Problems in Passing File Handle to DLL

I write a small application and using the following API to open the file:

hHandle = CreateFile(lpszFileName, GENERIC_READ, FILE_SHARE_READ | FILE_SHARE_WRITE,
NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);

Then I pass the hHandle to a DLL exported function, in that function, I will call

DWORD dwFlags = 0;

ASSERT(GetHandleInformation(hFile,&dwFlags) != 0)

to check if the handle is valid. However, the assert fails. And I use GetLastError to get the error code 6, which means invalid handle.

What is the problem with my codes?

Thanks

I'll answer your question by asking you two questions:

1) What is a handle in terms of C++? What if one module thinks a handle is really a struct with two int members, while another module thinks that a handle is really a void pointer? What do you think will happen if those two modules start sharing something they both call a "handle", but the internals of the handle are different?

2) What if the application and DLL have separate "handle" managers? What happens if you create a handle in the app, and then pass this "alien" handle to the DLL that uses a separate handle manager? The DLL's handle manager will assume the handle is invalid, since the DLL's handle manager didn't create the handle.

Regards,

Paul McKenzie

Is the dll used from two different processes? Is the handle created in one process and used in another process without calling DuplicateHandle?

Please see the attached sample that passes file handle to dll within the same process. The code does just fine:

Code:

---

D:\Temp\87>87.exe C:\hiberfil.sys
EXE: last error 32

D:\Temp\87>net helpmsg 32

The process cannot access the file because it is being used by another process.


D:\Temp\87>87.exe 87.exp
EXE: last error 2

D:\Temp\87>net helpmsg 2

The system cannot find the file specified.


D:\Temp\87>87.exe 87dll.exp
EXE: dwFlags = 0
DLL: dwFlags = 0

D:\Temp\87>87.exe 87.exe
EXE: dwFlags = 0
DLL: dwFlags = 0

---

But when I close handle before passing it to DLL...

Code:

---

#include <windows.h>
#include <tchar.h>
#include <stdio.h>

BOOL testHandle(HANDLE hFile);

int _tmain(int argc, LPCTSTR* argv)
{
        if (argc < 2)
        {
                _tprintf(TEXT("No path specified. Quit.\n"));
                return 1;
        }

        BOOL success = FALSE;
        LPCTSTR lpszFileName = argv[1];
        HANDLE hHandle = CreateFile(lpszFileName, GENERIC_READ, FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
        if (INVALID_HANDLE_VALUE != hHandle)
        {
                DWORD dwFlags = 0;
                success = GetHandleInformation(hHandle, &dwFlags);
                if (success)
                {
                        _tprintf(TEXT("EXE: dwFlags = %d\n"), dwFlags);
                        CloseHandle(hHandle);
                        success = testHandle(hHandle);
                }
        }

        if (!success)
        {
                DWORD err = GetLastError();
                _tprintf(TEXT("EXE: last error %d\n"), err);
                hHandle = NULL;
        }

        CloseHandle(hHandle);

        return 0;
}

---

... the handle is reported invalid :)

Code:

---

D:\Temp\87>87.exe 87dll.exp
EXE: dwFlags = 0
DLL: last error 6
EXE: last error 6

---

Good point. I didn't even think to mention not to close the file handle before using it.