Question

This must be a basic question but I am struggling with this problem for too long now.
I’ve looked everywhere on Google and found some similar problems and solutions but none that solved my specific problem.

I’ve written a very basic C++ DLL. In fact it is almost in C style because the DLL only has a main.cpp code file with a function in it so it’s not even using a class.

Then I have two header files :

MqlUtils.h :

#ifndef MQLUTILS_H
#define MQLUTILS_H

struct MqlStr
{
    int len;
    char *string;
};

enum TradeOperation
{
    OP_BUY = 0,
    OP_SELL = 1,
    OP_BUYLIMIT = 2,
    OP_SELLLIMIT = 3,
    OP_BUYSTOP = 4,
    OP_SELLSTOP = 5
};

#endif

main.h :

#ifndef _DLL_H_
#define _DLL_H_

#include "MqlUtils.h"

#define MT4_EXPFUNC __declspec(dllexport)

#define export extern "C" __declspec( dllexport )

 MT4_EXPFUNC int __stdcall  GetOrdersDetailsNoSymbol(const int orderCount, const char * MasterLicense, const char * SlaveLicense, int orderTicket[], int op[],
    double orderOpenPrice[], double orderStoploss[],
    double orderTakeProfit[], double orderLots[], int orderDateTime[],
    MqlStr * ordersymbol, MqlStr * ordercomments, int lotsCopyingMethod[], int returnedOrders[]);

#endif /* _DLL_H_ */

In fact, for creating my DLL, I started with existing code that someone else wrote, so the .cpp file for my DLL has some obscure syntax that I am not even sure what it is doing. Here’s an excerpt of what the .cpp looks like :

#include "main.h"

#define _UNICODE 1
#define UNICODE 1
#define WIN32_LEAN_AND_MEAN

// ...

EXTERN_C IMAGE_DOS_HEADER __ImageBase;


#if BUILDING_DLL
#define DLLIMPORT __declspec (dllexport)
#else /* Not BUILDING_DLL */
#define DLLIMPORT __declspec (dllimport)
#endif /* Not BUILDING_DLL */

// ...

#ifdef __cplusplus
extern "C"
{
#endif

// ... function code

#ifdef __cplusplus
}
#endif

I did not include everything that is in the .cpp file, where there are // ... there is something else but it’s basic stuff that I understand well so it shouldn’t be the source of my problem…I’ll be glad to post more as needed.

I am not an expert with all the obscure keywords like __declspec and such, but the DLL as it is, can be succesfully imported and the function GetOrdersDetailsNoSymbol can be used by some program, namely MetaTrader 4 (which is the main goal of my lib).

But now I would like to be able to test my library with a C++ program, so I created an empty console program, added the library project to the testing project’s references and linked the .obj and .h files through the testing project’s properties.

I am currently getting this when I compile the testing project :

Error   2   error LNK1120: 1 unresolved externals   Z:\Codes\Debug\TestsCpp.exe TestsCpp
Error   1   error LNK2019: unresolved external symbol "__declspec(dllimport) int __cdecl GetOrdersDetailsNoSymbol(int,char *,char *,int * const,int * const,double * const,double * const,double * const,double * const,int * const,struct MqlStr *,struct MqlStr *,int * const,int * const)" (__imp_?GetOrdersDetailsNoSymbol@@YAHHPAD0QAH1QAN2221PAUMqlStr@@311@Z) referenced in function "void __cdecl TestClient(void)" (?TestClient@@YAXXZ)    Z:\Codes\TestsCpp\main.obj  TestsCpp

Oh and here’s the main.cpp for the testing project :

#include "MqlUtils.h"
#include "main.h"

extern __declspec(dllimport) int GetOrdersDetailsNoSymbol(int orderCount, char * MasterLicense, char * SlaveLicense, int orderTicket[], int op[],
    double orderOpenPrice[], double orderStoploss[],
    double orderTakeProfit[], double orderLots[], int orderDateTime[],
    MqlStr* ordersymbol, MqlStr* ordercomments, int lotsCopyingMethod[], int returnedOrders[]);


void TestClient()
{
   char* Master = "7C83C4C2";
   char* Slave = "3B7C22A";

   int returnedOrderCount[1] = {0};

   double aStoredOrderOpenPrice[4];
   int aStoredOrderType[4];
   int aStoredOrderTicket[4];
   double aStoredOrderStopLoss[4];
   double aStoredOrdeTakeProfit[4];
   double aStoredOrderLots[4];
   int aStoredOrderDateTime[4];
   int aStoredLotsMethods[4];
   MqlStr* aStoredOrderComment[4];
   MqlStr* aStoredOrderSymbol[4];

   for (int i = 0; i < 4; i++)
   {
      aStoredOrderOpenPrice[i]= -1;
      aStoredOrderType[i]= -1;
      aStoredOrderTicket[i]= -1;
      aStoredOrderStopLoss[i]= -1;
      aStoredOrdeTakeProfit[i]= -1;
      aStoredOrderLots[i]= -1;
      aStoredOrderDateTime[i]= -1;
      aStoredLotsMethods[i]= -1;

      aStoredOrderComment[i]->len = 56;
      aStoredOrderComment[i]->string = "11111111111111111111111111111111111111111111111111111111";
      aStoredOrderSymbol[i]->len = 56;
      aStoredOrderSymbol[i]->string = "11111111111111111111111111111111111111111111111111111111";
   }


   GetOrdersDetailsNoSymbol(1, Master, Slave, aStoredOrderTicket,  aStoredOrderType,
                                            aStoredOrderOpenPrice, aStoredOrderStopLoss,
                                            aStoredOrdeTakeProfit, aStoredOrderLots, aStoredOrderDateTime,
                                            *aStoredOrderSymbol, *aStoredOrderComment,     aStoredLotsMethods, returnedOrderCount);
}

int main(int argc, char **argv)
{
   TestClient();

   return 0;
}

If anyone could help me solve this, I would be infinitely grateful.

Thanks for reading!

Answer 1

Your two declarations of GetOrdersDetailsNoSymbol do not match. In your header file you declare it with __stdcall and in main.cpp you don’t. You should have only a single declaration. It can use #define and #ifdef to apply the dllimport or dllexport keywords as appropriate.

EDIT: Also, get rid of the extern “C” statements. And then use the DLLIMPORT #define to declare your function and only #define BUILDING_DLL in the build of your DLL.