Hi,
I am trying to write a chat program with sockets as part of my project. I made a socket server, using fork ,I tried to connect it with different clients.Each client will send messages to others through server.So server has a shared memory storing every clients username and client id. But the problem facing is first client is able to sent messages to every other client formed after that.
But second client onwards it is not able to sent message to previously connected ones. Server is not able to write on those client id's. Please help me........

You may need to avoid using a 'forking server' style of service.

Instead a event driven server using 'select' will work better. However it is a very different style of programming, though perfectly equivalent. It is how most GUI programs actually work, so it can do one thing (such as updating the display) while it sends and receives messages at the same time.

Best idea is to start by getting some Event library to help you organise, and setup the events handling.

Without fork or thread how can i make a new process.. Without making new process, new client can't connect with server.

The problem in my program is I connect clients c1, c2, c3 using fork to the server. From c1 if i try to sent messages to c2 &c3 it will work(message from c1 goes to server, server finds the client_id's of c2&c3, write the msg to them. in the shared memory client names and corresponding id is stored).

But if c2 tries to sent to c1 & c3, server is able to write to c3. but it fails to write to c1(error: bad file descriptor).

if c3 tries to write to c1&c2, both will fail.

Now if i make new client c4, c1 is able to write to c2,c3,c4. c2 is able to write to c3&c4, not to c1.
c3 can write to c4 not to c1&c2. c4 can't write to c1,c2&c3.

I think it is the problem of server. sometimes server is able to identify its clientids &write on them, but sometimes not.
I dont know why it is happening like this.I am not getting a solution.

1. which language are you using ?
2. read http://beej.us/guide/bgnet/
3. you may want to ask the Mods (via the Report button) to move this to the Programming forum

fork is not nessary. when the client communication stream is 'accept()'ed you get a new separate file handle for that specific client.

select() can let you know when you have messages waiting on that stream file handle, or even when you have a new client waiting for an accept on the socket file handle.

The only reason 'fork()' method is taught is because it is simple to implement and understand. But it is not the only way, and not always the best way.

Hi,

select works better for you instead of fork or thread..

Giving you the code for multiuser chat server using select...

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>

#define PORT "9034" // 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)
{
fd_set master; // master file descriptor list
fd_set read_fds; // temp file descriptor list for select()
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);

// 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: %s\n", 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 bind\n");
exit(2);
}

freeaddrinfo(ai); // all done with this

// listen
if (listen(listener, 1) == -1) {
perror("listen");
exit(3);
}

// add the listener to the master set
FD_SET(listener, &master);

// keep track of the biggest file descriptor
fdmax = listener; // so far, it's this one

// main loop
for(; {
read_fds = master; // copy it
printf("Socket desc=%d\n",listener);
printf("Blocking in select.......\n");
if (select(fdmax+1, &read_fds, NULL, NULL, NULL) == -1) {
perror("select");
exit(4);
}

// run through the existing connections looking for data to read
printf("fdmax==%d\n",fdmax);
for(i = 0; i <= fdmax; i++) {
printf("i=%d\n",i);

if (FD_ISSET(i, &read_fds))
{ // we got one!!
printf("IN FD ISSET\n");
if (i == listener)
{
printf("handle new connection\n");
// handle new connections
addrlen = sizeof remoteaddr;
newfd = accept(listener,(struct sockaddr *)&remoteaddr, &addrlen);

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 %d\n",
inet_ntop(remoteaddr.ss_family,
get_in_addr((struct sockaddr*)&remoteaddr),
remoteIP, INET6_ADDRSTRLEN),
newfd);
}
}
else
{
printf("Hadndle data from client\n");
// 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 up\n", i);
}
else {
perror("recv");
}
close(i); // bye!
FD_CLR(i, &master); // remove from master set
}
else
{
printf("Data send to client");
// we got some data from a client
for(j = 0; j <= fdmax; j++)
{
// send to everyone!
if (FD_ISSET(j, &master))
{
// except the listener and ourselves
printf("listerner==%d:%d:%d\n",listener,j,i);
if (j != listener && j != i)
{
printf("send\n");
if (send(j, buf, nbytes, 0) == -1)
{
perror("send");
}
}
}
}
}
} // END handle data from client
} // END got new incoming connection
} // END looping through file descriptors
} // END for(;--and you thought it would never end!

return 0;
}

Please put code in a code block, you have lost all indentation in the code, making it harder to follow.