Using recvmsg() with non-unicast datagram (UDP) socket: inconsistent results

Lavrentiev, Anton (NIH/NLM/NCBI) [C] lavr@ncbi.nlm.nih.gov
Tue Jul 31 16:44:00 GMT 2012


Hi,

Observed is an inconsistent result when recvmsg() is used on a
UDP socket with or without ancillary control message parameters.

Here's is an example:

    static char buf[1 << 16];
    struct sockaddr_in sin;
    struct msghdr msg;
    struct iovec iov;
#ifndef CYGWIN_BUG
    char cmsgbuf[32];
#endif /*CYGWIN_BUG*/
    int n;

    iov.iov_base       = buf;
    iov.iov_len        = sizeof(buf);
    memset(&sin, 0, sizeof(sin));
    memset(&msg, 0, sizeof(msg));
    msg.msg_name       = (caddr_t) &sin;
    msg.msg_namelen    = sizeof(sin);
    msg.msg_iov        = &iov;
    msg.msg_iovlen     = 1;
#ifndef CYGWIN_BUG
    memset(cmsgbuf, 0, sizeof(cmsgbuf));
    msg.msg_control    = cmsgbuf;
    msg.msg_controllen = sizeof(cmsgbuf);
#endif /*CYGWIN_BUG*/

    if ((n = recvmsg(sock, &msg, 0/*no special flags*/)) <= 0) {
	   /* Handle error */
        return;
    }

    /* HERE: msg.msg_flags is inconsistent here for non-unicast UDP sockets */

For a non-unicast socket "sock", the returned msg.msg_flags will have proper WSA
bits (MSG_BCAST or MSG_MCAST) set if CYGWIN_BUG is not defined (i.e. the recvmag()
call is passed non-zero msg.msg_controllen).  Otherwise, msg.msg_flags will always
get returned 0.

The relevant code from CYGWIN source suggests that the native WSA flags should
have got preserved (fhandler_socket.cc, fhandler_socket::recvmsg):

	msg->msg_flags = wsamsg.dwFlags;

yet apparently this had slipped away in fhandler_socket::recv_internal, which
branches on the presence of msg_controllen (and opts out for use other WSA API
call if that is not set, even for non-stream sockets):

     if (wsamsg->Control.len > 0)
       {
         ...
         use_recvmsg = true;
       }

Can this please be rectified?

Thanks,

Anton Lavrentiev
Contractor NIH/NLM/NCBI


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