切换回阻塞套接字后无法连接到套接字

Question

有两台socket服务器，一台是不常开的主服务器，一台是备用服务器。

我的程序将尝试使用非阻塞连接连接主服务器（以便可以应用超时值），如果失败，它将使用阻塞连接连接备份服务器。

但是第二个connect函数大部分时候会返回“Invalid argument”错误码：

#define SERVER_URL "example.com"
#define SERVER_PORT_PRIMARY "1234"
#define SERVER_PORT_BACKUP "5678"

struct addrinfo *result = NULL;
struct addrinfo hints;

ZeroMemory(&hints, sizeof(hints));
hints.ai_family = AF_INET;
hints.ai_socktype = SOCK_STREAM;
hints.ai_protocol = IPPROTO_TCP;

if (getaddrinfo(SERVER_URL , SERVER_PORT_PRIMARY , &hints, &result) != 0) {
    WSACleanup();
    return;
}

SOCKET socketClient = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
if (socketClient == SOCKET_ERROR){
    WSACleanup();
    return;
}

//set the socket in non-blocking
unsigned long iMode = 1;
iResult = ioctlsocket(socketClient, FIONBIO, &iMode);
if (iResult != NO_ERROR){
    closesocket(socketClient);
    WSACleanup();
    return;
}

if (connect(socketClient, result->ai_addr, (int)result->ai_addrlen) == SOCKET_ERROR){
    if (WSAGetLastError() != WSAEWOULDBLOCK){
        closesocket(socketClient);
        WSACleanup();
        return;
    }
}

//switch it back to blocking socket
iMode = 0;
iResult = ioctlsocket(socketClient, FIONBIO, &iMode);
if (iResult != NO_ERROR){
    closesocket(socketClient);
    WSACleanup();
    return;
}
fd_set Write, Err;
FD_ZERO(&Write);
FD_ZERO(&Err);
FD_SET(socketClient, &Write);
FD_SET(socketClient, &Err);
TIMEVAL Timeout;
Timeout.tv_sec = 10;
Timeout.tv_usec = 0;
select(0, NULL, &Write, &Err, &Timeout);
if (FD_ISSET(socketClient, &Write) == false){
    //unable to connect to primary server within 10s, try to connect backup server
    ZeroMemory(&hints, sizeof(hints));
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_protocol = IPPROTO_TCP;
    if (getaddrinfo(SERVER_URL , SERVER_PORT_BACKUP, &hints, &result) != 0) {
        closesocket(socketClient);
        WSACleanup();
        return;
    }
    iResult = connect(socketClient, result->ai_addr, (int)result->ai_addrlen);
    if (iResult == SOCKET_ERROR){
        int a = WSAGetLastError(); ///<----Problem here, a == WSAEINVAL (Invalid argument)
        closesocket(socketClient);
        WSACleanup();
        return;
    }
}

正如我在上面的代码中评论的那样，第二次“连接”调用将在大多数情况下返回 SOCKET_ERROR，而 WSAGetLastError() 返回 WSAEINVAL（无效参数）。

如果我删除非阻塞代码，它将无错误地连接。那么我的代码有什么问题呢？

Answer 1

当connect() 仍在忙于连接时，您必须让套接字处于非阻塞模式。仅当connect() 返回WSAEWOULDBLOCK 时才调用select()，并且您还需要检查select() 的返回值。您还泄漏了getaddrinfo() 返回的内存。

试试类似的方法：

int connectTo(SOCKET s, const char *host, const char *port)
{
    struct addrinfo *result = NULL;
    struct addrinfo hints;

    ZeroMemory(&hints, sizeof(hints));
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_protocol = IPPROTO_TCP;

    int ret = getaddrinfo(host, port, &hints, &result);
    if (ret != 0) {
        return ret;
    }

    if (connect(socketClient, result->ai_addr, result->ai_addrlen) != SOCKET_ERROR) {
        freeaddrinfo(result);
        return 0;
    }

    ret = WSAGetLastError();
    freeaddrinfo(result);

    if (ret != WSAEWOULDBLOCK) {
        return ret;
    }

    fd_set Write, Err;
    FD_ZERO(&Write);
    FD_ZERO(&Err);
    FD_SET(s, &Write);
    FD_SET(s, &Err);

    TIMEVAL Timeout;
    Timeout.tv_sec = 10;
    Timeout.tv_usec = 0;

    ret = select(0, NULL, &Write, &Err, &Timeout);
    if (ret == SOCKET_ERROR) {
        return WSAGetLastError();
    }

    if (ret == 0) {
        return WSAETIMEDOUT;
    }

    if (FD_ISSET(s, &Err)) {
        u_long err;
        if (getsockopt(s, SOL_SOCKET, SO_ERROR, (char*)&err, sizeof(err)) == SOCKET_ERROR) {
            return WSAGetLastError();
        }
        return (int) err;
    }

    return 0;
}

那么你可以这样做：

#define SERVER_URL "example.com"
#define SERVER_PORT_PRIMARY "1234"
#define SERVER_PORT_BACKUP "5678"

SOCKET socketClient = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
if (socketClient == SOCKET_ERROR) {
    WSACleanup();
    return;
}

//set the socket in non-blocking
u_long iMode = 1;
iResult = ioctlsocket(socketClient, FIONBIO, &iMode);
if (iResult == SOCKET_ERROR) {
    closesocket(socketClient);
    WSACleanup();
    return;
}

if (connectTo(socketClient, SERVER_URL, SERVER_PORT_PRIMARY) != 0) {
    if (connectTo(socketClient, SERVER_URL, SERVER_PORT_BACKUP) != 0) {
        closesocket(socketClient);
        WSACleanup();
        return;
    }
}

//switch it back to blocking socket
iMode = 0;
iResult = ioctlsocket(socketClient, FIONBIO, &iMode);
if (iResult == SOCKET_ERROR) {
    closesocket(socketClient);
    WSACleanup();
    return;
}

// communicate with the server as needed ...