Question

This is my chat server :

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <string.h>

#define LISTEN_Q 20
#define MSG_SIZE 1024

struct userlist {
    int sockfd;
    struct sockaddr addr;

    struct userlist *next;
};

int main(int argc, char *argv[])    {
    // declare.
    int listFD, newFD, fdmax, i, j, bytesrecvd;
    char msg[MSG_SIZE], ipv4[INET_ADDRSTRLEN];
    struct addrinfo hints, *srvrAI;
    struct sockaddr_storage newAddr;
    struct userlist *users, *uptr, *utemp;
    socklen_t newAddrLen;
    fd_set master_set, read_set;

    // clear sets
    FD_ZERO(&master_set);
    FD_ZERO(&read_set);

    // create a user list
    users = (struct userlist *)malloc(sizeof(struct userlist));
    users->sockfd = -1;
    //users->addr = NULL;
    users->next = NULL;

    // clear hints
    memset(&hints, 0, sizeof hints);

    // prep hints
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_flags = AI_PASSIVE;

    // get srver info
    if(getaddrinfo("localhost", argv[1], &hints, &srvrAI) != 0) {
        perror("* ERROR | getaddrinfo()\n");
        exit(1);
    }

    // get a socket
    if((listFD = socket(srvrAI->ai_family, srvrAI->ai_socktype, srvrAI->ai_protocol)) == -1)    {
        perror("* ERROR | socket()\n");
        exit(1);    
    }

    // bind socket
    bind(listFD, srvrAI->ai_addr, srvrAI->ai_addrlen);

    // listen on socket
    if(listen(listFD, LISTEN_Q) == -1)  {
        perror("* ERROR | listen()\n");
        exit(1);            
    }

    // add listfd to master_set
    FD_SET(listFD, &master_set);

    // initialize fdmax
    fdmax = listFD;

    while(1)    {
        // equate
        read_set = master_set;

        // run select
        if(select(fdmax+1, &read_set, NULL, NULL, NULL) == -1)  {
            perror("* ERROR | select()\n");
            exit(1);            
        }

        // query all sockets
        for(i = 0; i <= fdmax; i++) {
            if(FD_ISSET(i, &read_set))  { // found active sockfd
                if(i == listFD) { // new connection
                    // accept
                    newAddrLen = sizeof newAddr;
                    if((newFD = accept(listFD, (struct sockaddr *)&newAddr, &newAddrLen)) == -1)    {
                        perror("* ERROR | select()\n");
                        exit(1);            
                    }

                    // resolve ip
                    if(inet_ntop(AF_INET, &(((struct sockaddr_in *)&newAddr)->sin_addr), ipv4, INET_ADDRSTRLEN) == -1)  {
                        perror("* ERROR | inet_ntop()");
                        exit(1);
                    }
                    fprintf(stdout, "* Client Connected | %s\n", ipv4);

                    // add to master list
                    FD_SET(newFD, &master_set);

                    // create new userlist component
                    utemp = (struct userlist*)malloc(sizeof(struct userlist));
                    utemp->next = NULL;
                    utemp->sockfd = newFD;
                    utemp->addr = *((struct sockaddr *)&newAddr);

                    // iterate to last node
                    for(uptr = users; uptr->next != NULL; uptr = uptr->next)    {
                    }

                    // add
                    uptr->next = utemp;

                    // update fdmax
                    if(newFD > fdmax)
                        fdmax = newFD;                  
                }
                else    {   // existing sockfd transmitting data
                    // read
                    if((bytesrecvd = recv(i, msg, MSG_SIZE, 0)) == -1)  {
                        perror("* ERROR | recv()\n");
                        exit(1);
                    }
                    msg[bytesrecvd] = '\0';

                    // find out who sent?
                    for(uptr = users; uptr->next != NULL; uptr = uptr->next)    {
                        if(i == uptr->sockfd)
                            break;
                    }

                    // resolve ip
                    if(inet_ntop(AF_INET, &(((struct sockaddr_in *)&(uptr->addr))->sin_addr), ipv4, INET_ADDRSTRLEN) == -1) {
                        perror("* ERROR | inet_ntop()");
                        exit(1);
                    }

                    // print
                    fprintf(stdout, "%s\n", msg);

                    // send to all
                    for(j = 0; j <= fdmax; j++) {
                        if(FD_ISSET(j, &master_set))    {
                            if(send(j, msg, strlen(msg), 0) == -1)
                                perror("* ERROR | send()");
                        }
                    }
                } // handle read from client
            } // end select result handle
        } // end looping fds
    } // end while

    return 0;
}

This is my client:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <string.h>

#define MSG_SIZE 1024

int main(int argc, char *argv[])    {
    // declare.
    int newFD, bytesrecvd, fdmax;
    char msg[MSG_SIZE];
    fd_set master_set, read_set;
    struct addrinfo hints, *srvrAI;

    // clear sets
    FD_ZERO(&master_set);
    FD_ZERO(&read_set);

    // clear hints
    memset(&hints, 0, sizeof hints);

    // prep hints
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_flags = AI_PASSIVE;

    // get srver info
    if(getaddrinfo(argv[1], argv[2], &hints, &srvrAI) != 0) {
        perror("* ERROR | getaddrinfo()\n");
        exit(1);
    }

    // get a socket
    if((newFD = socket(srvrAI->ai_family, srvrAI->ai_socktype, srvrAI->ai_protocol)) == -1) {
        perror("* ERROR | socket()\n");
        exit(1);    
    }

    // connect to server
    if(connect(newFD, srvrAI->ai_addr, srvrAI->ai_addrlen) == -1)   {
        perror("* ERROR | connect()\n");
        exit(1);    
    }

    // add to master, and add keyboard
    FD_SET(newFD, &master_set);
    FD_SET(STDIN_FILENO, &master_set);

    // initialize fdmax
    if(newFD > STDIN_FILENO)
        fdmax = newFD;
    else
        fdmax = STDIN_FILENO;

    while(1)    {
        // equate
        read_set = master_set;

        if(select(fdmax+1, &read_set, NULL, NULL, NULL) == -1)  {
            perror("* ERROR | select()");
            exit(1);
        }

        // check server
        if(FD_ISSET(newFD, &read_set))  {
            // read data
            if((bytesrecvd = recv(newFD, msg, MSG_SIZE, 0)) < 0 )   {
                perror("* ERROR | recv()");
                exit(1);
            }
            msg[bytesrecvd] = '\0';

            // print
            fprintf(stdout, "%s\n", msg);
        }

        // check keyboard
        if(FD_ISSET(STDIN_FILENO, &read_set))   {
            // read data from stdin
            if((bytesrecvd = read(STDIN_FILENO, msg, MSG_SIZE)) < 0)    {
                perror("* ERROR | read()");
                exit(1);
            }
            msg[bytesrecvd] = '\0';


            // send
            if((send(newFD, msg, bytesrecvd, 0)) == -1) {
                perror("* ERROR | send()");
                exit(1);
            }
        }
    }

    return 0;
}

The problem is with the part where the server recv()s data from an FD, then tries echoing back to all [send() ]; it just dies, w/o errors, and my client is left looping 🙁

Answer 1

Two issues I see with the server at least:

1. msg[bytesrecvd] = '\0'; overruns the buffer by one (this can be avoided by doing recv(i, msg, MSG_SIZE - 1, 0) instead;
2. the “send to all” part tries to write to server socket too.