How do I get the file HANDLE from the fopen FILE structure?

Question

The fopen function returns a pointer to a FILE structure, which should be considered an opaque value, without dealing with its content or meaning.

On Windows, the C runtime is a wrapper of the Windows API, and the fopen function relies on the CreateFile function. The CreateFile function return a HANDLE, which is used by other Windows API.

Now, I need to use Windows API deep inside of a library that uses fopen and FILE*. So: is there a way to get the HANDLE from the FILE structure? As this is compiler specific, I mean on the MSVC runtime library.

I understand that this would be an ugly, non-portable hack, and that could broke if Microsoft changes the internal format of FILE... but I'm developing on a closed system (i.e. on a Windows CE embedded system) and refactoring the library would be difficult and time consuming.

Answer 1

On Linux, there's the int fileno(FILE *); function that returns the file descriptor (the one that was returned by the low-level open function) from the FILE*.

I don't know if it applies to Windows and returns the HANDLE though?

Answer 2

Sure it's possible. Try IDA ;)

Answer 3

Didier is right. In Windows, _fileno is the standard way to do this. (A file descriptor is ~= a file HANDLE in Windows.)

Answer 4

Use _fileno followed by _get_osfhandle. Don't forget to _close it when you are done.

EDIT: it's not clear to me that _get_osfhandle is supported on WinCE. However the docs for WinCE _fileno say it returns a "file handle" rather than "descriptor". YMMV but this suggests that you can maybe just use _fileno return value directly as a handle on WinCE.

EDIT: #2 That theory is supported by this person's experience.

"If you take a look at the header files that I posted to the list on Jan 29 you can see how I handled the file creation/handle problem. I didn't have to replace all FILE* items with HANDLEs. See the following snippet from fileio.cpp:

#ifndef q4_WCE

  FlushFileBuffers((HANDLE) _get_osfhandle(_fileno(_file)));
  HANDLE h = ::CreateFileMapping((HANDLE)
_get_osfhandle(_fileno(_file)),
                        0, PAGE_READONLY, 0, len, 0);
#else

  FlushFileBuffers((HANDLE) _fileno(_file));
  HANDLE h = ::CreateFileMapping((HANDLE) _fileno(_file),
                    0, PAGE_READONLY, 0, len, 0);
#endif //q4_WCE

It turns out that _fileno returns a handle. You just have to cast it."