将字符串转换为 const char 然后再转换为 struct hostent 的问题

Question

我有生成 http-webrequest 的代码，但我不知道如何将 url 添加到主机常量，因为每次我使用代码执行此操作时都会显示此错误:

Additional information: Object reference not set to an instance of an object.

代码如下：

// originally posted to <a href="http://i.stack.imgur.com/hH1BG.png" rel="nofollow">http://i.stack.imgur.com/hH1BG.png</a>

#include <winsock2.h>
#include <windows.h>
#include <iostream>
#pragma comment(lib,"ws2_32.lib")
using namespace std;
void connect(string url)
{
  //Adding for the host
  const char *curl = url.c_str();
  WSADATA wsaData;
  //original code
  if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0) {
    cout << "WSAStartup failed.\n";
    system("pause");

  }
  SOCKET Socket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
  struct hostent *host;
  host = gethostbyname(curl);
  SOCKADDR_IN SockAddr;
  SockAddr.sin_port = htons(80);
  SockAddr.sin_family = AF_INET;
  SockAddr.sin_addr.s_addr = *((unsigned long*)host->h_addr);
  cout << "Connecting...\n";
  if (connect(Socket, (SOCKADDR*)(&SockAddr), sizeof(SockAddr)) != 0){
    cout << "Could not connect";
    system("pause");

  }
  cout << "Connected.\n";
  send(Socket, "GET / HTTP/1.1\r\nHost: www.google.com\r\nConnection: close\r\n\r\n", strlen("GET / HTTP/1.1\r\nHost: www.google.com\r\nConnection close\r\n\r\n"), 0);
  char buffer[10000];
  int nDataLength;
  while ((nDataLength = recv(Socket, buffer, 10000, 0)) > 0){
    int i = 0;
    while (buffer[i] > 32 || buffer[i] == '\n' || buffer[i] == '\r') {
      cout << buffer[i];
      i += 1;
    }
  }
  closesocket(Socket);
  WSACleanup();
  system("pause");

}

Answer 1

问题是您盲目地假设来自 gethostbyname 的 return 值不是 NULL，您应该在尝试之前检查主机名查找是否成功使用 host->h_addr.

If no error occurs, gethostbyname returns a pointer to the hostent structure described above. Otherwise, it returns a null pointer and a specific error number can be retrieved by calling WSAGetLastError.

src; msdn.microsoft.com - gethostbyname function (windows)

_建议修复

struct hostent *host;
host = gethostbyname(curl);

if (!host) {
   // hostname lookup failed, handle error
   std::cerr << "hostname error, code: " << WSAGetLastError () << "\n";
   return; // return from function, there's nothing more we can do
}

将字符串转换为 const char 然后再转换为 struct hostent 的问题

Problems coverting string into const char and then struct hostent

c++

winsock