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From: sking@nowhere.uucp (Steven King)
Newsgroups: comp.lang.c++
Subject: Re: How do I convert from ostream to FILE?
Keywords: ostream iostream FILE
Message-ID: <1991May17.041817.1618@nowhere.uucp>
Date: 17 May 91 04:18:17 GMT
References: <1991May16.011218.3161@agate.berkeley.edu> <732@taumet.com>
Organization: American Anarchist Union
Lines: 44

In article <732@taumet.com> steve@taumet.com (Stephen Clamage) writes:
>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.
>
>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;
>    }
>

   A few minor quibbles...

   The library distributed with cfront 2.0 has cin & cout instances
 of istream_withassign and ostream_withassign, repectively, and neither
 is defined as derived from filebuf or have a filebuf as member. ( I
 presume that the actual initialization of cin, etc. uses a filebuf ) 

   Objects derived from fstreambase ( ie. ofstream and ifstream ) have
 the member function rdbuf() which returns a pointer to a filebuf,
 rendering the cast unnecessary, ie:

    ifstream ins ;
    int fd = ins.rdbuf() -> fd() ;


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