
                    FD_CLR(i, &master); // remove from master set
                } else 
                {
                    // we got some data from a client
                    printf("Buffer we got %s ",buf);
                }
            } // END handle data from client
        } // END got new incoming connection
        memset(&buf[0], 0, sizeof(buf));
    } // END looping through file descriptors
} // END for(;;)--and you thought it would never end!

return 0;

}