Question

Found the following in the example code appserver.cpp distributed with UDT

hints.ai_flags = AI_PASSIVE;
hints.ai_family = AF_INET;
hints.ai_socktype = SOCK_STREAM;

Why would UDT use SOCK_STREAM and not SOCK_DGRAM?

Answer 1

This can be perfectly normal.

If I didn’t know anything about UDT, then I would assume that “hints” is likely an instance of addrinfo and used as the second parameter of getaddrinfo()

If the code is just trying to get the IP address of a server (i.e. DNS lookup), then it has to pass something into the hints structure for socktype. Otherwise, the result of getaddrinfo is likely to return 3x the the number of results. One result for SOCK_STREAM, another for SOCK_DGRAM, and a third for SOCK_RAW. But the ai_addr member for each will be the same address.

Now I did just take a peak at the UDT code. Never heard of it until now. But it does appear to have some code that is doing some SOCK_STREAM stuff and using getaddrinfo as a formal way to initialize a sockaddr for subsequent TCP connect.

memset(&hints, 0, sizeof(struct addrinfo));
hints.ai_flags = AI_PASSIVE;
hints.ai_family = AF_INET;
hints.ai_socktype = SOCK_STREAM; 

UDTSOCKET fhandle = UDT::socket(hints.ai_family, hints.ai_socktype,hints.ai_protocol); 

if (0 != getaddrinfo(argv[1], argv[2], &hints, &peer))
{
   cout << "incorrect server/peer address. " << argv[1] << ":" << argv[2] << endl;
   return -1;
}

// connect to the server, implict bind
if (UDT::ERROR == UDT::connect(fhandle, peer->ai_addr, peer->ai_addrlen))

But you’ll have to ask the UDT developers what it’s all about.