Win32 C++ DLL function gets garbage values in parameters - c++

Question

I have a Win32 C++ dll (A) that calls another Win32 C++ dll (B). (B) is loaded using LoadLibrary and contains a method:
Draw(HDC hDC, LPRECT lpRect, LPBUFFER buffer, LPOPTIONS options)
Buffer structure is defined as:
struct Buffer
{
char* pData;
long Length;
TCHAR FileName[MAX_PATH];
Extension Extension;
};
typedef Buffer BUFFER, *LPBUFFER;
(A) fills BUFFER with filename, length etc and calls the Draw function. The Draw function then uses the values from BUFFER. It all works fine when DLLs are compiled as 64-bit but if I compile them as 32-bit then I start getting garbage values in BUFFER fields in (B). Logs shows that the values are good in (A) but turn into garbage when they reach (B).
I tried changing the Structure Alignment Option /ZpX and calling convention for Draw method (__cdecl, __stdcall) but none helped. I think it is related to calling convention because if I change Draw function syntax and put BUFFER as first param then (B) gets correct values. What's going on here?
Function pointer type:
typedef bool (__cdecl *DrawFunc)(HDC hDC, LPRECT lpRect, LPBUFFER buffer, LPOPTIONS options);
Then in InitInstance:
pDrawFunc = (DrawFunc)GetProcAddress(dllHandle, "Draw");
UPDATE
1. As mentioned above, if I put BUFFER as first param then it receives correct values.
2. HDC being a single numeric value always receives correct value
3. RECT gets incorrect values, very large ones
I believe the problem has something to do with structs. Only structs get incorrect values.
UPDATE 2
OK I found out my own silly mistake, the declaration for Draw method had LPRECT whereas the implementation had RECT. My bad, sorry about that.
But I am still not sure why:
1. Other parameters were showing garbage values?
2. Why it worked in 64-bit?

Answer 1

Ok, I create a solution with 3 projects: library B, that contains Draw(), library A, that has Test(), that loads library B and call Draw() with some Buffer* and application test, that links with library A and calls Test(). Everything works fine, both for 32 bit and 64. Small snippet of Test():
#include "stdafx.h"
#include "A.h"
#include "../B/B.h"
namespace {
LPBUFFER CreateBuffer(const char* const data, LPCTSTR const name)
{
if(!data || !name)
return NULL;
LPBUFFER buffer = new BUFFER();
buffer->Length = static_cast<long>(strlen(data) + 1);
buffer->pData = new char[buffer->Length];
strcpy_s(buffer->pData, buffer->Length * sizeof(char), data);
buffer->Extension = 0;
::ZeroMemory(buffer->FileName, _countof(buffer->FileName) * sizeof(TCHAR));
_tcscpy_s(buffer->FileName, name);
return buffer;
}
void DestroyBuffer(LPBUFFER buffer)
{
delete [] buffer->pData;
buffer->Length = 0;
buffer->pData = NULL;
buffer->Extension = 0;
::ZeroMemory(buffer->FileName, _countof(buffer->FileName) * sizeof(TCHAR));
delete buffer;
}
} // namespace
A_API void Test()
{
HMODULE b_lib = ::LoadLibrary(_T("B.dll"));
if(!b_lib)
{
::OutputDebugString(_T("Can't load library\n"));
return;
}
typedef bool (*DrawFunction)(HDC hDC, LPRECT lpRect, LPBUFFER buffer, LPOPTIONS options);
DrawFunction draw = reinterpret_cast<DrawFunction>(::GetProcAddress(b_lib, "Draw"));
if(!draw)
{
::OutputDebugString(_T("Can't get address of Draw()"));
goto FINISH_LABEL;
}
LPBUFFER buffer = CreateBuffer("test", _T("path"));
draw(NULL, NULL, buffer, NULL);
DestroyBuffer(buffer);
FINISH_LABEL:
::FreeLibrary(b_lib);
b_lib = NULL;
}
And a whole solution: https://www.dropbox.com/s/5ei6ros9e8s94e2/B.zip