Do I have to bind a UDP socket in my client program to receive data? (I always get WSAEINVAL)

Question

I am creating a UDP socket (AF_INET, SOCK_DGRAM, IPPROTO_UDP) via Winsock and trying to recvfrom on this socket, but it always returns -1 and I get WSAEINVAL (10022). Why?

When I bind() the port, that does not happen, but I have read that it is very lame to bind the client's socket.

I am sending data to my server, which answers, or at least, tries to.

Inc::STATS CConnection::_RecvData(sockaddr* addr, std::string &strData) {     int ret;            // return code     int len;            // length of the data     int fromlen;        // sizeof(sockaddr)     char *buffer;       // will hold the data     char c;      //recv length of the message     fromlen = sizeof(sockaddr);     ret = recvfrom(m_InSock, &c, 1, 0, addr, &fromlen);     if(ret != 1)     { #ifdef __MYDEBUG__         std::stringstream ss;         ss << WSAGetLastError();         MessageBox(NULL, ss.str().c_str(), "", MB_ICONERROR | MB_OK); #endif         return Inc::ERECV;     }     ... 

This is a working example I wrote a few moments ago, and it works without the call to bind() in the client:

#pragma comment(lib, "Ws2_32.lib")  #define WIN32_LEAN_AND_MEAN  #include <WS2tcpip.h> #include <Windows.h> #include <iostream>  using namespace std;  int main() {     SOCKET sock;     addrinfo* pAddr;     addrinfo hints;     sockaddr sAddr;     int fromlen;     const char czPort[] = "12345";     const char czAddy[] = "some ip";      WSADATA wsa;     unsigned short usWSAVersion = MAKEWORD(2,2);      char Buffer[22] = "TESTTESTTESTTESTTEST5";     int ret;      //Start WSA     WSAStartup(usWSAVersion, &wsa);      //Create Socket     sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);      //Resolve host address     memset(&hints, 0, sizeof(hints));     hints.ai_family = AF_INET;     hints.ai_protocol = IPPROTO_UDP;     hints.ai_socktype = SOCK_DGRAM;      if(getaddrinfo(czAddy, czPort, &hints, &pAddr))     {         std::cerr << "Could not resolve address...\n";         std::cin.get();         return 1;     }      //Start Transmission     while(1)     {         ret = sendto(sock, Buffer, sizeof(Buffer), 0, pAddr->ai_addr,                      pAddr->ai_addrlen);         if(ret != sizeof(Buffer))         {             std::cerr << "Could not send data\n";             std::cin.get();             return 1;         }          fromlen = sizeof(SOCKADDR);         ret = recvfrom(sock, Buffer, sizeof(Buffer), 0, &sAddr, &fromlen);         if(ret != sizeof(Buffer))         {             std::cout << "Could not receive data -  error: " <<                            WSAGetLastError() << std::endl;             std::cin.get();             return 1;         }          Buffer[ret-1] = '\0';         std::cout << "Received: " << Buffer << std::endl;     }     return 0; } 

Answer 1

With UDP, you have to bind() the socket in the client because UDP is connectionless, so there is no other way for the stack to know which program to deliver datagrams to for a particular port.

If you could recvfrom() without bind(), you'd essentially be asking the stack to give your program all UDP datagrams sent to that computer. Since the stack delivers datagrams to only one program, this would break DNS, Windows' Network Neighborhood, network time sync....

You may have read somewhere on the net that binding in a client is lame, but that advice only applies to TCP connections.