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How call win32 func using pointer?

Posted on 2002-05-23
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Last Modified: 2010-04-02
I want to store the address of a win32 function (for example GetLocalTime) in a pointer var and then use the pointer var to call GetLocalTime. While assigning the offset of GetLocalTime to pointer var, I would like to add in an additional value like a random number, say 37, which I'll subtract back off before calling. My C/C++ is pretty weak but in ASM it looks like:

RNDNUM    = 37
mov       [ptrGLT],offset GetLocalTime + RNDNUM
sub       [ptrGLT],RNDNUM
call      [ptrGLT],offset SystemTime
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Question by:mortonsw
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9 Comments
 
LVL 86

Expert Comment

by:jkr
ID: 7030532
It is pretty straight foward.

First you need a 'typed function pointer', so the compiler knows about the function signature:

typedef VOID ( *FPTR_GetLocalTime)( LPSYSTEMTIME);

// Load dll

HINSTANCE hDll = LoadLibrary ( "kernel32.dll");

if ( !hDll) {

 // error, though _very_ unlikely
}

// get the entry point
FPTR_GetLocalTime pGetLocalTime = ( FPTR_GetLocalTime) GetProcAddress ( hDll, "GetLocalTime");

if ( !pGetLocalTime) {

 // error, though _very_ unlikely
}

// make the call

SYSTEMTIME st;

pGetLocalTime ( &st);
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LVL 86

Expert Comment

by:jkr
ID: 7030533
Ooops, and, of course, don't forget to 'FreeLibrary()' when you are done...
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Author Comment

by:mortonsw
ID: 7030546
thanks jkr, i edited the question though and you probably read the orig one.
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LVL 86

Expert Comment

by:jkr
ID: 7030559
Yes, I was wondering already :o)

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Expert Comment

by:fl0yd
ID: 7030576
Just out of curiostiy: what exactly are you trying to do? You got me a bit confused with this adding/subtracting of a random offset. Some sort of copy protection, maybe?
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Author Comment

by:mortonsw
ID: 7030887
-hiding the func call from a disassemble.
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Accepted Solution

by:
ambience earned 400 total points
ID: 7031165
typedef void  (__stdcall *FPTR)( LPSYSTEMTIME);


int main()
{
     SYSTEMTIME ti;
     
     FPTR ptr = GetLocalTime;
     ptr = (FPTR)((DWORD)ptr + (DWORD)37);
     ((FPTR) (((DWORD)ptr) - (DWORD)37 ))(&ti);

     cout << ti.wYear << endl;

     return 0;
}

Hope this helps
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Expert Comment

by:ambience
ID: 7031168
BTW can you really stop a hacker that way ?????? Is this enough !!
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Author Comment

by:mortonsw
ID: 7031287
Hardly; Just one trick in a bag.
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