Improve a server with select() function - c++

Question

I read about select() and also read many examples of it, but I can't understand when can I use it?
I understood that I can use it in the accept() function, in case I want few people to connect to the server, but it has confused me.
I need to built a server that receive data only from 2 clients, 1 every time. The first user sends to the server a string, the string responds and then the second user sends a string.
Can someone help me with combining the select() function in the recv() function?
I've added my server.cpp code. Thank you!
server:
#include <iostream>
#include <winsock2.h>
#include <string>
#include <windows.h>
#include <vector>
#pragma comment(lib,"ws2_32.lib")
#define MAX_NUMBER_OF_PLAYERS 1
#define BUFFER_SIZE 1024
#define LIMIT 1
// server side
#define INVALID_MOVE 00
#define PLEASE_ENTER_A_MOVE 15
#define PRINT_BOARD 20
#define END_GAME 30
// client side
#define MOVE 10
using namespace std;
int main()
{
WSADATA WsaDat;
SOCKET clientsock[2];
int minsock = 0;
int numsocks = MAX_NUMBER_OF_PLAYERS;
if (WSAStartup(MAKEWORD(2, 2), &WsaDat) != 0)
{
std::cout << "WSA Initialization failed!\r\n";
WSACleanup();
system("PAUSE");
return 0;
}
SOCKET serverSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
if (serverSocket == INVALID_SOCKET)
{
std::cout << "Socket creation failed.\r\n";
WSACleanup();
system("PAUSE");
return 0;
}
SOCKADDR_IN serverInf;
serverInf.sin_family = AF_INET;
serverInf.sin_addr.s_addr = INADDR_ANY;
serverInf.sin_port = htons(8888);
if (bind(serverSocket, (SOCKADDR*)(&serverInf), sizeof(serverInf)) == SOCKET_ERROR)
{
std::cout << "Unable to bind socket!\r\n";
WSACleanup();
system("PAUSE");
return 0;
}
listen(serverSocket, 5);
clientsock[0] = accept(serverSocket, NULL, NULL);
cout << "Client 1 has connected." << endl;
clientsock[1] = accept(serverSocket, NULL, NULL);
cout << "Client 2 has connected." << endl;
for (int i = 0; i < 2; i++)
{
cout << "Client " << i+1 << " Has Connected!" << endl;
}
char client1_buffer[BUFFER_SIZE];
char client2_buffer[BUFFER_SIZE];
char* clientBuffer;
// until there isn't a mate.
bool gameRunning = true;
// user represents if it's user1 (0), or user2(1)
bool user = 0;
while (gameRunning)
{
if (!user)
clientBuffer = client1_buffer;
else
clientBuffer = client2_buffer;
int in = recv(clientsock[user], clientBuffer, BUFFER_SIZE, 0);
cout << in << endl;
if (in > 0)
{
// CHECKS
// MOVE COMMAND
// IF worked, send the board to both clients. if current user = 1 ==> do user to 0 | if the user = 0 => do user to 11
// ELSE, send the current client (clientsock[user]) Error message and ask for a command again.
cout << clientBuffer << endl;
cout << " IN RECV";
char* szMessage = "15";
send(clientsock[user], szMessage, sizeof(szMessage), 0);
}
else if (in == 0)
{
// The connection has closed.
// REMEMBER : SAVE THE GAME SITUATION.
}
else
{
printf("recv failed: %d\n", WSAGetLastError());
// SEND ERROR MESSAGE TO BOTH CLIENTS
}
user = !user;
}
// Shutdown our socket
shutdown(serverSocket, SD_SEND);
// Close our socket entirely
closesocket(serverSocket);
WSACleanup();
system("pause");
return 0;
}

Answer 1

Ok, now that you corrected your code, I have to do some work.
I suggest you remove this
user = !user
and add this immediately after the beginning of the while loop:
{
int nfds = 0; // smallest number higher than all socket descriptors
fd_set set; // this contains garbage from the stack, thus ...
FD_ZERO(&set); // first clean it and then add both client sockets:
FD_SET(clientsock[0],&set); if(nfds<=clientsock[0]) nfds=clientsock[0]+1;
FD_SET(clientsock[1],&set); if(nfds<=clientsock[1]) nfds=clientsock[1]+1;
select(nfds,&set,0,0,0); // this uses and changes the content of set
bool next = !user; // next is the other user, and we try to serve it:
if(FD_ISSET(clientsock[next],&set)) user=next;
}
By the way, I like your creative way of using bool as an index, though once you have more than 2 clients, you might have to change that concept.
My code implements a little bit of scheduling policy: If there is data available from both clients, it reads from that client from which it did not read data the previous time. If, instead, you want to read lots of data from one client and then lots of data from the other, replace the lines containing next by
if(!FD_ISSET(clientsock[user],&set)) user=!user;
The first version tries to alternate as often as possible, while the second tries to read as much data as possible from the same client before switching to the other.