Winsock2 - how to open a TCP socket that allows recv() with MSG_WAITALL?

Question

In this code:

// error checking is omitted

// init Winsock2
WSADATA wsaData;
WSAStartup(MAKEWORD(2, 2), &wsaData);

// connect to server
struct addrinfo *res = NULL, *ptr = NULL, hints;
ZeroMemory(&hints, sizeof(hints));
hints.ai_family = AF_INET;
hints.ai_socktype = SOCK_STREAM;
hints.ai_protocol = IPPROTO_TCP;

getaddrinfo(server_ip, "9999", &hints, &res);
SOCKET client_socket = socket(res->ai_family, res->ai_socktype, res->ai_protocol);

connect(client_socket, res->ai_addr, (int)res->ai_addrlen);

freeaddrinfo(res);
res = NULL;

// read the data
unsinged int size1;
if (recv(client_socket, (char*)&size1, sizeof(int), MSG_WAITALL) == SOCKET_ERROR)
{
    return WSAGetLastError();
}

(note the MSG_WAITALL flag in recv()) everything works fine, expect for recv(). WSAGetLastError() returns WSAEOPNOTSUPP.
MSDN states that

Note that if the underlying transport does not support MSG_WAITALL, or if the socket is in a non-blocking mode, then this call will fail with WSAEOPNOTSUPP. Also, if MSG_WAITALL is specified along with MSG_OOB, MSG_PEEK, or MSG_PARTIAL, then this call will fail with WSAEOPNOTSUPP. This flag is not supported on datagram sockets or message-oriented sockets.

But it doesn't look like I'm doing something from this list. Why my recv() call doesn't work?

Answer 1

it doesn't look like I'm doing something from this list.

Yes, you are - the very first item on the list:

the underlying transport does not support MSG_WAITALL

Microsoft's default TCP transport provider does not support MSG_WAITALL. recv(), and Winsock in general, is not limited to just Microsoft's TCP provider. It supports 3rd party providers, and any transport protocols that the provider supports - TCP, UDP, IPX, ICMP, RAW, etc.

When using Microsoft's TCP, if you want recv() to wait until all of the requested TCP data has been received, you have to set the socket to blocking mode (its default mode) and then set the flags parameter of recv() to 0. But even that is not guaranteed, recv() can return with fewer bytes than requested, so you should be prepared to call recv() in a loop until all intended bytes have actually been received.