C/C++ Sockets: IPv6 TCP connection fails if I do anything before it

Question

I have been working on converting a small Windows C++ IPv4 program to be IPv6 compatible and have been banging my head against me desk for while. Something is making any IPv6 function fail.

I've written a simple server/client program that the replicates bug. If I do anything before the call to the client/server function the socket functions fail.

The server will get a WSA 10049 error on the bind function and the client will get a WSA 10049 error on the connect function.

But if the code is commented out it will succeed.

What on earth is going on? I feel like I must be just missing something really simple.

I'm using g++ to compile with the latest version of MINGW.

EDIT: It works always if I change this code back to using IPv4/AF_INET

server.cpp

#include <w32api.h>
#define WINVER                  WindowsVista
#define _WIN32_WINDOWS          WindowsVista
#define _WIN32_WINNT            WindowsVista

#include <winsock2.h>
#include <ws2tcpip.h>

int setupWinSock(){
    WSADATA wsa;

    if (WSAStartup(MAKEWORD(2,2),&wsa) != 0){
        return 1;
    }

    return 0;  
}

void simpleServer(int port){
    printf("SERVER START\n");
    int s, c;
    int reuseaddr = 1;
    struct sockaddr_in6 addr;
    int pid;

    s = socket(AF_INET6, SOCK_STREAM, 0);
    if (s == SOCKET_ERROR){
        printf("socket ERROR IPV6: %d\n", WSAGetLastError());
        return;
   }

   int optval = 1;
   setsockopt(s, SOL_SOCKET, SO_REUSEADDR, (const char*)&optval, sizeof optval);

    addr.sin6_family = AF_INET6;
    addr.sin6_port = htons(port);
    addr.sin6_addr = in6addr_any;

    if (bind(s, (struct sockaddr *)&addr, sizeof(addr)) < 0){
        printf("bind ERROR IPV6: %d\n", WSAGetLastError());
        return;
    }

    if (listen(s, 5) < 0){
        printf("listen ERROR IPV6: %d\n", WSAGetLastError());
        return;
    }


    c = accept(s, NULL, NULL);
    if (c == SOCKET_ERROR) printf("ACCEPT ERROR IPV6: %d\n", WSAGetLastError());
    else                   printf("It works!\n");
    closesocket(s);
    closesocket(c);
}

int main(){
    if (setupWinSock()){
        abort();
    }
    // ANYTHING HERE makes socket functions fail
    // Could be this 
    //double tmp = 100000;
    // tmp = tmp * tmp;
    // This to....
    //std::this_thread::sleep_for(std::chrono::milliseconds(2000));
    simpleServer(8080);
    WSACleanup();
}

client.cpp

#include <w32api.h>
#define WINVER                  WindowsVista
#define _WIN32_WINDOWS          WindowsVista
#define _WIN32_WINNT            WindowsVista

#include <winsock2.h>
#include <ws2tcpip.h>

int setupWinSock(){
    WSADATA wsa;

    if (WSAStartup(MAKEWORD(2,2),&wsa) != 0){
        return 1;
    }

    return 0;  
}


void simpleClient(int port){
    printf("CLIENT START\n");
    int s, x;
    struct sockaddr_in6 addr;

    s = socket(AF_INET6, SOCK_STREAM, 0);
     if (s == SOCKET_ERROR){
         printf("socket ERROR IPV6: %d\n", WSAGetLastError());
         return;
    }

    addr.sin6_family = AF_INET6;
    addr.sin6_port = htons(port);
    inet_pton(AF_INET6, "::1", &addr.sin6_addr);
    x = connect(s, (struct sockaddr *)&addr, sizeof(addr));
    if (x == SOCKET_ERROR) printf("CONNECT ERROR IPV6: %d\n", WSAGetLastError());
    else                   printf("It works!\n");
    closesocket(s);
}

int main(){
    if (setupWinSock()){
        abort();
    }
    // ANYTHING HERE makes socket functions fail
    // Could be this 
    //double tmp = 100000;
    // tmp = tmp * tmp;
    // This to....
    //std::this_thread::sleep_for(std::chrono::milliseconds(2000));
    simpleClient(8080);
    WSACleanup();
}

Answer 1

I think you should zero out addr prior to setting its fields. memset(&addr, 0, sizeof(addr)); Note that you are not initializing some of sockaddr_in6 fields , such as sin6_flowinfo or sin6_scope_id . Probably with extra actions prior to calling server/client functions stack gets polluted and fields left uninitialized are left with "more garbage" values than without those extra actions.