v7 man page for dup

DUP(2)								System Calls Manual							    DUP(2)

NAME
       dup, dup2 - duplicate an open file descriptor

SYNOPSIS
       dup(fildes)
       int fildes;

       dup2(fildes, fildes2)
       int fildes, fildes2;

DESCRIPTION
       Given  a  file  descriptor  returned from an open, pipe, or creat call, dup allocates another file descriptor synonymous with the original.
       The new file descriptor is returned.

       In the second form of the call, fildes is a file descriptor referring to an open file, and fildes2 is a non-negative integer less than  the
       maximum	value  allowed	for  file  descriptors	(approximately	19).  Dup2 causes fildes2 to refer to the same file as fildes.	If fildes2
       already referred to an open file, it is closed first.

SEE ALSO
       creat(2), open(2), close(2), pipe(2)

DIAGNOSTICS
       The value -1 is returned if: the given file descriptor is invalid; there are already too many open files.

ASSEMBLER
       (dup = 41.)
       (file descriptor in r0)
       (new file descriptor in r1)
       sys dup
       (file descriptor in r0)

       The dup2 entry is implemented by adding 0100 to fildes.

																	    DUP(2)