ultrix man page for dup2

dup(2)								System Calls Manual							    dup(2)

Name
       dup, dup2 - duplicate an open file descriptor

Syntax
       newd = dup(oldd)
       int newd, oldd;

       dup2(oldd, newd)
       int oldd, newd;

Description
       The  system  call  duplicates  an  existing  object descriptor.	The argument oldd is a small non-negative integer index in the per-process
       descriptor table.  The value must be less than the size of the table, which is returned by The new descriptor, newd, returned by  the  call
       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, and calls all move a single pointer into the file.  If a separate pointer into the file is desired, a
       different object reference to the file must be obtained by issuing an additional call.

       In the second form of the call, specify the value of newd needed. If this descriptor is already in use, the descriptor is first deallocated
       as if a call had been done.

Return Values
       The value -1 is returned if an error occurs in either call.  The external variable errno indicates the cause of the error.

Diagnostics
       The and system calls fail under the following conditions:

       [EBADF]	      The oldd or newd is not a valid active descriptor.

       [EMFILE]       Too many descriptors are active.

       [EINTR]	      The or function was terminated prematurely by a signal.

See Also
       accept(2), close(2), getdtablesize(2), lseek(2), open(2), pipe(2), read(2), socket(2), socketpair(2), write(2)

																	    dup(2)