V3/man/man2/dup.2

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.pa 1
.he 'DUP (II)'1/15/73'DUP (II)'
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NAME		dup -- duplicate an open file descriptor
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SYNOPSIS	(file descriptor in r0)
.br
sys dup	/ dup = 41.; not in assembler
.br
(file descriptor in r0)
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DESCRIPTION	Given
a file descriptor returned from
an open or creat call,
dup___
will allocate another file descriptor
synonymous
with
the original.
The new file descriptor is returned in r0.
.sp
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.
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SEE ALSO	creat(II), open(II), close(II)
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DIAGNOSTICS	The
error bit (c-bit)
is set if:
the given file descriptor is invalid;
there are already 10 open files.
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BUGS		--