PRB: Problems Using CFile::m_hFile When Porting MFC Apps

WINDOWS            | WINDOWS NT

The information in this article applies to:

The Microsoft Foundation Classes (MFC), included with:

• Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51, 1.52
• Microsoft Visual C++, 32-bit Edition, versions 1.0, 2.0, 2.1, 2.2, and

SYMPTOMS

When you port MFC applications that use CFile from Windows version 3.1 to Windows NT or Windows 95, you may encounter problems related to use of the m_hFile member variable.

CAUSE

In all versions of MFC, CFile:: m_hFile is defined as a UINT. In the 16-bit versions of MFC (versions 2.0 and 2.5x), m_hFile is a HANDLE that is created from a call to the C Runtime Library (CRT) function _dos_open(). In this case, other CRT functions that use the handle, such as _eof(), work correctly.

In the 32-bit versions of MFC (versions 2.1 and 3.x), m_hFile is a handle that is created from a call to the Win32 function CreateFile(), which is incompatible with the C Runtime functions. In this case, you need to use the Win32 API function set.

RESOLUTION

If your MFC application must use functions outside of the CFile class to obtain the functionality you need, note these differences and use compatible functions.

To obtain a C run-time file handle and set it to point to the operating system file handle m_hFile, you can use the _open_osfhandle() function. The handle returned by that function is compatible with _eof() and other C Runtime Library functions.

REFERENCES

Please see your online documentation for more information on compatible functions.