Question

In Unix, if you have a file descriptor (e.g. from a socket, pipe, or inherited from your parent process), you can open a buffered I/O FILE* stream on it with fdopen(3).

Is there an equivalent on Windows for HANDLEs? If you have a HANDLE that was inherited from your parent process (different from stdin, stdout, or stderr) or a pipe from CreatePipe, is it possible to get a buffered FILE* stream from it? MSDN does document _fdopen, but that works with integer file descriptors returned by _open, not generic HANDLEs.

Answer 1

Unfortunately, HANDLEs are completely different beasts from FILE*s and file descriptors. The CRT ultimately handles files in terms of HANDLEs and associates those HANDLEs to a file descriptor. Those file descriptors in turn backs the structure pointer by FILE*.

Fortunately, there is a section on this MSDN page that describes functions that “provide a way to change the representation of the file between a FILE structure, a file descriptor, and a Win32 file handle”:

• _fdopen, _wfdopen: Associates a stream with a file that was
  previously opened for low-level I/O and returns a pointer to the open
  stream.
• _fileno: Gets the file descriptor associated with a stream.
• _get_osfhandle: Return operating-system file handle associated
  with existing C run-time file descriptor
• _open_osfhandle: Associates C run-time file descriptor with an
  existing operating-system file handle.

Looks like what you need is _open_osfhandle followed by _fdopen to obtain a FILE* from a HANDLE.

Here’s an example involving HANDLEs obtained from CreateFile(). When I tested it, it shows the first 255 characters of the file “test.txt” and appends ” — Hello World! — ” at the end of the file:

#include <windows.h>
#include <io.h>
#include <fcntl.h>
#include <cstdio>

int main()
{
    HANDLE h = CreateFile("test.txt", GENERIC_READ | GENERIC_WRITE, 0, 0,
        OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, 0);
    if(h != INVALID_HANDLE_VALUE)
    {
        int fd = _open_osfhandle((intptr_t)h, _O_APPEND | _O_RDONLY);
        if(fd != -1)
        {
            FILE* f = _fdopen(fd, "a+");
            if(f != 0)
            {
                char rbuffer[256];
                memset(rbuffer, 0, 256);
                fread(rbuffer, 1, 255, f);
                printf("read: %s\n", rbuffer);
                fseek(f, 0, SEEK_CUR); // Switch from read to write
                const char* wbuffer = " --- Hello World! --- \n";
                fwrite(wbuffer, 1, strlen(wbuffer), f);
                fclose(f); // Also calls _close()
            }
            else
            {
                _close(fd); // Also calls CloseHandle()
            }
        }
        else
        {
            CloseHandle(h);
        }
    }
}

This should work for pipes as well.