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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