Using libcurl in a C++ program causes the Visual C++ compiler to emit errors

Question

I have compiled libcurl on Windows using the Visual C++ compiler into a dll. In addition to the compiled libcurl source code, that dll contains a simple test program that looks as follows:

Header (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;
  };
}

Implementation (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;
}

That test program compiles just fine.

(BTW: I'm aware that curl_global_init and curl_global_cleanup must only be invoked once. It's just a test program...)

Now, I want to reference that dll in another dll and create an instance of the HttpClient class and use it as follows:

#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/");

The compiler call used to compile that second dll looks as follows:

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>

However, attempting to compile the second dll causes the Visual C++ compiler to emit the following errors:

<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'

Following up with the first error message leads me to the following code fragment in curl.h:

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 */

So the compiler essentially complains about the following line:

134:  typedef SOCKET curl_socket_t;

However, I'm currently clueless at what exactly the problem is. The same code compiles fine with Clang for both Android and iOS.

Does anyone see what needs to be done to make this compile with Visual C++?

SOCKET is defined by Windows, that's most probably your issue. Is there a reason to use curl in Windows anyway? — Michael Chourdakis
– Michael Chourdakis, Commented Jun 12, 2019 at 7:57
I was thinking about that as well so I included windows.h and winsock.h separately but the problem is still there. Good question regarding why I'm using libcurl. The project I'm working on is a cross-platform app (Android, iOS, Windows UWP) that utilizes C++. For networking we use libcurl which allows us to build a cross-platform network layer. — ackh
– ackh, Commented Jun 12, 2019 at 8:24
Can your example be simplified? A minimal reproducible example makes it easier to locate problems. What happens if the shown code for your second DLL is replaced by simply #include "curl/curl.h"? Don't reference HttpClient at all. If you get the same error, you can eliminate much of the beginning of your question, since the code for your first DLL would no longer be relevant (other than being a project that does use curl in Windows successfully). — JaMiT
– JaMiT, Commented Jun 14, 2019 at 14:09
Try adding ``` #ifdef WIN32 #include <windows.h> #include <winsock2.h> #include <ws2tcpip.h> #define close closesocket #endif ``` at the top of your HttpClient.h — TinfoilPancakes
– TinfoilPancakes, Commented Jun 15, 2019 at 6:31

Bruno Bieri · Accepted Answer · 2019-06-17 06:42:35Z

Here's the code that will work. I did run it and fetched result also.

#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;
}

Also make sure to add the following:

Linker > Additional Dependencies > Normaliz.lib

Hope this helps.

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