c++ - 主机名未使用 Winsock 转换为 IP 地址

Question

getaddrinfo() 不会将主机名转换为 IP 地址，因此不会connect() 到服务器。我的实现有问题吗 - 编译时没有警告消息？

这个对 connect 的函数调用是否正确？

connect(client, result->ai_addr, result->ai_addrlen)

下面列出了完整的实现:

#include <winsock2.h>
#include <ws2tcpip.h>
#include <windows.h>
#include <stdio.h>
#include <iostream>

#pragma comment(lib, "Ws2_32.lib")

using namespace std;

int main (
          int argc,
          char* argv[])
{
if (argc != 3)
{
    cerr << "Usage: " << argv[0] << " [hostname] [port number]\n";
    exit(EXIT_FAILURE);
}

WSADATA wsaData;
WORD wVersionRequested;
int wError;

wVersionRequested = MAKEWORD(2, 2);

wError = WSAStartup(wVersionRequested, &wsaData);

if (wError != 0)
{
    cerr << "WSAStartup failed with error: " << wError << endl;
    exit (EXIT_FAILURE);
}

/*
* Confirm that the WinSock DLL supports 2.2.
* Note that if the DLL supports versions greater
* than 2.2 in addition to 2.2, it will still return 
* 2.2 in wVersion since that is the version we 
* requested. 
*/

if (LOBYTE(wsaData.wVersion) != 2 || HIBYTE(wsaData.wVersion) != 2)
{
    cerr << "Could not find a usable version of Winsock.dll." << endl;
    WSACleanup();
    exit(EXIT_FAILURE);
} else {
    cout << "The Winsock 2.2 dll was found." << endl;
}

SOCKET client;

if ((client = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)) == SOCKET_ERROR)
{
    cerr << "Error: socket() return value == SOCKET_ERROR" << endl;
    WSACleanup();
    exit (EXIT_FAILURE);
}

cout << "Created a socket." << endl;

struct addrinfo *result = NULL;
struct addrinfo hints;
memset(&hints, 0, sizeof(hints));
hints.ai_family = AF_INET;
hints.ai_socktype = SOCK_STREAM;
hints.ai_protocol = IPPROTO_TCP;

if ((wError = getaddrinfo(
                    argv[1],
                    argv[2],
                    &hints,
                    &result)) !=0 )
{
    freeaddrinfo(result);
    WSACleanup();
    if (wError == 11001)
    {
        cerr << "Error: occurred: getaddrinfo() failed "
             << wError << " - Host not found." << endl;
        exit(EXIT_FAILURE);
    }

    cerr << "Error: occurred: getaddrinfo() failed "
            << wError << endl;
    exit(EXIT_FAILURE);
}
/*
* Attempt to connect to the Server
*
*/

switch (wError = connect(client, result->ai_addr, result->ai_addrlen)) {
    case 0:
        cerr << "Resolved hostname." << endl;
        break;
    case SOCKET_ERROR:
        wError = WSAGetLastError();
        cerr << "Error: connet() failed "
                "Details: " << wError << endl;
        closesocket(client);
        freeaddrinfo(result);
        WSACleanup();
        exit(EXIT_FAILURE);
        break;
    default:
        cerr << "Fatal connect() error: unexpected "
                "return value." << endl;
        closesocket(client);
        freeaddrinfo(result);
        WSACleanup();
        exit(EXIT_FAILURE);
        break;
}

cout << "Connected to server." << endl;

closesocket(client);
freeaddrinfo(result);
WSACleanup();
exit(EXIT_SUCCESS);
}

Answer 1

getaddrinfo 可能为您提供了一个 IPv6 地址，或者机器可能有多个 IP 地址而您尝试连接到错误的地址。

此外，如果您的服务器正在监听 127.0.0.1 并且您尝试连接到真实 IP 地址，连接将失败。同样，如果服务器正在监听真实 IP 地址，而您尝试使用 127.0.0.1 进行连接，连接将失败。如果服务器监听 0.0.0.0，则两个地址都应该有效。

要在 0.0.0.0 上收听，您的代码将与此类似:

sockaddr_in sin;
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = INADDR_ANY;
sin.sin_port=htons( port_num );
bind( s, (sockaddr *)&sin, sizeof( sin ) );