Hello,
I want to know whats difference between following programs? Does they both are used to allow multiple clients in a LAN to connect to one single Server in that LAN and to have simulatneous communication.

****************************SERVER1***********************
#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <netinet/in.h>
#include <signal.h>
#include <unistd.h>

int main()
{
int server_sockfd, client_sockfd;
int server_len, client_len;
struct sockaddr_in server_address;
struct sockaddr_in client_address;

server_sockfd = socket(AF_INET, SOCK_STREAM, 0);

server_address.sin_family = AF_INET;
server_address.sin_addr.s_addr = htonl(INADDR_ANY);
server_address.sin_port = htons(9734);
server_len = sizeof(server_address);
bind(server_sockfd, (struct sockaddr *)&server_address, server_len);

listen(server_sockfd, 5);
signal(SIGCHLD, SIG_IGN);

while(1) {
char ch;
printf("server waiting\n");
client_len = sizeof(client_address);
client_sockfd = accept(server_sockfd,
(struct sockaddr *)&client_address, &client_len);

if(fork() == 0) {
read(client_sockfd, &ch, 1);
sleep(5);
ch++;
write(client_sockfd, &ch, 1);
close(client_sockfd);
exit(0);
}
else {
close(client_sockfd);
}
}
}

**************************************SERVER2******************
#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <netinet/in.h>
#include <sys/time.h>
#include <sys/ioctl.h>
#include <unistd.h>

int main()
{
int server_sockfd, client_sockfd;
int server_len, client_len;
struct sockaddr_in server_address;
struct sockaddr_in client_address;
int result;
fd_set readfds, testfds;

server_sockfd = socket(AF_INET, SOCK_STREAM, 0);

server_address.sin_family = AF_INET;
server_address.sin_addr.s_addr = htonl(INADDR_ANY);
server_address.sin_port = htons(9734);
server_len = sizeof(server_address);

bind(server_sockfd, (struct sockaddr *)&server_address, server_len);
listen(server_sockfd, 5);

FD_ZERO(&readfds);
FD_SET(server_sockfd, &readfds);

while(1) {
char ch;
int fd;
int nread;

testfds = readfds;

printf("server waiting\n");
result = select(FD_SETSIZE, &testfds, (fd_set *)0,
(fd_set *)0, (struct timeval *) 0);

if(result < 1) {
perror("server5");
exit(1);
}


for(fd = 0; fd < FD_SETSIZE; fd++) {
if(FD_ISSET(fd,&testfds)) {

if(fd == server_sockfd) {
client_len = sizeof(client_address);
client_sockfd = accept(server_sockfd,
(struct sockaddr *)&client_address, &client_len);
FD_SET(client_sockfd, &readfds);
printf("adding client on fd %d\n", client_sockfd);
}

else {
ioctl(fd, FIONREAD, &nread);

if(nread == 0) {
close(fd);
FD_CLR(fd, &readfds);
printf("removing client on fd %d\n", fd);
}

else {
read(fd, &ch, 1);
sleep(5);
printf("serving client on fd %d\n", fd);
ch++;
write(fd, &ch, 1);
}
}
}
}
}
}

The difference is in the technique they use to allow multiple clients. The first one uses fork(), so every user has a separate process, second program uses select() call. They have different use.

select() version uses less resources. If one request may take a long time, it's not a good thing - other clients will also be blocked. select() is good when you have a big number or calls, but they're rather short.

The way your first program is used is not good - separate processes eat too much resources. Use threads instead. But still, threads still add overhead - process switching. This version is good if the program can block when processing request data. It won't block other users.