function hook problem

[laconstantine]

I am playing with function hooking so I wrote simple program that calls
DisplayMessage Function which prints out Hello..

void DisplayMessage()
{
	cout << "Hello";
}

After this one i found the DisplayMessage offset its (0x00131000)
now my target is write a dll to change DisplayMessage function to say bye instead of hello....

and here comes the problem...
DLL code
dll.h'

#ifndef _DLL_H_
#define _DLL_H_

#if BUILDING_DLL
# define DLLIMPORT __declspec (dllexport)
#else /* Not BUILDING_DLL */
# define DLLIMPORT __declspec (dllimport)
#endif /* Not BUILDING_DLL */

DLLIMPORT void Hook(void);

#endif /* _DLL_H_ *

dll.cpp

void (*PHook)(void);
PHook = (void*)(0x00131000);
PHook(void)
{
	cout << "Bye";
}

void Hook()
{ 
	PHook();	       
          
}


BOOL APIENTRY DllMain (HINSTANCE hInst     /* Library instance handle. */ ,
                       DWORD reason        /* Reason this function is being called. */ ,
                       LPVOID reserved     /* Not used. */ )
{
    switch (reason)
    {
      case DLL_PROCESS_ATTACH:          
           Hook();
        break;

      case DLL_PROCESS_DETACH:
           
        break;

      case DLL_THREAD_ATTACH:
           
        break;

      case DLL_THREAD_DETACH:
          
        break;

    }
    /* Returns TRUE on success, FALSE on failure */
    return TRUE;
}

its wont compile no idea why
heres the errors

mpiling...
1>dllmain.cpp
1>.\dllmain.cpp(11) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
1>.\dllmain.cpp(11) : error C2373: 'PHook' : redefinition; different type modifiers
1>        .\dllmain.cpp(10) : see declaration of 'PHook'
1>.\dllmain.cpp(11) : error C2440: 'initializing' : cannot convert from 'void *' to 'int'
1>        There is no context in which this conversion is possible
1>.\dllmain.cpp(13) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
1>.\dllmain.cpp(13) : error C2365: 'PHook' : redefinition; previous definition was 'data variable'
1>        .\dllmain.cpp(10) : see declaration of 'PHook'
1>.\dllmain.cpp(15) : warning C4508: 'PHook' : function should return a value; 'void' return type assumed
1>.\dllmain.cpp(18) : warning C4273: 'Hook' : inconsistent dll linkage
1>        c:\users\zippo\documents\visual studio 2008\projects\dynamic link libary\dynamic link libary\dll.h(10) : see previous definition of 'Hook'
1>.\dllmain.cpp(19) : error C3861: 'PHook': identifier not found

it give compile errors

[thelamb]

void (*PHook)(void); // 1
PHook = (void*)(0x00131000); // 2
PHook(void) // 3
{
	cout << "Bye";
}

The first error is on line //3, you forget the return type of the PHook function.

But all-together what you are doing isn't going to work out, if you're looking at some easy way to hook have a look at Microsoft's Detours library, it provides an easy way for you to hook your functions.

[laconstantine]

can you explain why it wont work?because i already hooked regular variables this way
im totaly new to it..
and yes i heard about detours libary but is there anyway to do it without it? because i really want to understand how it works

[Ancient Dragon]

>>can you explain why it wont work?

It won't work because the addresses will change every time the program is run. Windows hook functions work because you pass it a pointer to the function, not the actually address value.

This should work

void PHook(void)
{
	cout << "Bye";
}
void (*fn)(void) = PHook;

void Hook()
{ 
	fn();	       
          
}

or this

void PHook(void)
{
	cout << "Bye";
}

void Hook(void (*fn)() )
{ 
	fn();	       
          
}

int main()
{
    Hook( PHook );
}

[laconstantine]

You didnt get my point right I wana change function of a program by injecting a dll into it.

for example we have 2 progs.
1 is .exe which has function DisplayMessage and main funcs call it..
2.a dynamic link libary

now i want that my dll will have a function pointer which points to .exe function DisplayMessage and then change it.

and the final step inject the dll into .exe

is it possible? without detours libary.

[Ancient Dragon]

you might search these links