Why do I get an Exception Exception thrown at 0x00000000 in ConsoleApplication1.exe: 0xC0000005: Access violation executing location 0x00000000

Question

I'm trying to create a very simple dll and load it to another file for learning purpose. I'm using VC++ same version for both the DLL and the C++ file. Here is my source code:

load.cpp:

#include <iostream>
#include <windows.h>

using namespace std;

typedef int(__stdcall* nsum)(int a, int b);

int main(void)
{
    HINSTANCE myDll = LoadLibrary(L".\\DLL1.dll");
    nsum sum = (nsum)GetProcAddress(myDll, "sum");

    if (!myDll) {
        cout << "could not load the dynamic library" << endl;
        return EXIT_FAILURE;
    }


    int xfinal = sum(10, 20);
    cout << xfinal << endl;

    return 0;
}

and dll.cpp

#include <windows.h>
#include "pch.h"

using namespace std;

int __declspec(dllexport) __stdcall sum(int a, int b)
{
    return a + b;
}

pch.h

// pch.cpp: source file corresponding to the pre-compiled header

#include "pch.h"

// When you are using pre-compiled headers, this source file is necessary for compilation to succeed.

But still I'm getting the exception. I searched for answers but found none, so I am writing this for help

In the load.cpp, in HINSTANCE myDll, I can peek and see the value being set to 0x000000000. Is it the reason? If so, How can i fix it?

Answer 1

Your problem is quite simple. The exported name of your function is not sum because you are not use a C ABI. Instead, it's a mangled name ?sum@@YGHHH@Z which of course is not found by your use of GetProcAddress(myDLL, "sum"); There is no such function sum. As such, you are trying to invoke a function at address 0, i.e. exactly what the exception tells you.

Microsoft calls these Decorated Names rather than "mangled" but they both mean the same thing: the type information for the parameter types, calling-convention, and return value is encoded into special symbols that make up the name.

A few options here to fix this problem:

• Use the import library instead of explicit linking which will use the mangled name so you can stick with a C++ ABI.

• Use a C ABI via extern "C" keeping the existing __stdcall calling convention in which case the name will be _sum@8 when building for x86 (32-bit):

extern "C" int __declspec(dllexport) __stdcall sum(int a, int b)

• Use a C ABI with extern "C" but use __cdecl which will actually result in sum for x86 (32-bit):

extern "C" int __declspec(dllexport) __cdecl sum(int a, int b)

For x64 there is one standard calling convention __fastcall (__cdecl, __stdcall are just aliases for __fastcall). In that, just using extern "C" will result in the name sum for any of those calling conventions. The only case that results in 'mangled' exports using a C ABI for x64 then is extern "C" __vectorcall.

Be sure to update your typedef to match the calling convention you are using if you change it.

Second, you should add more error-checking:

HMODULE myDll = LoadLibrary(L".\\DLL1.dll");
if (!myDll) {
    cout << "could not load the dynamic library" << endl;
    return EXIT_FAILURE;
}

nsum sum = (nsum)GetProcAddress(myDll, "sum");
if (!sum) {
    cout << "could not find the export" << endl;
    return EXIT_FAILURE;
}

If you are using C++11 or later (Visual Studio 2012 or later), then auto sum = reinterpret_cast<nsum>(reinterpret_cast<void*>(GetProcAddress(myDll, "sum"))); is better.