Intercept function calls made to a DLL by an Application

Question

The scenario is the following:

Lets say I have this application "App" that depends on this Library "library.dll". I would like to know the function calls "App" does while its running. Assume I don't have access to the source code of "App" or "library.dll", but I know the name and arguments of every function that exists is "library.dll". Is there any way I can somehow find out which of the functions from "library.dll" are being called by "App"?

I saw a similar questions in stackoverflow: How to intercept dll method calls?

An answer my Mr. Ates Goral intrigued me, he mention writting a wrapperDLL that forwards function calls to the real DLL. I was hoping someone could provide me with some insight as to how this could be accomplished or point me to a place where were I could get information in the matter.

The two parts I am most interested in is having my application load my .dll and how to actually forward the function to the original "library.dll"

Thank You

Answer 1

The wrapper DLL works perfect - here is how it works:

Let's assume, the library.dll exports int somefunct(int i, void* o) - you now create your own DLL, with something like

#include <windows.h>

//Declare this for every function prototype
typedef int (*int_f_int_pvoid)(int,void*);

//Declare this for every function
int_f_int_pvoid lib_somefunct


//this snipplet goes into dllmain
...
HINSTANCE hlibdll = LoadLibrary("X:\PATH\TO\renamed_library.dll");
//For every function
lib_somefunct=(int_f_int_pvoid)GetProcAddress(hlibdll,"somefunct");
...


//Again for every function    
int somefunct(int i, void* o)
{
    //Log the function call and parameters
    //...

    //Call library.dll
    int result=lib_somefunct(i, o);


    //Log the result 
    //...

    return result;
}

Export your functions, name the resulting DLL library.dll after renaming the original to renamed_library.dll

Now the target EXE will load (your) library.dll, which in turn will load the (original, but renamed) renamed_library.dll - and whenever the target program calls a function, it will run through your loggin code.

Caveat: Your traget EXE might be multithreaded, so be prepared to have a thread-safe logging mechanism.

I have successfully used this method to debug a strange MAPI issue.