Modify the client.c and server.c in this way:

After the client receives This server has been contacted 1 times, instead of exit, the client sends back This client has been contacted 1 times to the server. The server then sends back This server has been contacted 2 times, the client sends back This client has been contacted 2 times to the server, and so on. That is, both the client and the server do not exit. They alternatively send messages to each other.

/////////////////////////////////////////////////////////////////////////////

client:

/* Create a socket. */

sd = socket(PF_INET, SOCK_STREAM, ptrp->p_proto);

if (sd < 0) {

fprintf(stderr, "socket creation failed ");

exit(1);

}

/* Connect the socket to the specified server. */

if (connect(sd, (struct sockaddr *)&sad, sizeof(sad)) < 0) {

fprintf(stderr,"connect failed ");

exit(1);

}

/* Repeatedly read data from socket and write to user.s screen. */

n = recv(sd, buf, sizeof(buf), 0);

while (n > 0) {

write(1,buf,n);

printf("n: %d ", n);

n = recv(sd, buf, sizeof(buf), 0);

}

printf("n: %d ", n);

/* Close the socket. */

closesocket(sd);

/* Terminate the client program gracefully. */

exit(0);

}

////////////////////////////////////////////////////////////////////////////////////////

server:

/* Create a socket */

sd = socket(PF_INET, SOCK_STREAM, ptrp->p_proto);

if (sd < 0) {

fprintf(stderr, "socket creation failed ");

exit(1);

}

/* Bind a local address to the socket */

if (bind(sd, (struct sockaddr *)&sad, sizeof(sad)) < 0) {

fprintf(stderr,"bind failed ");

exit(1);

}

/* Specify size of request queue */

if (listen(sd, QLEN) < 0) {

fprintf(stderr,"listen failed ");

exit(1);

}

/* Main server loop - accept and handle requests */

while (1) {

alen = sizeof(cad);

if ( (sd2=accept(sd, (struct sockaddr *)&cad, &alen)) < 0) {

fprintf(stderr, "accept failed ");

exit(1);

}

visits++;

sprintf(buf,"This server has been contacted %d time%s ",

visits,visits==1?".":"s.");

send(sd2,buf,strlen(buf),0);

closesocket(sd2);

}