i am exporting dll but GetProcAddress returning 127 error ,how to resolve it ?

Question

#include "Calc.h"
#include<iostream>
#include <windows.h>
#include <WINERROR.H.>

typedef void (WINAPI * PCTOR) ();
int main()
{
    HMODULE hMod = LoadLibrary (L"Calci.dll");
    if (NULL == hMod)
    {
        printf ("LoadLibrary failed\n");
        return 1;
    }
    CCalc *pCCalc = (CCalc *) malloc (sizeof (CCalc));
    if (NULL == pCCalc)
    {
        printf ("memory allocation failed\n");
        return 1;
    }
    PCTOR pCtor = (PCTOR) GetProcAddress (hMod, "CCalc");//127 error
    int err = GetLastError();
    if (NULL == pCtor)
    {
        printf ("GetProcAddress failed\n");
        return 1;
    }
    __asm { MOV ECX, pCCalc };
    pCtor ();
    return 0;
}

//dll file
#include <tchar.h>


#ifdef CALC_EXPORTS
#define CALC_API __declspec (dllexport)
#else
#define CALC_API __declspec (dllimport)
#endif

#define SOME_INSTN_BUF        260

class CALC_API CCalc
{
private:
char m_szLastUsedFunc[SOME_INSTN_BUF];

public:
    CCalc (); 

    int Add (int i, int j);
    int Sub (int i, int j);
    TCHAR* GetLastUsedFunc ();

};

Answer 1

Use dumpbin.exe to check the exact name of the export in the DLL. Maybe it doesn't exist at all?

If you have a chance to use import library instead of LoadLibrary API, it is better.

Answer 2

You're invoking GetProcAddress (hMod, "CCalc"), however "CCalc" isn't the name of a function: it's the name of a class.

You're trying to load the address of the CCalc::CCalc default constructor: to do that, use a tool (e.g. dumpbin) to discover the "decorated" name of the constructor.

However instead of trying to dynamic-load and invoke the constructor, a more usual way to implement this functionality would be to create a static factory method in the DLL, e.g. like this:

class CALC_API CCalc 
{ 
public:
    static CCalc* create() { return new CCalc(); }

private:
    //doesn't need to be public because users instantiate this class using 
    //the static create method
    CCalc();  

public:
    virtual int Add (int i, int j); 
    virtual int Sub (int i, int j); 
    virtual TCHAR* GetLastUsedFunc ();

    virtual ~CCalc() {}
};

Then use GetProcAddress to get the address of the static CCalc::create function, which because it's static you can invoke without using assembly to mess with ECX.

Answer 3

You can't use GetProcAddress for classes. This does not work. Only functions you can resolve their names are unmangled "C" functions.

For example:

extern "C" __declspec(dllexport) CCalc *create_calc()
{
     return new CCalc;
}

Now, you can resolve it using.

GetProcAddress(halnder,"create_calc");

As create_calc is not-mangled function.

Also you will have to provide abstract API class without implementation and make CCalc inherit ACalc, otherwise you'll get unresolved symbols tying to compile your application. Because actual add and remove member functions are not known to the application.

class ACalc {
public:
      virtual add(int i,int j) = 0;
      ...
      virtaul ~ACalc() {}
};

class CCalc : public ACalc {
public:
      virtual add(int i,int j) { ... };
      ...
};

And in the main program

ACalc *ptr= call_for_dll_function