在 C++ 程序中使用 libcurl 会导致 Visual C++ 编译器发出错误

Question

我已在 Windows 上使用 Visual C++ 编译器将 libcurl 编译为 dll。除了编译好的 libcurl 源代码之外，该 dll 还包含一个简单的测试程序，如下所示：

标头（HttpClient.h）：

#pragma once

#include <string>

#include "curl/curl.h"


namespace My::Project
{
  class HttpClient
  {
  public:
    HttpClient();
    ~HttpClient();
    std::string Get(std::string address);

  private:
    CURL* easy_handle;
  };
}

实施 (HttpClient.cpp)：

#include "HttpClient.h"

#include <iostream>

using namespace My::Project


HttpClient::HttpClient()
{
  easy_handle = curl_easy_init();
  curl_global_init(CURL_GLOBAL_ALL);
}


HttpClient::~HttpClient()
{
  curl_global_cleanup();
}


static size_t WriteCallback(void* contents, size_t size, size_t nmemb, void* userp)
{
  ((std::string*)userp)->append((char*)contents, size * nmemb);
  return size * nmemb;
}


std::string HttpClient::Get(std::string address)
{
  std::string html;

  curl_easy_setopt(easy_handle, CURLOPT_URL, address.c_str());
  curl_easy_setopt(easy_handle, CURLOPT_FOLLOWLOCATION, 1L);
  curl_easy_setopt(easy_handle, CURLOPT_WRITEFUNCTION, WriteCallback);
  curl_easy_setopt(easy_handle, CURLOPT_WRITEDATA, &html);
  CURLcode returnValue = curl_easy_perform(easy_handle);

  if(returnValue != CURLE_OK)
  {
    html = "Failed to retrieve HTML";
  }

  curl_easy_cleanup(easy_handle);
  return html;
}

那个测试程序编译得很好。

（顺便说一句：我知道curl_global_init 和curl_global_cleanup 只能调用一次。这只是一个测试程序......）

现在，我想在另一个 dll 中引用该 dll 并创建 HttpClient 类的实例并按如下方式使用它：

#include "<relative-path>/HttpClient.h" 

using namespace My::Project;

// Within a constructor of another class
HttpClient client;
std::string html = client.Get("https://www.google.com/");

用于编译第二个 dll 的编译器调用如下所示：

C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.21.27702\bin\HostX86\x64\CL.exe
/c
/I<project path>
/I"Generated Files\\"
/Iobj\Debug\x64\
/I<path to curl source>\curl\include
/ZI
/JMC
/ZW
/ZW:nostdlib
/nologo
/W3
/WX-
/diagnostics:column
/sdl
/MP
/Od
/Oy-
/D _WINRT_DLL
/D _WINDLL
/D _UNICODE
/D UNICODE
/D _DEBUG
/D WINAPI_FAMILY=WINAPI_FAMILY_APP
/D __WRL_NO_DEFAULT_LIB__
/Gm-
/EHsc
/RTC1
/MDd
/GS
/fp:precise
/Zc:wchar_t
/Zc:forScope
/Zc:inline
/std:c++17
/Yu"pch.h"
/Fp"obj\Debug\x64\pch.pch"
/Fo"obj\Debug\x64\\"
/Fd"obj\Debug\x64\vc142.pdb"
/Gd
/TP
/wd28204
/FU"C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.21.27702\lib\x86\store\references\platform.winmd"
/FU"<other winmd files>
/analyze-
/FC
/errorReport:prompt
/bigobj
<path to cpp file invoking HttpClient>

但是，尝试编译第二个 dll 会导致 Visual C++ 编译器发出以下错误：

<path to curl source>\curl\include\curl\curl.h(134,29): error C4430:  missing type specifier - int assumed. Note: C++ does not support default-int
<path to curl source>\curl\include\curl\curl.h(134,16): error C2146:  syntax error: missing ';' before identifier 'curl_socket_t'
<path to curl source>\curl\include\curl\curl.h(397,52): error C2061:  syntax error: identifier 'curl_socket_t'
<path to curl source>\curl\include\curl\curl.h(407,23): error C2079:  'curl_sockaddr::addr' uses undefined struct 'sockaddr'
<path to curl source>\curl\include\curl\curl.h(411,3): error C2065:  'curl_opensocket_callback': undeclared identifier
<path to curl source>\curl\include\curl\curl.h(411,27): error C4430:  missing type specifier - int assumed. Note: C++ does not support default-int
<path to curl source>\curl\include\curl\curl.h(411,27): error C2378:  'curl_socket_t': redefinition; symbol cannot be overloaded with a typedef
<path to curl source>\curl\include\curl\curl.h(134): message :  see declaration of 'curl_socket_t'
<path to curl source>\curl\include\curl\curl.h(411,28): error C2513:  'int': no variable declared before '='
<path to curl source>\curl\include\curl\curl.h(411,28): error C2143:  syntax error: missing ';' before '('
<path to curl source>\curl\include\curl\curl.h(411,34): error C2062:  type 'void' unexpected
<path to curl source>\curl\include\curl\curl.h(416,59): error C2061:  syntax error: identifier 'curl_socket_t'
<path to curl source>\curl\include\curl\multi.h(113,19): error C3646:  'fd': unknown override specifier
<path to curl source>\curl\include\curl\multi.h(113,19): error C4430:  missing type specifier - int assumed. Note: C++ does not support default-int
<path to curl source>\curl\include\curl\multi.h(157,47): error C2061:  syntax error: identifier 'fd_set'
<path to curl source>\curl\include\curl\multi.h(271,51): error C2061:  syntax error: identifier 'curl_socket_t'
<path to curl source>\curl\include\curl\multi.h(292,76): error C2061:  syntax error: identifier 'curl_socket_t'
<path to curl source>\curl\include\curl\multi.h(296,62): error C2061:  syntax error: identifier 'curl_socket_t'
<path to curl source>\curl\include\curl\multi.h(410,55): error C2061:  syntax error: identifier 'curl_socket_t'

跟进第一条错误消息将我带到curl.h 中的以下代码片段：

131:  #ifndef curl_socket_typedef
132:  /* socket typedef */
133:  #if defined(WIN32) && !defined(__LWIP_OPT_H__) && !defined(LWIP_HDR_OPT_H)
134:  typedef SOCKET curl_socket_t;
135:  #define CURL_SOCKET_BAD INVALID_SOCKET
136:  #else
137:  typedef int curl_socket_t;
138:  #define CURL_SOCKET_BAD -1
139:  #endif
140:  #define curl_socket_typedef
141:  #endif /* curl_socket_typedef */

所以编译器基本上抱怨以下行：

134:  typedef SOCKET curl_socket_t;

但是，我目前对到底是什么问题一无所知。对于 Android 和 iOS，相同的代码可以在 Clang 中正常编译。

有人知道用 Visual C++ 编译需要做什么吗？

Answer 1

正如所指出的，SOCKET 是由 Windows 定义的。更具体地说，它在WinSock2.h 中定义。我包含了那个特定的标题，但没有解决问题。但是，第二个 dll 使用预编译的头文件 (pch.h)。一旦我将WinSock2.h 包含在pch.h 文件中，编译器错误终于得到解决。

但是，要为 Windows UWP 应用程序构建 libcurl，还需要完成另一个关键步骤。在pch.h 文件中包含WinSock2.h 只会解决编译器错误，但会导致两个链接器错误，如下所示：

error LNK2019: unresolved external symbol __imp_VerSetConditionMask referenced in function Curl_verify_windows_version
error LNK2019: unresolved external symbol __imp_VerifyVersionInfoA referenced in function Curl_verify_windows_version
fatal error LNK1120: 2 unresolved externals

这些术语可以通过指示链接器包含附加库OneCore.lib 来解决。 Microsoft 对这个库的描述如下：

为方便起见，Microsoft Windows 软件开发工具包 (SDK) 中提供了一个名为 OneCore.lib 的总括库，该工具包提供所有 Windows 10 设备通用的 Win32 API 子集的导出。将您的经典桌面应用程序或驱动程序与 OneCore.lib（而不是其他库）链接以访问这些 API。如果您将应用或驱动程序链接到 OneCore.lib，并且只调用该库中的 Win32 API，那么您的应用或驱动程序将在所有 Windows 10 设备上成功加载。

有关OneCore.lib 的更多详细信息，请参阅此link。

Answer 2

下面是有效的代码。我确实运行它并获取了结果。

#include <iostream>
#include <stdlib.h>
#define CURL_STATICLIB
#include "curl\curl.h"
#include <string>

#pragma comment(lib, "crypt32")
#pragma comment(lib, "ws2_32.lib")
#pragma comment(lib, "wldap32.lib")


class HttpClient
{
public:
    HttpClient();
    ~HttpClient();
    std::string Get(std::string address);

private:
    CURL* easy_handle;
};

HttpClient::HttpClient()
{
    easy_handle = curl_easy_init();
    curl_global_init(CURL_GLOBAL_ALL);
}

HttpClient::~HttpClient()
{
    curl_global_cleanup();
}

static size_t WriteCallback(void* contents, size_t size, size_t nmemb, void* userp)
{
    ((std::string*)userp)->append((char*)contents, size * nmemb);
    return size * nmemb;
}

std::string HttpClient::Get(std::string address)
{
    std::string html;

    curl_easy_setopt(easy_handle, CURLOPT_URL, address.c_str());
    curl_easy_setopt(easy_handle, CURLOPT_FOLLOWLOCATION, 1L);
    curl_easy_setopt(easy_handle, CURLOPT_WRITEFUNCTION, WriteCallback);
    curl_easy_setopt(easy_handle, CURLOPT_WRITEDATA, &html);
    CURLcode returnValue = curl_easy_perform(easy_handle);

    if (returnValue != CURLE_OK)
    {
        html = "Failed to retrieve HTML";
    }

    curl_easy_cleanup(easy_handle);
    return html;
}

int main()
{
    HttpClient client;
    std::string html = client.Get("https://www.google.com/");
    std::cout << html;
    std::cin.get();
    return 0;
}

还要确保添加以下内容：

链接器 > 附加依赖 > Normaliz.lib

希望这会有所帮助。