将字符串覆盖到const char然后构造hostent时出现问题

Question

我有执行http-webrequest的代码，但是我不知道如何向主机const添加url，因为每次使用此代码执行时，都会显示此错误：

附加信息：对象引用未设置为对象的实例。

这是代码：

// originally posted to http://i.stack.imgur.com/hH1BG.png

#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gethostbyname的返回值不为NULL ，在尝试使用host->h_addr之前，应进行检查以确保主机名查找成功。

如果未发生错误，则gethostbyname返回指向上述托管结构的指针 。 否则，它将返回一个空指针，并且可以通过调用WSAGetLastError来检索特定的错误号。

SRC; msdn.microsoft.com -gethostbyname函数（Windows）

---

---

_{建议的修复}

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
}