Software :: Receiving UDP Multicast - Blocks As If There Is No Data

I am trying to port some MS Visual Studio C++ code to Linux/wxWidgets. The code is socket-based and very similar between Windows and Linux. In Linux the socket, bind, and setsocketopt all return with no error. See:

Code:
BOOL CAV15Input::SetupSocket(BOOL Out)
{ if(pU->PrDebug) {
sprintf(_send,"CAV15Input::SetupSocket for port %d",pSonar->dPort);
pU->OutMsg(_send,1,0); } pSonar->UDP_Sock=0; pSonar->UDPError=0;
pSonar->UDP_Sock=socket(AF_INET,SOCK_DGRAM,0);
if(pSonar->UDP_Sock<0 ) {
pU->OutMsg("socket failed on SetupSocket",11,0);
pSonar->UDP_Sock=0;
pSonar->UDPError|=4;
return FALSE; }
// the AF_INET address family, which contains IP addresses,
local.sin_family=AF_INET;
local.sin_addr.s_addr=inet_addr(pSonar->IP_Add_PC);
local.sin_port=htons(pSonar->dPort);
// from Dave Goffe:
//> int t = 1;
//>
//>setsockopt(fd,SOL_SOCKET,SO_REUSEADDR,&t,sizeof(t));
int t=1; if(setsockopt(pSonar->UDP_Sock,
SOL_SOCKET, SO_REUSEADDR,
(char *)&t,sizeof(t)) <0) {
CloseSock();
pU->OutMsg("set SO_REUSEADDR failed in SetupSocket",11,0);
pSonar->UDPError|=4;
return FALSE; }
if(bind(pSonar->UDP_Sock,(sockaddr *)&local,sizeof(local)))
{ CloseSock();
pU->OutMsg("bind failed in SetupSocket",11,0);
pSonar->UDPError|=8; return FALSE; }
if(pU->PrDebug) {
sprintf(_send,"socket setup & bind OK for port %d",pSonar->dPort);
pU->OutMsg(_send,1,0); }
// Join multicast group
if(pSonar->datatype) // for element data
mreq.imr_multiaddr.s_addr=inet_addr(pSonar->IP_Add_ELD);
else // for beam data
mreq.imr_multiaddr.s_addr=inet_addr(pSonar->IP_Add_HRB);
mreq.imr_interface.s_addr=inet_addr(pSonar->IP_Add_PC);
if(setsockopt(pSonar->UDP_Sock,
IPPROTO_IP, IP_ADD_MEMBERSHIP,
(char *)&mreq,sizeof(mreq)) <0) {
CloseSock();
pU->OutMsg("join multicast failed in SetupSocket",11,0);
pSonar->UDPError|=16;
return FALSE;
} if(pU->PrDebug) {
if(pSonar->datatype) // for element data
sprintf(_send,"join multicast OK on %s",pSonar->IP_Add_ELD);
else // for beam data
sprintf(_send,"join multicast OK on %s",pSonar->IP_Add_HRB);
pU->OutMsg(_send,1,0); }
jMCast=1; return TRUE; }

But then when I try
recv(pSonar->UDP_Sock,&pUDP->u_dg_cM.cM[8],reedlen,0);
it blocks as if there is no data. (In Windows this works fine.) Is there something else in Linux and/or wxWidgets that I'm missing that could prevent UDP datagrams from being received? Is there some way in Linux to monitor incoming UDP traffic?