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  1. #1
    Join Date
    Dec 2002

    Question How to give back hand to screen in C ?

    Hello everybody,

    I have a little problem..

    A program I'm modifying has the following behavior :

    If an option is given, it will write information messages to the screen, but if an other option is given, it will write information messages to a File named LogFile.txt ...

    Now, what's done in the code is this :

    freopen("LogFile.txt", "a+", stdout);
    To close stdout and assign LogFile.txt to the standard output.

    After everything is done, I need to give back the hand to the screen to write a message about the success of the program, based on some text in an other file.

    Everything I saw about the freopen function didn't talk about giving back the hand to the screen..

    I use
    freopen("NUL", "a+", stdout);
    to close the stream stdout assigned to LogFile ( it is shared by a lot of programs called from the main one) , but I can't write anything on the screen from then on.

    Does anybody know how I can write back to the screen without leaving the program ?

    Last edited by sylvain_nurun; December 3rd, 2002 at 01:02 PM.

  2. #2
    Join Date
    Oct 2003

    How to give back hand to screen - Answer

    Hi I have been looking for this one for a while, the doc for freopen doesn't mention any of it however it can be found under _dup2 documentation.

    Anyway here's the code from it:

    /* DUP.C: This program uses the variable old to save
    * the original stdout. It then opens a new file named
    * new and forces stdout to refer to it. Finally, it
    * restores stdout to its original state.

    #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" */
    if( old == -1 )
    perror( "_dup( 1 ) failure" );
    exit( 1 );
    write( old, "This goes to stdout first\r\n", 27 );
    if( ( new = fopen( "data", "w" ) ) == NULL )
    puts( "Can't open file 'data'\n" );
    exit( 1 );

    /* stdout now refers to file "data" */
    if( -1 == _dup2( _fileno( new ), 1 ) )
    perror( "Can't _dup2 stdout" );
    exit( 1 );
    puts( "This goes to file 'data'\r\n" );

    /* Flush stdout stream buffer so it goes to correct file */
    fflush( stdout );
    fclose( new );

    /* Restore original stdout */
    _dup2( old, 1 );
    puts( "This goes to stdout\n" );
    puts( "The file 'data' contains:" );
    system( "type data" );


    This goes to stdout first
    This goes to file 'data'

    This goes to stdout

    The file 'data' contains:

    This goes to file 'data'

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