Re: switching stdout to a file

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On Fri, 06 May 2005 04:03:51 GMT, Norm Dresner wrote:

> /* The MSDN Library 10/2001 gives this example for the use of dup()/dup2()
> to make "stdout" refer to a program-determined file and then restores it
> NOTE: All error checking deleted for "clarity" Also, the type and
> arguments
> of main are not proper either (that's the way it was there) */
>
> #include <io.h>
> #include <stdlib.h>
> #include <stdio.h>
>
> void main( void )
> {
> int old;
> FILE *new;
>
> old = _dup( 1 ); /* "old" now refers to "stdout" */
> /* Note: file handle 1 == "stdout" */
>
> new = fopen( "data", "w" );
>
> /* stdout now refers to file "data" */
> _dup2( _fileno( new ), 1 );
>
> puts( "This goes to file 'data'\r\n" );
>
> fclose( new );
>
> /* Restore original stdout */
> _dup2( old, 1 );
> }
>
>
>
>
> /* BUT I'd like to know what's wrong with using this method instead */
>
> void main(void)
> {
> FILE *savestdout = stdout;
> stdout = fopen( "data" , "w" );
> ...
> fclose( stdout );
> stdout = savestdout;
> }
>
>
>
> TIA
> Norm

The second method isn't guaranteed to work. First, stdout is only required
to be an *expression* of type FILE*. It needn't be an actual pointer
lvalue. In fact, VC's <stdio.h> defines it like this:

#define stdout (&_iob[1])

That's an rvalue, and you can't assign to it. Second, and this is getting
even more into implementation details, stdout is hardcoded to file
descriptor 1. So code that uses _write(1) wouldn't notice your change to
stdout, nor would code that directly uses &_iob[1], such as one weird
function in <wchar.h>. Your next question should be, "What about *stdout =
*fp?" That's actually suggested in a KB article or two IIRC, but I'm trying
to forget it. As FILE is an "object type", the assignment should work, but
the C Standard says, "The address of the FILE object used to control a
stream may be significant; a copy of a FILE object may not necessarily
serve in place of the original." So I'd regard it as very non-portable at
best. To conclude, use the venerable dup/dup2 method, which is non-standard
but very portable. There's also the standard freopen, but that function
irreversibly closes the stream you're trying to redirect.

--
Doug Harrison
Microsoft MVP - Visual C++
.

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