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_dup, _dup2

Create a second file descriptor for an open file (_dup), or reassign a file descriptor (_dup2).

int _dup( 
   int fd 
int _dup2( 
   int fd1,
   int fd2 


fd, fd1
File descriptors referring to open file.
Any file descriptor.

Return Value

_dup returns a new file descriptor. _dup2 returns 0 to indicate success. If an error occurs, each function returns –1 and sets errno to EBADF if the file descriptor is invalid, or to EMFILE if no more file descriptors are available.

See _doserrno, errno, _sys_errlist, and _sys_nerr for more information on these, and other, return codes.


The _dup and _dup2 functions associate a second file descriptor with a currently open file. These functions can be used to associate a predefined file descriptor, such as that for stdout, with a different file. Operations on the file can be carried out using either file descriptor. The type of access allowed for the file is unaffected by the creation of a new descriptor. _dup returns the next available file descriptor for the given file._dup2 forces fd2 to refer to the same file as fd1. If fd2 is associated with an open file at the time of the call, that file is closed.

Both _dup and _dup2 accept file descriptors as parameters. To pass a stream (FILE *) to either of these functions, use _fileno. The fileno routine returns the file descriptor currently associated with the given stream. The following example shows how to associate stderr (defined as FILE * in STDIO.H) with a file descriptor:

cstderr = _dup( _fileno( stderr ));


Routine Required header Compatibility
_dup <io.h> Win 98, Win Me, Win NT, Win 2000, Win XP
_dup2 <io.h> Win 98, Win Me, Win NT, Win 2000, Win XP

For additional compatibility information, see Compatibility in the Introduction.


All versions of the C run-time libraries.


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

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

int main( void )
   int old;
   FILE *DataFile;

   old = _dup( 1 );   /* "old" now refers to "stdout" */
                      /* Note:  file descriptor 1 == "stdout" */
   if( old == -1 )
      perror( "_dup( 1 ) failure" );
      exit( 1 );
   write( old, "This goes to stdout first\n", 26 );
   if( ( DataFile = fopen( "data", "w" ) ) == NULL )
      puts( "Can't open file 'data'\n" );
      exit( 1 );

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

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

   /* 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 stdout

The file 'data' contains:
This goes to file 'data'

See Also

Low-level I/O Routines | _close | _creat | _open | Run-Time Routines and .NET Framework Equivalents