DUP (II)                     1/15/73                     DUP (II)

NAME            dup -- duplicate an open file descriptor

SYNOPSIS        (file descriptor in r0)

                sys dup / dup = 41.; not in assembler

                (file descriptor in r0)

DESCRIPTION     Given a file descriptor returned from an open or

                creat call, dup will allocate another file de-

                scriptor synonymous with the original.  The new

                file descriptor is returned in r0.

                Dup is used more to manipulate the value of file

                descriptors than to genuinely duplicate a file

                descriptor.  Since the algorithm to allocate file

                descriptors is known to use the lowest available

                value between 0 and 9, combinations of dup and

                close can be used to manipulate file descriptors

                in a general way.  This is handy for manipulating

                standard input and/or standard output.

SEE ALSO        creat(II), open(II), close(II)

DIAGNOSTICS     The error bit (c-bit) is set if: the given file

                descriptor is invalid; there are already 10 open


BUGS            --