How to Load External DLL from Dissector

Question

I have a DLL file named, Test2.dll which loads fine in anything but wireshark, but when I try to open it within my wireshark dissector it fails to load, this is how I am loading it:

#define LIBRARYLOCATION "C:\\Docum...\\Test2.dll"

...

static void dissect_foo(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree){
HINSTANCE DLLInstance = LoadLibrary((LPCWSTR)LIBRARYLOCATION);

if(DLLInstance == NULL){
proto_tree_add_text(tree, NULL, 0, 0, "Cannot Load DLL: %*s", 0, LIBRARYLOCATION);}
else{
proto_tree_add_text(tree, NULL, 0, 0, "Loaded DLL: %*s", 0, LIBRARYLOCATION);}

The .h file for the DLL is thus:

#ifndef TEST2_H
#define TEST2_H

#ifdef __cplusplus

extern "C"
{
#endif
__declspec(dllexport) void Foo ();
#ifdef __cplusplus
}
#endif
#endif

Any ideas as to why this wouldn't work? The DLL is compiled in VC++ 2005 (No option there) and the dissector is compiled with VC++ 2010 EE.

Thank you for your time, Brandon

Answer 1

1

Bug: You're passing a single-char string as a wide-char string to LoadLibrary, but you actually need to pass in a TCHAR string (which is set at compile-time to be either wide-char or single-char).

Fix: Use _T for the literal #define, and remove the LPCWSTR cast (presumably done to mute the compiler warning you should've heeded).

from:

#define LIBRARYLOCATION "C:\\Docum...\\Test2.dll"
/* ... */
HINSTANCE DLLInstance = LoadLibrary((LPCWSTR)LIBRARYLOCATION);

to:

//#include <tchar.h>
#define LIBRARYLOCATION _T("C:\\Docum...\\Test2.dll")
/* ... */
HINSTANCE DLLInstance = LoadLibrary(LIBRARYLOCATION);

link

answered 25 Jul '11, 17:55

helloworld
2.6k●2●17●39
accept rate: 27%

That did the trick! I also really liked how you included the example!

Thanks! Brandon

(26 Jul '11, 07:06) officialhopsof

Good spot, I missed the cast.

More portably you could define the string literal as a wide character e.g. L"\path\to\your\dll".

No need to use Windows generic types then.

(26 Jul '11, 10:00) grahamb ♦

Using L would cause the same problem if UNICODE was not defined (in which case LoadLibrary expects a single-char string instead). If portability were a goal, I would use GLib's dynamic module loading (designed to be cross-platform).

(26 Jul '11, 11:24) bstn

How to Load External DLL from Dissector

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