man/man2/dup.2

.pa 1
.he 'DUP (II)'1/15/73'DUP (II)'
.ti 0
NAME            dup -- duplicate an open file descriptor
.sp
.ti 0
SYNOPSIS        (file descriptor in r0)
.br
sys dup / dup = 41.; not in assembler
.br
(file descriptor in r0)
.sp
.ti 0
DESCRIPTION     Given
a file descriptor returned from
an open or creat call,
dup\b\b\b___
will allocate another file descriptor
synonymous
with
the original.
The new file descriptor is returned in r0.
.sp
Dup\b\b\b___
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\b\b\b___
and close\b\b\b\b\b_____ can be
used to
manipulate file descriptors
in a general way.
This is handy for
manipulating standard input and/or
standard output.
.sp
.ti 0
SEE ALSO        creat(II), open(II), close(II)
.sp
.ti 0
DIAGNOSTICS     The
error bit (c-bit)
is set if:
the given file descriptor is invalid;
there are already 10 open files.
.sp
.ti 0
BUGS            --