Getting function by name in c++ - c++

Question

Code:
#include <tchar.h>
#include <windows.h>
typedef INT (WINAPI * lolMessageBoxA)(HWND,LPCSTR,LPCSTR,UINT);
int WINAPI WinMain (HINSTANCE hThisInstance,
HINSTANCE hPrevInstance,
LPSTR lpszArgument,
int nCmdShow)
{
lolMessageBoxA UchwytMessageBoxA;
UchwytMessageBoxA = lolMessageBoxA(GetProcAddress(GetModuleHandle("User32.dll"),"MessageBoxA"));
UchwytMessageBoxA(NULL,"TEST WINDOWS","TEXT 2", MB_OKCANCEL | MB_ICONWARNING);
return 0;
}
When i called MessageBoxA normally before making hook the hooking part worked good. But when i erased that part and leaved only the 'hooking' part it fails...
Solved:
#include <tchar.h>
#include <windows.h>
typedef INT (WINAPI * lolMessageBoxA)(HWND,LPCSTR,LPCSTR,UINT);
int WINAPI WinMain (HINSTANCE hThisInstance,
HINSTANCE hPrevInstance,
LPSTR lpszArgument,
int nCmdShow)
{
lolMessageBoxA UchwytMessageBoxA;
LoadLibraryA("User32.dll");
UchwytMessageBoxA = lolMessageBoxA(GetProcAddress(GetModuleHandle("User32.dll"),"MessageBoxA"));
UchwytMessageBoxA(NULL,"TEST WINDOWS","TEXT 2", MB_OKCANCEL | MB_ICONWARNING);
return 0;
}

Answer 1

As presented here, there's no reason for the process to have loaded user32.dll. So the call to GetModuleHandle will return NULL. Then the call to GetProcAddress will also fail and return NULL. And well, you can see what happens next.
When you called MessageBoxA directly, the linker would have produced an import table that forced the loader to load user32.dll. But when you removed the call to MessageBoxA, there was no need for it to do so.
You should replace the call to GetModuleHandle with a call to LoadLibrary, and so force the library to be loaded.
Finally, the real reason for you having asked this question is that your code neglects error checking. Don't do that. Check the values returned by the Win32 function calls. Had you done so you would have been able to work this out for yourself.