c++ select only checks last client socket

Question

i am writing a server for a chat client i am making the problem is the following Select only lifts its block if a net client connects and if the last connected client is writing. example: i have 4 clients connected the server will keep blocking until client no. 4 writes if client 1-3 writes it keeps blocking what am i doing wrong? for analysing here is my code might contain ugly and useless code but that's just analysing its behaviour

Server.c++

#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 <boost/thread/thread.hpp>
#include <sys/time.h>
#include <sys/ioctl.h>

using namespace std;

void error(char *msg, int socket) {
    perror(msg);
    close(socket);
    exit(1);
}

int main(int argc, char** argv) {
    int sockfd, newsockfd, portno, n, highsock;
    socklen_t clilen;
    fd_set readfds;
    list<int> CliSocks;
    FD_ZERO(&readfds);
    /*
     * Sockfd, newsockfd contain values returned by the socket
     * portno stores the port number on which the server accepts connections
     * clilen stores the size of the address of the client
     * n contains the amount of character written of read
     */
    char buffer[256];
    /* buffer contains the characters read from the socket*/

    struct sockaddr_in serv_addr, cli_addr;
    /* 
     * sockaddr_in contains an internet address
     * serv_addr contains the servers address
     * cli addr contains the clients address
     */
    if (argc < 2) {
        fprintf(stderr, "ERROR no port provided");
        exit(1);
    }
    /*
     * error if no argument
     */
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    highsock = sockfd;
    FD_SET(sockfd, &readfds);
    int opt = 1;
    ioctl(sockfd, FIONBIO, &opt);

    if (sockfd < 0) {
        error("ERROR opening socket", sockfd);
    }
    /*
     * socket() creates a new socket
     * argument 1 contains the address domain
     * argument 2 contains the socket type
     * argument 3 contains the protocol should be 0
     * socket() returns a reference for itself
     */
    bzero((char*) &serv_addr, sizeof (serv_addr));
    /* empty the serv_addr variable*/
    portno = atoi(argv[1]);
    /*converts the port argument from string  to int*/
    serv_addr.sin_family = AF_INET;
    /*set the code for the address family*/
    serv_addr.sin_port = htons(portno);
    /*htons converts the portno to network bytes and gives it to the server address*/
    serv_addr.sin_addr.s_addr = INADDR_ANY;
    /*set the server ip to the ip of the running machine*/
    if (bind(sockfd, (struct sockaddr *) &serv_addr, sizeof (serv_addr)) < 0)
        error("ERROR on binding", sockfd);
    /*
     * bind() binds a socket to an address, in this case the
     * addess of the current host
     */
    listen(sockfd, 5);

    /*the listen system call allows the process to listen on the socket for connections*/


    while (1) {
        int sockcount = select(highsock + 1, &readfds, NULL, NULL, NULL);

        clilen = sizeof (cli_addr);
        newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen);
        /*
         * accept() lets the system wait until a client connects to the server
         */

        if (newsockfd > 0) {
            ioctl(newsockfd, FIONBIO, &opt);
            FD_SET(newsockfd, &readfds);
            highsock = newsockfd;
            CliSocks.push_back(newsockfd);
        }


        for (list<int>::iterator it = CliSocks.begin(); it != CliSocks.end(); it++) {
            bzero(buffer, 256);
            n = read(*it, buffer, 255);
            if(buffer[0] != 0){
            printf("Here is the message: %s", buffer);

            /*
             * bzero empties the buffer
             * read obviously reads data from the new socket descriptor
             */
            n = write(*it, "I got your message", 18);
            if (n < 0) error("ERROR writing to socket", sockfd);
                        }
        }
    }
    close(sockfd);
    return 0;
}

Answer 1

The problem is that you must reset readfds every time in the loop before the call to select. This is because the call to select modifies its parameters.

while (1) {
    FD_ZERO(&readfds);
    FD_SET(sockfd, &readfds);
    highsock = sockfd;
    for (list<int>::iterator it = CliSocks.begin(); it != CliSocks.end(); it++) {
        FD_SET(*it, &readfds);
        highsock = *it > highsock ? *it : highsock;
    }
    int sockcount = select(highsock + 1, &readfds, NULL, NULL, NULL);

    ...
}