Syntax

#include <io.h>
int dup(int handle);

dup associates a second file handle with a currently open file. You can carry out operations on the file using either file handle because all handles associated with a given file use the same file pointer. Creation of a new handle does not affect the type of access allowed for the file.

For example, given:

   handle2 = dup(handle1)

handle2 will have the same file access mode (text or binary) as handle1. In addition, if handle1 was originally opened with the O_APPEND flag (described in open), handle2 will also have that attribute.

Warning: Both handles share a single file pointer. If you reposition a file using handle1, the position in the file returned by handle2 will also change.

If you duplicate a file handle for an open stream, the resulting file handle has the same restrictions as the original file handle.

dup returns the next available file handle for the given file. It returns -1 if an error occurs and sets errno to one of the following values:

Value

Meaning
EBADF
The file handle is not valid.
EMFILE
No more file handles are available.
EOS2ERR
The call to the operating system was not successful.

This example makes a second file handle, fh3, refer to the same file as the file handle fh1 using dup. The file handle fh2 is then associated with the file edopen.da2, and finally fh2 is forced to associate with edopen.da1 dup2.

#include <io.h>#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys\stat.h>

int main(void)
{
   int fh1,fh2,fh3;

   if (-1 == (fh1 = open("edopen.da1", O_CREAT|O_TRUNC|O_RDWR, S_IREAD|S_IWRITE)
      )) {
      perror("Unable to open edopen.da1");
      return EXIT_FAILURE;
   }
   if (-1 == (fh3 = dup(fh1))) {     /* fh3 refers to the sample file as fh1  */
      perror("Unable to dup");
      close(fh1);
      return EXIT_FAILURE;
   }
   else
      printf("Successfully performed dup handle.\n");
   if (-1 == (fh2 = open("edopen.da2", O_CREAT|O_TRUNC|O_RDWR, S_IREAD|S_IWRITE)
      )) {
      perror("Unable to open edopen.da2");
      close(fh1);
      close(fh3);
      return EXIT_FAILURE;
   }
   if (-1 == dup2(fh1, fh2)) {  /* Force fh2 to the refer to the same file     */
                                /* as fh1.                                    */
      perror("Unable to dup2");
   }
   else
      printf("Successfully performed dup2 handle.\n");
   close(fh1);
   close(fh2);
   close(fh3);
   return 0;

   /****************************************************************************
      The output should be:

       Successfully performed dup handle.
       Successfully performed dup2 handle.
   ****************************************************************************/
}

• close
  
• creat
  
• dup2
  
• open
  
• sopen

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