Winsock blocking recv() not return, while WireShark says there are incoming segments in the TCP stream - tcp

Question

I am working with some protocol on my Windows 10 pro with VC++ 2013 Community, basically includes three steps:
client sends a GET header (e.g. authentication, etc)
server returns a HTTP header (e.g. status code 200 if everything is fine)
then server keeps sending binary data stream after the HTTP header
I send the header, and call recv() in blocking mode to receive data from server through the TCP stream. However, the recv() blocks, never return.
I use WireShark to "follow" the TCP stream, and it shows that server does keep sending binary data, and I do see ACK message from client side to acknowledge every segment it receives. However, recv() still blocks, and never returns.
I tried to use:
pure C implementation over WinSock
C# using TcpClient
C++ with Boost Asio
non-blocking WinSock (as in this article)
The first version was implemented in WinHTTP, and eventually got Timeout.
None of them can receive any data. However, the WireShark can still tell that the server keeps sending binary data.
I tried to turn off my firewall, but the problem still there.
The most weird thing is my first implementation actually did successfully get data from recv(), about two days ago. On that day, recv() returned three times, and then blocked again. The next day, the same implementation, recv() never be able to return anything.
I am really confused. Thank you!
Here is the code, blocking Winsock version:
WSADATA wsaData;
int iResult;
SOCKET ConnectSocket = INVALID_SOCKET;
struct sockaddr_in clientService;
char recvbuf[DEFAULT_BUFLEN];
int recvbuflen = DEFAULT_BUFLEN;
//----------------------
// Initialize Winsock
iResult = WSAStartup(MAKEWORD(2, 2), &wsaData);
if (iResult != NO_ERROR) {
printf("WSAStartup failed: %d\n", iResult);
return 1;
}
//----------------------
// Create a SOCKET for connecting to server
ConnectSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
if (ConnectSocket == INVALID_SOCKET) {
printf("Error at socket(): %ld\n", WSAGetLastError());
WSACleanup();
return 1;
}
//----------------------
// The sockaddr_in structure specifies the address family,
// IP address, and port of the server to be connected to.
clientService.sin_family = AF_INET;
auto ip = gethostbyname(name);
clientService.sin_addr = *(reinterpret_cast<struct in_addr *>(ip->h_addr));
clientService.sin_port = htons(DEFAULT_PORT);
//----------------------
// Connect to server.
iResult = connect(ConnectSocket, (SOCKADDR*)&clientService, sizeof(clientService));
if (iResult == SOCKET_ERROR) {
closesocket(ConnectSocket);
printf("Unable to connect to server: %ld\n", WSAGetLastError());
WSACleanup();
return 1;
}
// Send an initial buffer
iResult = send(ConnectSocket, sendbuf, (int)strlen(sendbuf), 0);
if (iResult == SOCKET_ERROR) {
printf("send failed: %d\n", WSAGetLastError());
closesocket(ConnectSocket);
WSACleanup();
return 1;
}
printf("Bytes Sent: %ld\n", iResult);
// shutdown the connection since no more data will be sent
iResult = shutdown(ConnectSocket, SD_SEND);
if (iResult == SOCKET_ERROR) {
printf("shutdown failed: %d\n", WSAGetLastError());
closesocket(ConnectSocket);
WSACleanup();
return 1;
}
// Receive until the peer closes the connection
do {
iResult = recv(ConnectSocket, recvbuf, recvbuflen, 0);
if (iResult > 0)
printf("Bytes received: %d\n", iResult);
else if (iResult == 0)
printf("Connection closed\n");
else
printf("recv failed: %d\n", WSAGetLastError());
} while (iResult > 0);
// cleanup
closesocket(ConnectSocket);
WSACleanup();
return 0;

Answer 1

Since the shutdown code is a copy/paste from the microsoft page (https://msdn.microsoft.com/en-us/library/windows/desktop/ms740121(v=vs.85).aspx) - I suppose that is how they indicate sending has completed. I believe this explains the issue you're having (from the above page):
Note When issuing a blocking Winsock call such as recv, Winsock may need to wait for a network event before the call can complete. Winsock performs an alertable wait in this situation, which can be interrupted by an asynchronous procedure call (APC) scheduled on the same thread. Issuing another blocking Winsock call inside an APC that interrupted an ongoing blocking Winsock call on the same thread will lead to undefined behavior, and must never be attempted by Winsock clients.
I actually just realized that I've only ever used non-blocking sockets and always have the receiving code on its own thread. So try adding this:
iResult = ioctlsocket(m_socket, FIONBIO, &iMode);
From here:
https://msdn.microsoft.com/en-us/library/windows/desktop/ms738573(v=vs.85).aspx
Note: You're going to get mostly a series of failed attempts to receive data from the socket - so you'll need to handle those and you'll also not want to tight loop.