letting a DLL call a exe function with function pointer - c++

Question

Can anyone tell me what I'm doing wrong?
I'm trying to run a custom main on a different thread.
This is the code.
.exe
main.cpp
#include "dll_class.h"
#include <iostream>
int main(void);
DllClass object(main);
int main(void)
{
std::cout << "Enter the main code.\n";
std::getchar();
}
.dll
dll_class.h
#include "platform.h"
#include <iostream>
class DLL_API DllClass //DLL_API is just a macro for import and export.
{
public:
DllClass(int(*main)(void))
{
std::cout << "Start application.\n";
platform = new Platform(main);
}
~DllClass(void)
{
delete platform;
}
private:
Platform* platform;
};
platform.h
class DLL_API Platform
{
public:
Platform(main_t main_tp);
~Platform(void){}
};
platform.cpp
#include "platform.h"
#include "Windows.h"
#include <iostream>
HHOOK hookHandle;
int(*main_p)(void);//This will hold a the main function of the the .exe.
LRESULT CALLBACK keyHandler(int nCode, WPARAM wParam, LPARAM lParam);
DWORD WINAPI callMain(_In_ LPVOID lpParameter)
{
std::cout << "Calling the main function.\n";
main_p();
return 0;
}
Platform::Platform(int(*main_tp)(void))
{
main_p = main_tp;
CreateThread(NULL, 0, callMain, NULL, 0, NULL);
std::cout << "Setting hooks.\n";
hookHandle = SetWindowsHookEx(WH_MOUSE_LL, keyHandler, NULL, 0);
std::cout << "Enter message loop.\n";
MSG message;
while(GetMessage(&message, (HWND)-1, 0, 0) != 0){
TranslateMessage( &message );
DispatchMessage( &message );
}
}
LRESULT CALLBACK keyHandler(int nCode, WPARAM wParam, LPARAM lParam)
{
std::cout << "Inside the hook function.\n" << std::endl;
return CallNextHookEx(hookHandle, nCode, wParam, lParam);
}
It runs great, till a certain moment.
This is the output.
Start application.
Setting hooks.
Calling the main function.
Enter message loop.
Inside the hook function. (A lot of times of course).
but it never says:
Enter the main code.
Is it impossible to let dll call a exe function?

Answer 1

It is very much POSSIBLE to call a function in the executable file from a shared library. However, the C standard, as mentioned in the other answer, doesn't allow you to call main. This has to do with the fact that the C runtime [to guard against language lawyers: insert "sometimes" here] relies on a certain order of things, and if you try to call main before the C runtime has done the right initialization BEFORE main, you get problems.
If your goal is to actually subvert what main does, then you will have to find a different way of achieving this - at least if you expect it to work for more than one particular executable.

Answer 2

The C++ standard doesn't allow calling main() or taking its address, which is what you're doing here. See this thread which quotes line and verse. So, what you're doing is undefined.