select() help

rrlangly · 10-05-2011, 11:50 AM

Once I send a single UDP msg from my client to this server, it receives the msg, but then acts as if it's stuck in a loop and FD_ISSET always returns true thereafter. And when the recvmsg() below is called, it always has the same msg. It acts like there's always something on the incoming socket to read.

What am I doing wrong?

Code:

    struct timeval timeout;
    fd_set read_flags;

    timeout.tv_sec = 1;
    timeout.tv_usec = 0;

    fd_flags = fcntl(sfd, F_GETFL);
  
    err = fcntl(sfd, F_SETFL, fd_flags |= O_NONBLOCK);

    if (err < 0) { ... }
    
    while (running) {
        FD_ZERO(&read_flags);
        FD_SET(sfd, &read_flags);

        if (FD_ISSET(sfd, &read_flags)) {
            // this returns true, always falls here.
        }
        
        ready = select(sfd + 1, &read_flags, NULL, NULL, &timeout);
                       
        if (ready < 0) {
            fprintf(...);
            continue;  
        }
        else if (ready == 0) 
            continue;

        if (FD_ISSET(sfd, &read_flags)) {
			// always returns true, always falls here.
			my_read();
        }
    }

int MyTest::my_read()
{
    char msgbuf[512];

    struct sockaddr_storage src_addr;

    memset(msgbuf, 0x0, MSG_SZ);

    memset(iov, 0x0, sizeof(iovec));
    iov[0].iov_base = msgbuf;
    iov[0].iov_len = sizeof(msgbuf);

    memset(&msg, 0x0, sizeof(msghdr));
    msg.msg_name = &src_addr;
    msg.msg_namelen = sizeof(src_addr);
    msg.msg_iov = iov;
    msg.msg_iovlen = 1;
    msg.msg_control = 0;
    msg.msg_controllen = 0;

    received = recvmsg(sfd, &msg, fd_flags);

    if (received < 0) {
        goto out;
    }
    else if (msg.msg_flags & MSG_TRUNC) {
        goto out;
    }

    // print msgs
    // same msg always appears, even though cli sent it only 
    // once and shuts down after it sends.

    return received;
}

dwhitney67 · 10-05-2011, 03:42 PM

What's the point of setting the socket to be non-blocking if you plan to use select()?

Comment out this line to see if you get better results:

Code:

err = fcntl(sfd, F_SETFL, fd_flags |= O_NONBLOCK);

Also, and this may be the main issue... you need to reset the 'timeout' variable each iteration through the while-loop. It may be decremented after the call to select().

ta0kira · 10-05-2011, 08:59 PM

Quote:

Originally Posted by rrlangly

Code:

        if (FD_ISSET(sfd, &read_flags)) {
            // this returns true, always falls here.
        }

What does this comment mean? You realize you're testing to see if sfd is set right after you explicitly set it, right?

Quote:

Originally Posted by rrlangly

Code:

    received = recvmsg(sfd, &msg, fd_flags);

Why are you using fcntl flags where the recvmsg flags go? There's a chance that one of the bits in fd_flags coincides with MSG_PEEK. On my system O_RDWR==MSG_PEEK, and since it's a socket I'm sure this would cause your problem if the same equality holds on your system.
Kevin Barry