TITLE: Getting file descriptors from fstream


PROBLEM: morss@ssd.kodak.com (Charlie Morss)

I recently asked a question in regard to record locking using
the fstream class. Steve Clamage replied and suggested that
I derive a class from fstream called lfstream in which I could
then have a lock and unlock member function. Now the question
is how do you get the file descriptor from the fstream class? 


RESPONSE: steve@taumet.com (Steve Clamage)
          Vice Chair, ANSI C++ Committee, X3J16


Check <fstream.h>.  Class filebuf usually has a member function
fd() which returns the file descriptor.

This is all rather specific to Unix-like implementations.  If you
are trying to do file locking on a radically different system,
there may be slightly different functions.  Presumably the filebuf
class provides some way to get at the underlying file in a way
consistent with the file locking routines.

Example:


#include <fstream.h>

main()
{
    fstream f("foo.bar", ios::in|ios::out);

    // use fstream::rdbuf() to get the filebuf
    // then filebuf::fd() to get the file descriptor
    cout << "file descriptor = " << (f.rdbuf())->fd() << endl;

    return 0;
}