TCP socket server with multi-client support with poll()

main.cpp

// NOTE : you need the library fmt to format strings
// Command to use : `g++ server.cpp -o server -lfmt`

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#include <vector>
#include <string>
#include <iostream>
#include <fmt/core.h>

#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/socket.h>

#include <poll.h>

struct client_t {
    int fd;
    std::string strBuffer;
};

// send a message to every client, except `except_fd` if non-zero
void send_to_clients(const std::vector<client_t>& clients, const std::string& msg, int except_fd=0){
    for(const auto& loop_client : clients){
        if(loop_client.fd==except_fd)continue;
        write(loop_client.fd, msg.c_str(), msg.size());
    }
}

int main(){
    int sock_server = socket(AF_INET, SOCK_STREAM, 0);
    if(sock_server<0){
        perror("socket() failed");
        return EXIT_FAILURE;
    }

    int yes = 1;
    if (setsockopt(sock_server, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int))<0){
        perror("setsockopt() failed");
        return EXIT_FAILURE;
    }

    struct sockaddr_in address;
    address.sin_family = AF_INET;
    address.sin_addr.s_addr = INADDR_ANY;
    address.sin_port = htons(8080);

    if(bind(sock_server, (struct sockaddr*) &address, sizeof(struct sockaddr_in))<0){
        perror("bind() failed");
        return EXIT_FAILURE;
    }

    if(listen(sock_server, 3) < 0) {
        perror("listen() failed");
        return EXIT_FAILURE;
    }

    std::cout << "Server started and listening !" << std::endl;
    
    std::vector<client_t> clients;

    while(1){

        // generate the structures for the fds we want to poll
        std::vector<pollfd> descriptors;

        pollfd pollfd_inst;
        pollfd_inst.fd = sock_server;
        pollfd_inst.events = POLLIN;
        descriptors.push_back(pollfd_inst);

        for(auto& client : clients){
            pollfd_inst.fd = client.fd;
            descriptors.push_back(pollfd_inst);
        }

        // blocking function, wait for a file descriptor to have an event
        if(!poll(descriptors.data(), descriptors.size(), -1)){
            perror("poll failed");
            return EXIT_FAILURE;
        }

        int i=0;
        for(auto pollfd_inst : descriptors){

            if(pollfd_inst.revents & POLLIN){            
                if(pollfd_inst.fd==sock_server){
                    // means we have a connection
                    std::cout << "Server socket got a new client !" << std::endl;
                    
                    struct sockaddr_in client_addr;
                    socklen_t socklen;
                    int sock_client = accept(sock_server, (struct sockaddr*) &client_addr, &socklen);
                    if(sock_client<0){
                        perror("accept() failed");
                        return EXIT_FAILURE;
                    }

                    client_t client;
                    client.fd = sock_client;
                    client.strBuffer = "";
                    clients.push_back(client);
                            
                    std::cout << "Address : " << inet_ntoa(client_addr.sin_addr) << ":" << ntohs(client_addr.sin_port) << std::endl;

                    // send welcome message
                    std::string welcome_msg = fmt::format("Welcome to the server ! Your ID is: {}\n", sock_client);

                    write(sock_client, welcome_msg.c_str(), welcome_msg.size());

                    send_to_clients(clients, fmt::format("[SYSTEM] [{}] joined the server\n", sock_client), sock_client);


                }else{
                    // means we have a message
                    #define READ_BLOCK_SIZE 1024
                    char buffer[READ_BLOCK_SIZE];

                    client_t& client = clients[i-1];

                    int bytes_read = read(pollfd_inst.fd, buffer, READ_BLOCK_SIZE-1);
                    if(bytes_read==-1){
                        // connection error
                        perror("read failed");
                        return -1;
                    }else if(bytes_read==0){
                        // connection finished
                        close(client.fd);

                        printf("Client ID=%i left\n", client.fd);
                        send_to_clients(clients, fmt::format("[SYSTEM] [{}] quit the server\n", client.fd), client.fd);
                        
                        clients.erase(clients.begin()+i-1);

                    }else{
                        buffer[bytes_read] = 0;

                        std::string& strBuffer = client.strBuffer;
                        strBuffer.append(buffer);

                        size_t nlPos;
                        while((nlPos = strBuffer.find('\n')) != std::string::npos){
                            std::string msg = strBuffer.substr(0, nlPos+1);
                            strBuffer.erase(0, nlPos+1);

                            // format message for chat
                            std::string msg_with_id = fmt::format("[CHAT] [{}] {}", client.fd, msg);
                            
                            std::cout << msg_with_id; // send to the console

                            send_to_clients(clients, msg_with_id, client.fd); // send to the clients
                        }
                    }
                }
            }
            i++;
        }
    }

    return EXIT_SUCCESS;
}