How to create C++ DLL (with DEV -C++) and use in MT4 script (Step by step)

Question

I am trying to create my own DLL with DEV-C++ IDE tool and trying to use it inside MT4 script. I tried to study the example file [MT4_HOME]\MQL4\Scripts\Examples\DLL\DLLSample.cpp available in any MT4 installation and I tried to follow the same logic with other script but without sucess. Below I am describing in great details steps i followed just to be clear. I would like to understand why following the described steps my own dll doesn't work.

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System configuration

1. Laptop with windows 10 ;
2. Dev- cpp installed
3. MT4 installed

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Goals

1. to write my own dll by using "dev–c++" IDE tool;
2. compile the dll;
3. use the dll into a simple script in mt4.

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Steps

1. First I create a folder on my desktop named mydll;
2. I start dev-cpp IDE tool;
3. File -> New -> Project;
4. I select project type -> DLL
5. I write project name: mydll
6. I press OK button
7. Then I choose the folder in which to save the project (the folder mydll created in desktop at step a) and press save
8. At this point Dev –C++ showes me two file templates (dllmain.cpp, dll.h) but I ignore them and close them without saving them into the project. After closing them I also remove them from IDE tool (write click with mouse and click remove file for each of them)
9. Now I rigth clik over devc++ project -> New File
10. Now I paste into this file the source code of my own DLL. (the below code) Note: For people who are familiar with metatrader 4, please notice that this code is a fragment of the file [MT4_HOME]\MQL4\Scripts\Examples\DLL\DLLSample.cpp of standard MT4 installation

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#include <windows.h>
#include <stdlib.h>
#include <stdio.h>
//---
#define MT4_EXPFUNC __declspec(dllexport)


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 int __stdcall GetIntValue(const int ipar)
  {
   printf("GetIntValue takes %d\n",ipar);
   return(ipar);
  }

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11. I save this file into my DEV-C++ project folder with name mydll.cpp
12. Now, in DEV-C++ ide tool I press F9 button to compile this file.
13. Observations: a. the compilation process completes succesfully without any errors and any warnings b. some files appears into the DEV-C++ project (mydll.dll, libmydll.def, libmydll.a, mydll.o, Makefile.win, mydll.layout).
14. Now, I copy and paste the mydll.dll into [MT4_HOME]\MQL4\Libraries directory of MT4
15. Now, I create an empty folder [MT4_HOME]\MQL4\Scripts\Examples\mydll
16. I copy and paste the files mydll.cpp and libmydll.def into the [MT4_HOME]\MQL4\Scripts\Examples\mydll folder
17. Finally, I create a new file named mydllTester.mq4 into the [MT4_HOME]\MQL4\Scripts\Examples\mydll folder. Below is the source code

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#import "DLLTutorial.dll"
int    _Z11GetIntValuei(int);  
#import

void OnStart()
{
   int cnt=_Z11GetIntValuei(int(10)); 
   Comment(cnt);
}

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18. I open the file mydllTester.mq4 with the MT4 code editor and I compile the file.
19. Final test As the final step, I make a test to check if this works. I open Metatrader4 , I open a new chart and I simply click over my script . My expectation is that the number 10 appears on the top left bottom of the chart buti t doesn’t work. Can you help me to understand which is the step I am making mistakes? Thank you very much, Best Regards

Porker

Which errors do you get? What did you observe when you stepped through your code line by line with the debugger? – Ratib

Unfortunatelly I am a beginner with C++ so I don't know how to debug the compiled DLL. However, when I run the final tester script in MT4 (step 19 above ) I get nothing: no errors, no output messages in MT4 chart . On the other hand, If I try to run the default script available in MT4 installation it perfectly works . – Porker

Maybe some of these links are helpful. – Ratib

Answer 1

Finally I found a solution of my issue and now I am able to write a simple DLL and call it from MT4 with success. Below the steps:

1. Create file mydll.cpp
2. Write the content of file mydll.cpp

---

#include <stdlib.h> 

#ifdef __cplusplus 
extern "C" 
{ 
#endif 
__declspec(dllexport) int __stdcall DLLAdd(int i, int j) ; 
#ifdef __cplusplus 
} 
#endif 

__declspec(dllexport) int __stdcall DLLAdd(int i, int j) 
{ 
    return i+j; 
} 

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3. Compile the file mydll.cpp (pay attention to compile using "TDM-GCC 32 bit-release" compiler because MT4 is 32 bit application and it only understand 32 bit compiled files). The compiler will produce files mydll.dll , libmydll.def

4. Copy the file mydll.dll into the [MT4_HOME]\MQL4\Libraries directory of MT4

5. Create a folder "test_script" into the [MT4_HOME]\MQL4 directory of MT4 (or wherever you want inside the MT4 main folder)

6. Copy and paste inside the "test_script" folder the libmydll.def file

7. Create a new script "mydlltester.mq4" inside the "test_script" folder

8. Write the content of "mydlltester.mq4" file as below

---

#property strict

#import "mydll.dll"
    int DLLAdd(int i, int j); 
#import

void OnStart()
{ 
   Comment(DLLAdd(2,3));
}

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9. Open the "mydlltester.mq4" file with the MT4 compiler and compile it
10. Final test: if you now open a chart in mt4 and run the script mydlltester you will see the sum 5 appearing on the top left corner of the chart. Cheers!!

Porker