#include <unistd.h>

int dup(int oldd)
int dup2(int oldd, int newd)
 DESCRIPTION
 Dup duplicates an existing descriptor.
The argument oldd is a small non-negative integer index in
the per-process descriptor table. The value must be less
than OPEN_MAX, the size of the table.
The new descriptor returned by the call, let's name it
 newd, is the lowest numbered descriptor that is
not currently in use by the process.

The object referenced by the descriptor does not distinguish
between references using oldd and newd in any way.
Thus if newd and oldd are duplicate references to an open
file,
 read (2),  write (2) and
 lseek (2) calls all move a single pointer into the file,
and append mode, non-blocking I/O and asynchronous I/O options
are shared between the references.
If a separate pointer into the file is desired, a different
object reference to the file must be obtained by issuing an
additional
 open (2) call.
The close-on-exec flag on the new file descriptor is unset.

In the second form of the call, the value of
 newd desired is specified. If this descriptor is already
in use, the descriptor is first deallocated as if a
 close (2) call had been done first.
 Newd is not closed if it equals
 oldd . 
 "RETURN VALUE
The value -1 is returned if an error occurs in either call.
The external variable
 errno indicates the cause of the error.
 "ERRORS
 Dup and
 dup2 fail if:
 15
[EBADF]
Oldd or
newd is not a valid active descriptor
 15
[EMFILE]
Too many descriptors are active.
 NOTES
 Dup and
 dup2 are now implemented using the
 F_DUPFD function of
 fcntl (2), although the old system call interfaces still exist to support old programs.
 "SEE ALSO"
 open (2),  close (2),  fcntl (2),  pipe (2).