I have this code that uses epoll and it has a problem. When I run it, it gives output: Server-socket() is OK... Server-bind() is OK... 3 4 accept: Invalid argument

I'm running it on ubuntu linux, system updated, both as limited user and root what is wrong with the input to accept? What should I change?

struct epoll_event ev, events[MAX_EVENTS];
   struct sockaddr_in serveraddr;
   int listen_sock, conn_sock, nfds, epollfd;
   int yes = 1;

   /* Set up listening socket, 'listen_sock' (socket(),
   bind(), listen()) */

 if((listen_sock = socket(AF_INET, SOCK_STREAM, 0)) == -1) 
 {
  perror("Server-socket() error lol!");
  //just exit lol!
  exit(1);
 }
 printf("Server-socket() is OK...\n");
 //"address already in use" error message 
 /*if(setsockopt(listen_sock, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int)) == -1) 
 {
  perror("Server-setsockopt() error lol!");
  exit(1);
 }
 printf("Server-setsockopt() is OK...\n");*/
 // bind 
 serveraddr.sin_family = AF_INET;
 serveraddr.sin_addr.s_addr = INADDR_ANY;
 serveraddr.sin_port = htons(PORT);
 memset(&(serveraddr.sin_zero), '\0', 8);
 if(bind(listen_sock, (struct sockaddr *)&serveraddr, sizeof(serveraddr)) == -1) 
 {
  perror("Server-bind() error lol!");
  exit(1);
 }
 printf("Server-bind() is OK...\n");

 epollfd = epoll_create(MAX_EVENTS);
 if (epollfd == -1) {
    perror("epoll_create");
    exit(EXIT_FAILURE);
 }

 ev.events = EPOLLIN;
 ev.data.fd = listen_sock;
 if (epoll_ctl(epollfd, EPOLL_CTL_ADD, listen_sock, &ev) == -1) {
    perror("epoll_ctl: listen_sock");
    exit(EXIT_FAILURE);
 }

 for (;;) {
    nfds = epoll_wait(epollfd, events, MAX_EVENTS, -1);
    if (nfds == -1) {
     perror("epoll_pwait");
     exit(EXIT_FAILURE);
    }

    for (int n = 0; n < nfds; ++n) {
     if (events[n].data.fd == listen_sock) {
      struct sockaddr_in clientaddr;
      socklen_t addrlen = sizeof(clientaddr);
      cout <<listen_sock <<'\n' <<epollfd <<'\n';
      conn_sock = accept(listen_sock, (struct sockaddr *) &clientaddr, &addrlen);
      if (conn_sock == -1) {
       perror("accept");
       exit(EXIT_FAILURE);
      }
      fcntl(conn_sock, F_SETFL, fcntl(conn_sock, F_GETFD, 0)|O_NONBLOCK);
      ev.events = EPOLLIN | EPOLLET;
      ev.data.fd = conn_sock;
      if (epoll_ctl(epollfd, EPOLL_CTL_ADD, conn_sock,
         &ev) == -1) {
       perror("epoll_ctl: conn_sock");
       exit(EXIT_FAILURE);
      }
     } else {
      printf("%d\n", events[n].data.fd);
     }
    }
 }

Correct me if I'm wrong, but shouldn't you first accept incoming connection and this connection put into epoll? Epoll will tell when the socket is ready to read or write.

As far as i see, epoll_wait is to early...

After bind you should accept.