TITLE: A non-portable way to detect redirection of stdio 


PROBLEM: gieseke@Informatik.Uni-Marburg.de (GEORG GIESEKE)

How can one detect wether `cout' is redirected or not. In the latter case
some `paging' is reqired, because a long list should be displayed( by my
application). The only solution I know, is to call `sync_with_stdio()' and 
to inspect the `FILE'-structure of `stdout'. But is there a elegant
solution using only streamfunctions? 


RESPONSE: kanze@us-es.sel.de (James Kanze), 24 May 94

There is, in fact, no portable solution.

The following works on my system (Sun sparc, with both Sun CC and
g++): 


    #include    <iostream.h>
    #include    <fstream.h>
    #include    <unistd.h>

    int
    main()
    {
        if ( isatty( ((filebuf*)(cout.rdbuf()))->fd() ) )
            cout << "To tty" << endl ;
        else
            cout << "To file" << endl ;
        return 0 ;
    }

I suspect that it will work on all Unix systems, but I know of no
guarantee that cout in fact uses a filebuf, and not some special type
of streambuf.  (You really need dynamic_cast for this.)