Question

The program is supposed to accept telnet connections on port 8888 and then send and messages from each telnet client using poll(), send() and recv() but it doesn’t quite work 100%. It seems certain connections can always send messages to anyone and the program works fine but there is always at least one client who cannot send messages. All clients can always receive (poll doesn’t register incoming data).

This code runs on it’s own so if you put it in a file and compile it with gcc -o app filename.c then you can telnet to localhost on port 8888 and see it not working. This code was written for Fedora but shouldn’t have anything non-Linux specific in it. Any help would really be appreciated.

#include <stdio.h> #include <stdlib.h> #include <poll.h> #include <unistd.h> #include <sys/time.h> #include <sys/types.h> #include <string.h> #include <signal.h> #include <sys/socket.h> #include <netinet/in.h> #include <arpa/inet.h> #include <errno.h>  #define PORT 8888 #define MAX_CONN 10 #define SECOND 1000 #define TIMEOUT (30 * SECOND)  static int listen_socket();  int main(int argc, char **argv) {     struct pollfd **my_fds;                  //array of pollfd structures for poll()     struct pollfd *curr, *new_conn;          //so I can loop through     int num_fds;                             //count of how many are being used     int i, j;                                //for loops     char buff[255], buff2[255];              //for sending and recieving text     struct sockaddr_in my_addr, their_addr;  // my address information     socklen_t sin_size;     int buff_sz;                             //size of data recieved      printf('App Started\n');      //allocate space for 10      my_fds = (struct pollfd**) malloc(sizeof(struct pollfd*) * MAX_CONN);      //set all the pointers to NULL     for (i = 0; i < MAX_CONN; i++)         *(my_fds + i) = NULL;      //I call listen_socket() which creates a socket to listen to     //this is anchored into my_fds array at element 0.     curr = (struct pollfd*) malloc (sizeof(struct pollfd));     curr->fd = listen_socket();     curr->events = POLLIN;     curr->revents = 0;      *my_fds = curr;      printf('Listening socket fd locked always at position zero in array: %d\n', curr->fd);      //num_fds, the count of items in the array is set to 1     //because the listen socket is already present     num_fds = 1;      //This is the main loop.     //While (true)     //  set all struct pollfd items revents to 0     //  call poll     //  loop through, see if there is data to read     //      read the data     //          loop through all sockets (except the listen_socket()) and send the data.     while (1)     {          //reset all event flag         for (i = 1; i < num_fds; i++)         {             curr = *(my_fds + i);             curr->events = POLLIN | POLLPRI;             printf('%i: fd %i\n', i, curr->fd);             curr->revents = 0;             send(curr->fd, 'Enter some text:\n', 18, 0);         }          //put all this into poll and wait for something magical to happen         printf('calling poll (%d sockets)\n', num_fds);         if (poll(*my_fds, num_fds, TIMEOUT) == -1)         {             perror('poll');             exit(0);         }          printf('poll returned!\n');          //First item is the accepting socket....check it independently of the rest!         curr = *my_fds;         if (curr->revents != 0)         {             printf('We have a new connection.\nAccept goes here...\n');              //Accept the connection             sin_size = sizeof their_addr;             new_conn = (struct pollfd*) malloc(sizeof(struct pollfd));             new_conn->fd = accept(curr->fd, (struct sockaddr *)&their_addr, &sin_size);             new_conn->events = POLLIN;             new_conn->revents = 0;              printf('Connection from %s\n', inet_ntoa(their_addr.sin_addr));             sprintf(buff, 'Your %i\n', num_fds);             send(new_conn->fd, buff, 7, 0);              //Add it to the poll call             *(my_fds + num_fds) = new_conn;             num_fds++;          }         else         {             //skip first one, we know that's the accepting socket (handled above).             for (i = 1; i < num_fds; i++)             {                 curr = *(my_fds + i);                 if (curr->revents != 0)                 {                     buff_sz = recv(curr->fd, &buff, 254, 0);                     buff[buff_sz] = '\0';                     printf('Recieved: %s', buff);                      //send the message to everyone else                     for (j = 1; j < num_fds; j++)                     {                         printf('i = %i, j = %i\n', i, j);                         if (j != i)                         {                             new_conn = *(my_fds + j);                             sprintf(buff2, '%i sent you %i: %s', i, j, buff);                             send(new_conn->fd, buff2, strlen(buff2) + 1, 0);                         }                     }                 }             }         }     }      printf('App Ended\n'); }  static int listen_socket() {     struct sockaddr_in a;     int s;     int yes;      if ((s = socket(AF_INET, SOCK_STREAM, 0)) < 0) {         perror('socket');         return -1;     }     yes = 1;     if (setsockopt(s, SOL_SOCKET, SO_REUSEADDR,             (char *) &yes, sizeof(yes)) < 0) {         perror('setsockopt');         close(s);         return -1;     }     memset(&a, 0, sizeof(a));     a.sin_port = htons(PORT);     a.sin_family = AF_INET;     if (bind(s, (struct sockaddr *) &a, sizeof(a)) < 0) {         perror('bind');         close(s);         return -1;     }     printf('Accepting connections on port %d\n', PORT);     listen(s, 10);     return s; } 

Answer 1

Isn’t poll supposed to take an array of structures? From the man page:

   int poll(struct pollfd fds[], nfds_t nfds, int timeout); 

And you’re passing an array of pointers to structures:

struct pollfd **my_fds;  

I think you need to have

struct pollfd *my_fds = calloc(sizeof(pollfd), MAX_CONN);  

Then your code has a chance to work.