Passing a string to a DLL Function

[Christian Stern]

Hey all,

maybe you could help me solving this problem. I have create a simple DLL, like as described here:

https://www.mql5.com/en/articles/18

The DLL work when i use as parameter a Integer but when i switch to string, MT5 come up with the message, he couldn't find the method:

2017.11.18 11:24:35.507    Test(EURUSD,M1)    Cannot find 'testWchar_Ref' in 'Test\variableTest.dll'

This is my import in MT5:

#import "Test\variableTest.dll"
   void testInt_Ref   (int    &intTest_ref);
   void testWchar_Ref (string &wchar_tTest_Ref);
#import  

Implementation in MT5:

   string stringTest = "Hello you...";  
   int    myInt = 0;
   
   testInt_Ref(myInt);
   Print("New INT: ", myInt );
   
   Print("PRE: " + stringTest);
   testWchar_Ref(u_stringTest);
   Print("POST: " + stringTest);

Export in DLL:

#ifdef VARIABLETEST_EXPORTS  
#define QVAR_API __declspec(dllexport)   
#else  
#define QVAR_API __declspec(dllimport)   
#endif

extern QVAR_API void testInt_Ref(int &intTest_ref);
extern QVAR_API void testWchar_Ref(wchar_t &wchar_tTest_Ptr);

DLL Function:

// Include header files
#include "stdafx.h"

using namespace std;

// add new INT value
void testInt_Ref(int &intTest_ref) {
    intTest_ref = 222;
}

// add new WCHAR_T value
void testWchar_Ref(wchar_t &wchar_tTest) {

    wchar_t *msg = L"This is cool.....";
    wchar_t *wchar_tTest_Ref = &wchar_tTest;

    //--- replace it
    memcpy(wchar_tTest_Ref, msg, wcslen(msg) * sizeof(wchar_t));
}

Result:

2017.11.18 11:24:35.507    Test(EURUSD,M1)    New INT: 222
2017.11.18 11:24:35.507    Test(EURUSD,M1)    PRE: Hello you...
2017.11.18 11:24:35.507    Test(EURUSD,M1)    Cannot find 'testWchar_Ref' in 'Test\variableTest.dll'
2017.11.18 11:24:35.507    Test(EURUSD,M1)    unresolved import function call

it's compiled correctly. In my c++ Testprogram it works. Even when i leave the DLL Method empty it appears. It must be the parameter.

How can i pass a string to a DLL, modify it and use it back in MT5 ?

Thanks in Advanced

[Stanislav Korotky]

What you're asking for is not passing a string to DLL, but getting a string from DLL!

To pass a string to DLL just declare a function with LPWSTR parameter (not a reference!) in DLL and call it with a string.

To read a string you can allocate a uchar buffer in MQL and pass it as a pointer to the DLL (something like GetString(/*in|out*/uchar *buffer, /*in*/const int bufferLen) in MQL import), which will fill it with chars. Then you can reconstruct the string using CharArrayToString.

Or you can have a static (or preallocated) buffer in the DLL and return wchar_t* (LPWSTR) for it from a function (like string GetString() in MQL import). Probably you can find a solution in this source codes (unfortunately, the description is in Russian only).

[Mikhail Dovbakh]

In the standard delivery of MT4 (\ MQL4 \ Scripts \ Examples \ DLL) there is an example ( & it works fine in MT5) :

//+------------------------------------------------------------------+

#include <windows.h>

#include <stdlib.h>

#include <stdio.h>

//---

#define MT4_EXPFUNC __declspec(dllexport)

//+------------------------------------------------------------------+

#pragma pack(pop)

//---

struct MqlStr

  {

   int               len;

   char             *string;

  };

static int CompareMqlStr(const void *left,const void *right);

//+------------------------------------------------------------------+

//|                                                                  |

//+------------------------------------------------------------------+

BOOL APIENTRY DllMain(HANDLE hModule,DWORD ul_reason_for_call,LPVOID lpReserved)

  {

//---

   switch(ul_reason_for_call)

     {

      case DLL_PROCESS_ATTACH:

      case DLL_THREAD_ATTACH:

      case DLL_THREAD_DETACH:

      case DLL_PROCESS_DETACH:

         break;

     }

//---

   return(TRUE);

  }

//+------------------------------------------------------------------+

MT4_EXPFUNC wchar_t* __stdcall GetStringValue(wchar_t *spar)

  {

   spar=L" it ...";

   return(spar);

  }

#import "DLLSample.dll"

string GetStringValue(string);

#import

//+------------------------------------------------------------------+

int init()

  {

   string sret;

   int    cnt;

//--- simple dll-functions cal

   sret=GetStringValue("some string");

   Print("Returned value is ",sret);

//---

   return(0);

  }

2017.11.18 23:08:23.306 DLL_Sample (EURUSD,M1) Returned value is  it ...

[Christian Stern]

Hi Mikhail ,Thank you.  Yeah i know that but it doesn't match my needings. I have a mysql string result which i want to handover to MT. I dont know what i do wrong...see below my full replay

Mikhail Dovbakh:

In the standard delivery of MT4 (\ MQL4 \ Scripts \ Examples \ DLL) there is an example ( & it works fine in MT5) :

...

[Christian Stern]

Stanislav Korotky:

What you're asking for is not passing a string to DLL, but getting a string from DLL!

To pass a string to DLL just declare a function with LPWSTR parameter (not a reference!) in DLL and call it with a string.

To read a string you can allocate a uchar buffer in MQL and pass it as a pointer to the DLL (something like GetString(/*in|out*/uchar *buffer, /*in*/const int bufferLen) in MQL import), which will fill it with chars. Then you can reconstruct the string using CharArrayToString.

Or you can have a static (or preallocated) buffer in the DLL and return wchar_t* (LPWSTR) for it from a function (like string GetString() in MQL import). Probably you can find a solution in this source codes (unfortunately, the description is in Russian only).

Hi Stanislav , thank you and thank you for the hint. Yes you are right, i want to pass a string reference to a DLL function and within the DLL i want to assign data, like a mysql result. Which means i need to refer a string array. After all that reading i had some problems sorting my mind :D ...I understand what you are saying but somehow it won't work. I guess more, i have some compiler settings missing, because also the examples found here or the mysql_wrapper from russell ( https://www.mql5.com/en/code/8623 ) wont work.

He just provides a 32bit version and i compiled it for 64bit with VS2017. But also very very simple function where you just pass the arument won't work. I dont have problems passing a string, this works. I also can return a wchar_t value but i can't handle a string reference.

In MT4 there use, so far i know, char and in mt5 wchar_t

I tried all this:

#pragma pack(push,1)
struct MqlStr_mt4 {
    int      len;       // 32 bit integer, contains the size of the buffer allocated for the string
    char*    string;     // 32 bit address of the buffer that contains the string
    int      reserved;   // 32 bit integer, reserved, do not use
};

struct MqlStr_mt5 {
    int      len;        // 32 bit integer, contains the size of the buffer allocated for the string
    wchar_t* string;     // 32 bit address of the buffer that contains the string
    int      reserved;   // 32 bit integer, reserved, do not use
};
#pragma pack(pop,1)

extern MYSQLWRAPPER_API wchar_t* MT5_mysql_wrapper_version    (); << works

extern MYSQLWRAPPER_API void     MT5_mysql_wrapper_version    (int &foobar); << int works

extern MYSQLWRAPPER_API void     MT5_mysql_wrapper_version    (wchar_t *foobar); << Passing only

extern MYSQLWRAPPER_API void     MT5_mysql_wrapper_version    (wchar_t *&foobar); << Error in MT

extern MYSQLWRAPPER_API void     MT5_mysql_wrapper_version    (MqlStr_mt4 &foobar); << Error in MT

extern MYSQLWRAPPER_API void     MT5_mysql_wrapper_version    (MqlStr_mt4 *&foobar); << Error in MT

extern MYSQLWRAPPER_API void     MT5_mysql_wrapper_version    (MqlStr_mt5 &foobar); << Error in MT

extern MYSQLWRAPPER_API void     MT5_mysql_wrapper_version    (MqlStr_mt5 *&foobar); << Error in MT

extern MYSQLWRAPPER_API void     MT5_mysql_wrapper_version    (LPWSTR &foobar); << Error in MT

extern MYSQLWRAPPER_API void     MT5_mysql_wrapper_version    (LPWSTR *&foobar); << Error in MT

extern MYSQLWRAPPER_API void     MT5_mysql_wrapper_version    (wchar_t &foobar); << Error in MT

The function also has been exported with "C" but it wont change.

Also the example from Mikhail don't work. I spend the entiere weekend with MT documentary, google and coffee and i have no clue what i do wrong.  Within C++ it works. NO Errors, no NULL Pointer Exception ....all fine.

[Mikhail Dovbakh]

Hello Christian !
If you do not have the DLL that I packed into the attachment, then you have a 32 bit MT5.
If this is not the case, I can not reason why this assembly of DLL (by the way it was made directly in the META Editor) does not work.

[Christian Stern]

Mikhail Dovbakh:
Hello Christian !
If you do not have the DLL that I packed into the attachment, then you have a 32 bit MT5.
If this is not the case, I can not reason why this assembly of DLL (by the way it was made directly in the META Editor) does not work.

Hi Mikhail ,

no, your DLL get's loaded and it works. I meaned the DLL from the one documentation i mention. But as i said, i need to pass a string reference in order to handle mysql results.

[Christian Stern]

Now, somehow i am a bit futher but still not at the goal. Current this works but the string is displayed incorretly:

Since i export the function as "C", MT stops showing me the unresolve able error message.

C++:

extern "C" MYSQLWRAPPER_API wchar_t*    returnString    ();
extern "C" MYSQLWRAPPER_API void        changeString    (wchar_t* &stringTest);
extern "C" MYSQLWRAPPER_API void        changeString1   (char* &stringTest);
extern "C" MYSQLWRAPPER_API void        fnReplaceString (wchar_t *text, wchar_t *from, wchar_t *to);

wchar_t* __stdcall returnString() {
    return(L"this is a wchar_t string");
}

void  __stdcall changeString(wchar_t* &stringTest) {
    stringTest = L"Hello you ....";
}

void  __stdcall changeString1(char* &stringTest) {
    stringTest = "Hello you ....";
}

void __stdcall fnReplaceString(wchar_t *text, wchar_t *from, wchar_t *to) {
    wchar_t *cp;
    //--- parameters check
    if (text == NULL || from == NULL || to == NULL) return;
    if (wcslen(from) != wcslen(to))             return;
    //--- search for substring
    if ((cp = wcsstr(text, from)) == NULL)         return;
    //--- replace it
    memcpy(cp, to, wcslen(to) * sizeof(wchar_t));
}

MT5:

#import "Quantum\MySQL\mysql_wrapper.dll"
   string   returnString   ();
   void     changeString   (string &value);
   void     changeString1   (string &value);
#import  

string return_value1 = returnString();
string return_value2 = "";
   
Print("1. Return value is: " + return_value1 );
   

// wchar_t
changeString(return_value1);

// char
changeString1(return_value2);

Print("2. Return value is: " + return_value1 );
Print("3. Return value is: " + return_value2 );

//--- modify the string
string text="A quick brown fox jumps over the lazy dog";
  
fnReplaceString(text,"fox","cat");
Print("Replace: ",text);

Result:

2017.11.19 20:40:55.816    Test (EURUSD,M1)    1. Return value is: this is a wchar_t string
2017.11.19 20:41:06.085    Test (EURUSD,M1)    2. Return value is: ㎘߾
2017.11.19 20:41:07.636    Test (EURUSD,M1)    3. Return value is: ㎸߾
2017.11.19 20:41:11.343    Test (EURUSD,M1)    Replace: A quick brown fox jumps over the lazy dog

Also in the example, from a example ( https://www.mql5.com/en/articles/18 ) , the hasn't changed. 

But since i overwrite the pointer at the end in the DLL function fnReplaceString:

void __stdcall fnReplaceString(wchar_t* text, wchar_t *from, wchar_t *to) {
    wchar_t *cp;
    //--- parameters check
    if (text == NULL || from == NULL || to == NULL) return;
    if (wcslen(from) != wcslen(to))             return;
    //--- search for substring
    if ((cp = wcsstr(text, from)) == NULL)         return;

    //--- replace it
    memcpy(cp, to, wcslen(to) * sizeof(wchar_t));
    text = cp;
}

The result looks great:

2017.11.19 20:50:34.581    QuantumEA (EURUSD,M1)    Replace: A quick brown cat jumps over the lazy dog