c++ UDP relay application latency - c++

Question

I want to receive UDP packets from a server application (on the same computer) and forward it to an UDP receive app on a different port.
The server app is not mine, nor do i have the source code.
The UDP receive app is a Java application.
If i bind the Java application directly to the server app port, there is very low latency, but if i connect it to the relay app´port i get almost one second of delay.
The receiving port has to be non-blocking.
#define rcv_length 160
fd_set fds;
int n;
struct timeval tv;
void CMClient::startUDPListener(){
CMport_number = 32200;
remoteAddrLen = sizeof(struct sockaddr_in);
if (WSAStartup(0x0101, &CMw) != 0)
{
fprintf(stderr, "Could not open Windows connection.\n");
exit(0);
}
CMsd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
if (CMsd == INVALID_SOCKET)
{
fprintf(stderr, "Could not create socket.\n");
WSACleanup();
exit(0);
}
CMserver.sin_family = AF_INET;
CMserver.sin_port = htons(CMport_number);
CMserver.sin_addr.s_addr = inet_addr("127.0.0.1");
long rc=bind(CMsd,(SOCKADDR*)&CMserver,sizeof(SOCKADDR_IN));
if(rc==SOCKET_ERROR)
{
printf("Error: bind code: %d\n",WSAGetLastError());
}
}
void CMClient::updateData(UDPServer* svr, int CMnr){
FD_ZERO(&fds);
FD_SET(CMsd, &fds);
tv.tv_sec = 0;
tv.tv_usec = 1;
n = select ( CMsd, &fds, NULL, NULL, &tv ) ;
if (FD_ISSET(CMsd, &fds))
{
FD_CLR(CMsd,&fds);
char* rcvBuffer = new char[rcv_length];
long rc=recv(CMsd,rcvBuffer,rcv_length,0); //receive
if(rc!=SOCKET_ERROR)
{
rcvBuffer[0] = CMnr;
sendto(svr->sd, (char*)rcvBuffer, rcv_length, 0, (struct sockaddr*)&svr->server, sizeof(svr->server)) != (int)sizeof(rcvBuffer); //send
}
if(rcvBuffer)
delete [] rcvBuffer;
}
}
The UDP server to send to the Java application is initialized as follows:
void UDPServer::startUDPServer(){
if (WSAStartup(0x0101, &w) != 0)
{
fprintf(stderr, "Could not open Windows connection.\n");
exit(0);
}
sd = socket(AF_INET, SOCK_DGRAM, 0);
if (sd == INVALID_SOCKET)
{
fprintf(stderr, "Could not create socket.\n");
WSACleanup();
exit(0);
}
server.sin_family = AF_INET;
server.sin_port = htons(port_number);
server.sin_addr.s_addr = inet_addr("127.0.0.1");
if (connect(sd, (struct sockaddr *)&server, sizeof(struct sockaddr_in)) == -1)
{
fprintf(stderr, "Could not connect name to socket.\n");
stopUDPServer();
}
}
The main calls
UDPServer* udpsvr;
udpsvr = new UDPServer;
udpsvr->startUDPServer();
while(1){
CM->updateData(udpsvr, CMid);
}
Any help would be appreciated why i get that much delay.
The data received from the Java app is correct but ~1 second delay.
Thank you,
Regards

Answer 1

Don't use bind() on client. Use connect().
Summing up, for UDP:
Server:
socket()
bind()
Client:
socket()
connect()
Example of code for server:
// socket()
fd_ = socket(AF_INET, SOCK_DGRAM, 0);
if (fd_ < 0) {
throw std::runtime_error ("Client socket error");
}
// connect()
struct sockaddr_in serv_addr;
bzero((char *) &serv_addr, sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
serv_addr.sin_port = htons(port);
struct in_addr addr;
inet_aton(address.c_str(), &addr);
bcopy(&addr, &serv_addr.sin_addr.s_addr, sizeof(addr));
if (connect(fd_, (struct sockaddr *) &serv_addr,
sizeof(serv_addr)) < 0) {
::close(fd_);
throw std::runtime_error ("Client socket error");
}
Example of code for client:
// socket()
fd_ = socket(AF_INET, SOCK_DGRAM, 0);
if (fd_ < 0) {
throw std::runtime_error ("Client socket error");
}
// connect()
struct sockaddr_in serv_addr;
bzero((char *) &serv_addr, sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
serv_addr.sin_port = htons(port);
struct in_addr addr;
inet_aton(address.c_str(), &addr);
bcopy(&addr, &serv_addr.sin_addr.s_addr, sizeof(addr));
if (connect(fd_, (struct sockaddr *) &serv_addr,
sizeof(serv_addr)) < 0) {
::close(fd_);
throw std::runtime_error ("Client socket error");
}
Finally, you may want to have a look at flushing UDP buffers.