clean window socket internal buffer - c++

Question

I am wondering if there is a way to clean up window socket internal buffer, because what I want to achieve is this
while(1){
for(i=0;i<10;i++){
sendto(...) //send 10 UDP datagrams
}
for(i=0;i<10;i++){
recvfrom (Socket, RecBuf, MAX_PKT_SIZE, 0,
(SOCKADDR*) NULL, NULL);
int Status = ProcessBuffer(RecBuf);
if (Status == SomeCondition)
MagicalSocketCleanUP(Socket); //clean up the rest of stuff in the socket, so that it doesn't effect the reading for next iteration of the outer while loop
break; //occasionally the the receive loop needs to terminate before finishing off all 10 iteration
}
}
so I am asking for is there a function to clean up whatever remaining in the socket so that it won't effect my next reading? Thank you

Answer 1

The way to clean up data from the internal receive socket buffer is to read data until there is no more data to read. If you do this in a non-blocking way, you do not need to wait for more data in select(), because the EWOUDBLOCK error value means the internal receive socket buffer is empty.
int MagicalSocketCleanUP(SOCKET Socket) {
int r;
std::vector<char> buf(128*1024);
do {
r = recv(Socket, &buf[0], buf.size(), MSG_DONTWAIT);
if (r < 0 && errno == EINTR) continue;
} while (r > 0);
if (r < 0 && errno != EWOULDBLOCK) {
perror(__func__);
//... code to handle unexpected error
}
return r;
}
But this is not exactly safe. The other end of the socket may have sent good data into the socket buffer too, so this routine may discard more than what you want to discard.
Instead, the data on the socket should be framed in such a way that you know when the data of interest arrives. So instead of a cleanup API, you could extend ProcessBuffer() to discard input until it finds data of interest.
A simpler mechanism would be a message exchange between the two sides of the socket. When the error state is entered, the sender sends a "DISCARDING UNTIL <TOKEN>" message. The receiver sends back "<TOKEN>" and knows that only the data after the "<TOKEN>" message will be processed. The "<TOKEN>" can be a random sequence.