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C++ redirect standard output for printf

Posted on 2003-11-14
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Last Modified: 2008-01-16
Hi
I am building a small command shell in C++ in a Unix environment and I want to be able to redirect standard IO.  I have figured out how to do this for cout statements, but is there a simple way to do it for printf statements as well?  Thank you.
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Question by:k23m
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2 Comments
 
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Accepted Solution

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g0rath earned 50 total points
ID: 9750775

to a file?

FILE *stream;
int temp_fd;

// get a copy of the stdout file decriptor
temp_fd = dup(fileno(stdout));
// redirect stdout to a file
stream = freopen("redirect_file.txt", "w", stdout);

if (stream == NULL)
   exit(1);

printf("We're going to a file now\n");
// copy old stdout back, closing any open file descriptors while we're at it
dup2(temp_fd, fileno(stdout));
printf("We're going to the screen now\n");

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LVL 86

Expert Comment

by:jkr
ID: 9750783
If you want to redirect/read the output of an external program, use 'popen()'. If you want to do that inside the code of a program, use 'freopen()', e.g.

/* FREOPEN.C: This program reassigns stderr to the file
 * named FREOPEN.OUT and writes a line to that file.
 */

#include <stdio.h>
#include <stdlib.h>

FILE *stream;

void main( void )
{
   /* Reassign "stderr" to "freopen.out": */
   stream = freopen( "freopen.out", "w", stderr );

   if( stream == NULL )
      fprintf( stdout, "error on freopen\n" );
   else
   {
      fprintf( stream, "This will go to the file 'freopen.out'\n" );
      fprintf( stdout, "successfully reassigned\n" );
      fclose( stream );
   }
   system( "type freopen.out" );
}


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