How to handle multi client with winsock32 - c++

Question

I am learning to make a client-server communication model through socket use Winsock (C++) in my school.
And I am facing problem how to make server serve multi client with limit resources, and my mission is handle this problem using multithreading and exploration technical and Specific here is use two function in C++ and winSock is: select() and _beginthreadex(). (with limited assumptions about resources)
If I use only select() function so my server only serve max 1024 client and if I use only _beginthreadex() so it will be limited on resources.
But I need to serve more than 4096 clients just use select() and _beginthreadex() (can not use only one of them because the above reason)
This is my struct of program use select():
SOCKET client[FD_SETSIZE], connSock;
fd_set readfds, initfds; //use initfds to initiate readfds at the begining of every loop step
sockaddr_in clientAddr;
int nEvents, clientAddrLen;
bool ret;
for (int i = 0; i < FD_SETSIZE; i++)
client[i] = 0; // 0 indicates available entry
FD_ZERO(&initfds);
//Step 5: Communicate with clients
while (1) {
FD_SET(listenSock, &initfds);
for (int i = 0; i < FD_SETSIZE; ++i) {
if (client[i] > 0) {
FD_SET(client[i], &initfds);
}
}
readfds = initfds; /* structure assignment */
nEvents = select(0, &readfds, 0, 0, 0);
if (nEvents < 0) {
printf("\nError! Cannot poll sockets: %d", WSAGetLastError());
break;
}
//new client connection
if (FD_ISSET(listenSock, &readfds)) {
clientAddrLen = sizeof(clientAddr);
if ((connSock = accept(listenSock, (sockaddr *)&clientAddr, &clientAddrLen)) < 0) {
printf("\nError! Cannot accept new connection: %d", WSAGetLastError());
break;
}
else {
printf("You got a connection from %s\n", inet_ntoa(clientAddr.sin_addr)); /* prints client's IP */
cout << "\tconnSocket: " << (int)connSock << endl;
int i;
for (i = 0; i < FD_SETSIZE; i++)
if (client[i] == 0) {
client[i] = connSock;
FD_SET(client[i], &initfds);
break;
}
if (i == FD_SETSIZE) {
printf("\nToo many clients.");
closesocket(connSock);
}
if (--nEvents == 0)
continue; //no more event
}
}
//receive data from clients
for (int i = 0; i < FD_SETSIZE; i++) {
if (client[i] == 0)
continue;
if (FD_ISSET(client[i], &readfds)) {
ret = business(client[i]); // the function handle logic of my server
if (ret == false ) {
FD_CLR(client[i], &initfds);
closesocket(client[i]);
client[i] = 0;
}
}
if (--nEvents <= 0)
continue; //no more event
}
}
//Step 5: Close socket
closesocket(listenSock);
With program above my server can not serve more than 4096 client. Somebody can give a solution for my problem ? Thank you very much!

Answer 1

One thread can handle 4096 as you show. That is because a FD_SET is limited to 4096 entries. However you can have multiple threads, and multiple FD_SETs.
Each thread would have its FD_SET to read from and reply, handling up to 4k clients. When the client 4097 arrives, you create a new thread to handle a new set of sockets. You'll need synchronisation around accepting and connecting. But apart from that, the thread handling the first 4k can run independently of the thread handling the next 4k clients.
A full example is out of scope. :-)