socket recv() 没有接收到数据

Question

I have a server daemon listening on a TCP unix domain/local socket. Multiple clients running on the same machine connect to it. The daemon is also bound to a UDP Inte.net socket. Whenever the daemon receives any data from one of the local clients, it sends the same data to all the connected clients except the sending client. If the daemon receives data on the UDP inte.net socket, it needs to send that data to all the local connected clients. The sending/receiving of data works perfectly when the daemon receives data on the local socket. However, the clients do not receive any data when the server sends them data received on the UDP inte.net socket. The clients receive that inte.net data either after the server daemon is exited and the connection is closed, or, when any of the clients sends data locally to the server. The inte.net data is received by the clients along with the local data. I have set both local and .net sockets as blocking using fcntl() . Here is the daemon code that I have (I have removed all the unnecessary code):

while(1)
{
  FD_SET(sockfd, &read_fds);
  FD_SET(inet_sock, &read_fds);
  for (i = 0; i < nclients; i++)
  {
    FD_SET(clients[i], &read_fds);
  }

  select(maxfd + 1, &read_fds, &write_fds, &except_fds, NULL);

  /* Check for events on inet sock */
  if (FD_ISSET(inet_sock, &read_fds))
  {
    /* Read from inet sock */
    socklen = sizeof(dest_sin);
    rval = recvfrom(inet_sock, buf, BUFLEN-1, MSG_DONTWAIT,
                    (struct sockaddr *) &dest_sin, &socklen);        
    buf[rval]=0;
    fprintf(stderr, "Received: %d (%d) bytes containing %s", rval, strlen(buf), buf);

    /* Send the message to every other client */
    for(j=0; j < nclients; j++)
    {
      send(clients[j], buf, strlen(buf), MSG_DONTWAIT);
    }
  }

  /* A read event on the local socket is a new connection */
  if (FD_ISSET(sockfd, &read_fds))
  {
    socklen = sizeof(dest_sun);
    /* Accept the new connection */
    rval = accept(sockfd, (struct sockaddr *) &dest_sun, &socklen);

    /* Add client to list of clients */
    clients[nclients++] = rval;
    if (rval > maxfd) maxfd = rval;
    snprintf(s, BUFLEN, "You are client %d [%d]. You are now connected.\n\0",
        nclients, rval);
    send(rval, s, strnlen(s, BUFLEN), MSG_DONTWAIT);
  }

  /* Check for events from each client */
  for (i = 0; i < nclients; i++)
  {
    fprintf(stderr,"Checking client %d [%d] for read indicator.\n",i, clients[i]);

    /* Client read events */
    if (FD_ISSET(clients[i], &read_fds))
    {
      fprintf(stderr, "Client %d [%d] marked for read.\n", i, clients[i]);

      /* Read from client */
      rval=recv(clients[i], buf, BUFLEN-1, MSG_DONTWAIT);

      buf[rval]=0;
      fprintf(stderr, "Received: %d (%d) bytes containing %s", rval, strlen(buf), buf);

      /* Send the message to every other client */
      for(j=0; j < nclients; j++)
      {
        /* Skip the sender */
        if (j == i) continue;
        /* Send the message */
        send(clients[j], s, strlen(s, BUFLEN), MSG_DONTWAIT);
      }
    }
  } 
}

Here is the client code that I have:

while(1)
{
  FD_SET(fileno(stdin), &read_fds);
  FD_SET(sockfd, &read_fds);
  select(fileno(stdin) > sockfd ? fileno(stdin)+1 : sockfd+1,
&read_fds, &write_fds, &except_fds, NULL);

  if (FD_ISSET(sockfd, &read_fds))
  {
    /* Read from socket and display to user */
    mlen = recv(sockfd, (void *)buf, BUFLEN-1, MSG_DONTWAIT);
    buf[mlen]=0;
    printf("Received %d bytes: %s", mlen, buf);
  }

  if (FD_ISSET(fileno(stdin), &read_fds))
  {
    fgets(buf, BUFLEN, stdin);
    fprintf(stderr, "Sent %d octets to server.", 
    send(sockfd, (void *)buf, (size_t) strnlen(buf, BUFLEN), 0));
  }
}

The goal is to have the clients receive data immediately that the daemon sends them (the data which the daemon receives on its .net socket).

EDIT: I have figured that when the daemon sends the data, the select() on the client side returns that the socket is readable, but recv() is blocking, that's the reason I'm not getting data on the client side. Any suggestions on how to fix this?

Answer 1

Here's the send() calls from your code, extracted and aligned:

send(clients[j], buf, strlen(buf),        MSG_DONTWAIT);
send(rval,       s,   strnlen(s, BUFLEN), MSG_DONTWAIT);
send(clients[j], s,   strlen(s, BUFLEN),  MSG_DONTWAIT);

I see some inconsistencies here. Sometimes you call strlen() , sometimes strnlen() , and sometimes strlen() with two arguments (I don't even know what that's going to do).

The problem you're seeing may be related to the fact that you're not sending any information on the socket that shows where the boundaries between messages are. Over a stream socket, message boundaries are not preserved and you should take care to include appropriate framing information in your protocol so that the receiver can extract the individual messages. You cannot rely on exactly the same number of bytes coming through a recv() call as there was in a send() call. You will get the same total number of bytes in the same order (that's the point of a stream socket), but the messages might get consolidated or split up and you have no control over that.