AccessViolationException while calling DLL

[calderon]

I created a simple DLL with the following code:

#include "stdafx.h"
#include "winNT.h"
#include <stdio.h>
#include <windows.h>

UINT A_AddOne (UINT32 Base)
{
return Base + 1;
}

I created a .def file an from VB.net I can call this DLL without problems. Now I created a second DLL that calls the first:

#include "stdafx.h"
#include "winNT.h"
#include <stdio.h>
#include <windows.h>

typedef UINT (*A_ADDONE)(UINT32);

UINT B_AddOne (UINT32 uiBase)
{
HINSTANCE hinstLib;
A_ADDONE ProcAdd;
BOOL fFreeResult = FALSE;
UINT32 param;
int Result;
Result = 0;
param = uiBase;
hinstLib = LoadLibrary(TEXT("C:\Visual Studio Projects\ESD\debug\DLL_A.dll"));
if (hinstLib = NULL) return 101;
ProcAdd = (A_ADDONE) GetProcAddress(hinstLib, TEXT("A_AddOne"));
if (ProcAdd = NULL) return 102;
Result = ProcAdd (param);
fFreeResult = FreeLibrary(hinstLib);
return Result;
}

When I try to call this function an AccessViolationException is thrown as I reach the line Result = ProcAdd (param);

Can anyone explain me why?
Thanks

 

[xwb]

    if (ProcAdd =[COLOR=RED]=[/COLOR] NULL) return 102;

 

[calderon]

Thanks xwb,
This shurely is an important syntax error. Now I don´t get the AccessViolationError anymore.

The problem is that the LoadLirary call fails (resulting in my error code 101). The DLL path is correct and from VB.net I can call DLL_A perfectly using the declare keyword.

Any more hints?

 

[xwb]

It is not a syntax error as such. You can avoid it by switching the check the other way round

if (NULL == ProcAdd) return 102;

If you'd dropped one of the =, it would be syntax error. Doesn't read as well though.

Very strange that your code worked in VB.net Anyway, you could try changing the declaration to

__declspec (dllexport) UINT A_AddOne (UINT32 Base)

 

[calderon]

I tried this, but it doesn't work. I think that DLL_A works fine, since I can use it from VB. To export the routine both DLL´s have a .def file. The dllexport apears to be a substitute for the .def file, but is not necasary if you have a .def file. Anyway it seems that DLL_B can not find DLL_A for some reason, so it will not even be able to search for the routine A_AddOne...

 

[xwb]

Presumably you have compiled it as a C library and not a C++ one. Check that you do not have the /TP flag set in compilation. This will compile it as C++ and will require name mangling. Make sure it is /TC.

 

[jkwilborn]

You may want to follow the previous advise since you also have:

if (hinstLib = NULL) return 101;

When I worked at a large institution I got to write the coding rules and the NULL was ALWAYS on the left (unless an assignment was going on).

So rewrite it properly and it should work!