2.9BSD/usr/man/man2/dup.2

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.TH DUP 2 
.UC
.SH NAME
dup, dup2 \- duplicate an open file descriptor
.SH SYNOPSIS
.B dup(fildes)
.br
.B int fildes;
.PP
.B dup2(fildes, fildes2)
.br
.B int fildes, fildes2;
.SH DESCRIPTION
Given
a file descriptor returned from
an
.I open,
.I pipe,
or
.IR creat (2)
call,
.I dup
allocates another file descriptor
synonymous
with
the original.
The new file descriptor is returned.
.PP
In the second form of the call,
.I fildes
is a file descriptor referring to an open file, and
.I fildes2
is a non-negative integer less than the maximum value
allowed for file descriptors (approximately 19).
.I Dup2
causes
.I fildes2
to refer to the same file as
.I fildes.
If
.I fildes2
already referred to an open file, it is closed first.
.SH RETURN VALUE
Upon successful completion, a value of 0 is returned.  Otherwise, a
value of \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.IR Dup " and " dup2
will fail if:
.TP 20
[EBADF]
.I Fildes " or " fildes2
is not a valid active descriptor.
.TP 20
[EMFILE]
Too many descriptors are active.
.SH "SEE ALSO"
close(2), creat(2), open(2), pipe(2)
.SH ASSEMBLER
(dup = 41.)
.br
(file descriptor in r0)
.br
(new file descriptor in r1)
.br
.B sys dup
.br
(file descriptor in r0)
.PP
The
.I dup2
entry is implemented by adding 0100 to
.I fildes.