Dll injection cant load a function address

Question

Im trying to hook the keyboard with a dll injection in C. When i try GetProcAddress on the KeyboardProc function the GetProcAddress return NULL and GetLastError returns error 131. after that I get a DLL_PROCESS_DETACH. In windows website it says:

ERROR_NEGATIVE_SEEK
131 (0x83)
An attempt was made to move the file pointer before the beginning of the file.

I dont understand what is the problem in my code.

The injector I am using:

#include <Windows.h>
#include <stdio.h>

int main(int argc, char *argv[])
{

    HMODULE dll = LoadLibrary((LPCSTR) "dll.dll");
    if (dll == NULL)
    {
        printf("The DLL could not be found.\n");
        FreeLibrary(dll);
        return -1;
    }
    printf("The DLL was found.\n");
    HOOKPROC addr = (HOOKPROC)GetProcAddress(dll, "KeyboardProc");
    if (addr == NULL)
    {
        printf("The function was not found.\n");
        FreeLibrary(dll);
        return -1;
    }
    printf("The function was  found.\n");
    HHOOK handle = SetWindowsHookEx(WH_KEYBOARD, addr, dll, 0);
    if (handle == NULL)
    {
        printf("The KEYBOARD could not be hooked.\n");
        FreeLibrary(dll);
    }

    printf("Program successfully hooked.\nPress enter to unhook the function and stop the program.\n");
    getchar();
    UnhookWindowsHookEx(handle);
    FreeLibrary(dll);

    return 0;
}

The dll Im using:

#include <windows.h>
#include <stdio.h>

INT APIENTRY DllMain(HMODULE hDLL, DWORD Reason, LPVOID Reserved)
{

    switch (Reason)
    {
    case DLL_PROCESS_ATTACH:
        MessageBox(0, (LPCSTR) "DLL attach function called.", (LPCSTR) "Dll injection", MB_OK);
        break;
    case DLL_PROCESS_DETACH:
        MessageBox(0, (LPCSTR) "DLL detach function called.", (LPCSTR) "Dll injection", MB_OK);
        break;
    case DLL_THREAD_ATTACH:
        MessageBox(0, (LPCSTR) "DLL thread attach function called.", (LPCSTR) "Dll injection", MB_OK);
        break;
    case DLL_THREAD_DETACH:
        MessageBox(0, (LPCSTR) "DLL thread detach function called..", (LPCSTR) "Dll injection", MB_OK);

        break;
    }
    return TRUE;
}

extern __declspec(dllexport) LRESULT CALLBACK KeyboardProc(int code, WPARAM wParam, LPARAM lParam)
{
    if (code < 0)
    {
        return CallNextHookEx(NULL, code, wParam, lParam);
    }

    FILE *LOG;
    LOG = fopen("LOG.txt", "a+");
    if (wParam == WM_KEYDOWN)
    {
        fputs((char *)lParam, LOG);
        fclose(LOG);
    }
    return CallNextHookEx(NULL, code, wParam, lParam);
}

Im using win10 and mingw. Both injector and dll are compiled as C.

Answer 1

Problem is simple:

extern __declspec(dllexport) LRESULT CALLBACK KeyboardProc(int code, WPARAM wParam, LPARAM lParam)

Most probably is defined in cpp file and as a result it is C++ function. C++ to allow function overloading is using name mangling, so your function is visible under a name which is result of mangling.

You have to force it to be a C function so name mangling is disabled.

So add extern "C" or make source to have C specific extension (compile it as C code). Your code is pure C.