Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!uunet!taumet!steve
From: steve@taumet.com (Stephen Clamage)
Newsgroups: comp.lang.c++
Subject: Re: How do I convert from ostream to FILE?
Keywords: ostream iostream FILE
Message-ID: <732@taumet.com>
Date: 16 May 91 18:05:15 GMT
References: <1991May16.011218.3161@agate.berkeley.edu>
Organization: Taumetric Corporation, San Diego
Lines: 45

holub@violet.berkeley.edu (Allen Holub) writes:

>Is there an official way to convert from an ostream either to a FILE pointer
>or file descriptor? I need to use a few C routines that take file-descriptor
>arguments in a stream-based C++ application compiled under Borland C++ 2.0.

You don't say whether you are using AT&T version 1.2-style (old) streams
or version 2.x-style (new) iostreams.

Old-style streams were built on C Standard I/O, and you can just pick up
the FILE pointer.

New-style iostreams do not use C Standard I/O at all (except for
stdiostream, which is recommended against).  So there is no FILE
pointer available.

Beyond that, an ostream is not necessarily file-based.  It uses a
streambuf-derived class for its output, and this streambuf-derived
class might be a filebuf (using a file), a strstreambuf (using a
char array), or some user-defined class.  So there is no guarantee
that an ostream will have a file associated with it.

In the event that an ostream-derived class uses a filebuf (as do
cout and cerr), you can use the public member function ios::rdbuf()
to get the associated streambuf, then invoke public member function
fd() to get the file descriptor.  Of course, fd() is not a member
of streambuf, but of filebuf, so you need a cast, which is not
safe unless you know you are dealing with a filebuf-derived
buffer class.  Example:

    #include <fstream.h>
    main()
    {
	cout << "fd for cout is "
	     << ((filebuf*)cout.rdbuf())->fd() // UNSAFE CAST
	     << endl;
	return 0;
    }

This makes sense on systems which have the Unix-like concept of small
integers representing a file as known to the operating system.  (DOS is
such a system.)
-- 

Steve Clamage, TauMetric Corp, steve@taumet.com