Socket question - "active open" vs "passive open"
Posted on 2013-05-14
I am new to sockets. Trying to communicate with a peice of equipment whose documentation is not very good. It is set up to use multiple socket connections (one for 'connectivity heartbeat', another for 'main control', another for 'image transmission', etc). All but one of these is working fine. For all these sockets, I do the following to configure a socket and make it do what I would call a "blocking listen":
SOCKET s, S;
SOCKADDR_IN addr, ADDR;
WSAStartup(MAKEWORD(1, 1), &wsaData);
s = socket(AF_INET, SOCK_STREAM, 0);
addr.sin_family = AF_INET;
addr.sin_port = htons(ThePort);
addr.sin_addr.s_addr = htonl(INADDR_ANY);
bind(s, (SOCKADDR *)&addr, sizeof(addr);
S = accept(s, (SOCKADDR *)&ADDR, &len);
int len = recv(S, lpszBuffer, 4096, 0);
It blocks at the "recv" call until something comes in. Thats all fine. But I can't get the one last socket to work (the "image transmission" socket). Finally, I noticed that in the crappy documentation, it shows the other sockets (that I have working) as "passive open" for my side, but the one I'm having trouble with they show as "active open" from my side. So, I'm not sure what is meant, but I'm guessing it means I should be the 'client' instead of the 'server' for this connection. It is a connection on which I am to receive a bunch of data, and (apparently) I needn't send out anything -- so I thought that my normal sequence (code above) to open the socket and listen for data would be correct. But, nothing ever comes, and it looks like I never get past the "accept" call.
So.... is there a different way I should set up this socket? Looking at some other socket code, it looks like I see the same preamble as in my code above, but in place of the "bind" call, just do a "connect" call at that point. But, I don't really know what I'm doing, so I'm hoping a socket expert might have an idea for me!