.\"	$NetBSD: dup.2,v 1.4 1995/02/27 12:32:21 cgd Exp $
.\"
.\" Copyright (c) 1980, 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\"    must display the following acknowledgement:
.\"	This product includes software developed by the University of
.\"	California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"     @(#)dup.2	8.1 (Berkeley) 6/4/93
.\"
.Dd December 1, 2010
.Dt DUP 2
.Os BSD 4
.Sh NAME
.Nm dup ,
.Nm dup2
.Nd duplicate an existing file descriptor
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft int
.Fo dup
.Fa "int fildes"
.Fc
.Ft int
.Fo dup2
.Fa "int fildes"
.Fa "int fildes2"
.Fc
.Sh DESCRIPTION
.Fn Dup
duplicates an existing object descriptor
and returns its value to the calling process
.Fa ( fildes2
=
.Fn dup fildes ) .
The argument
.Fa fildes
is a small non-negative integer index
in the per-process descriptor table.
The value must be less than the size of the table,
which is returned by
.Xr getdtablesize 2 .
The new descriptor returned by the call
is the lowest numbered descriptor
currently not in use by the process.
.Pp
The object referenced by the descriptor does not distinguish
between
.Fa fildes
and
.Fa fildes2
in any way.
Thus if
.Fa fildes2
and
.Fa fildes
are duplicate references to an open
file,
.Xr read 2 ,
.Xr write 2
and
.Xr lseek 2
calls all move a single pointer into the file,
and append mode, non-blocking I/O and asynchronous I/O options
are shared between the references.
If a separate pointer into the file is desired,
a different object reference to the file must be obtained
by issuing an additional
.Xr open 2
call.
The close-on-exec flag on the new file descriptor is unset.
.Pp
In 
.Fn dup2 ,
the value of the new descriptor
.Fa fildes2
is specified.
If
.Fa fildes
and
.Fa fildes2
are equal, then
.Fn dup2 
just returns
.Fa fildes2 ;
no other changes are made to the existing descriptor.
Otherwise, if descriptor
.Fa fildes2
is already in use, it is first deallocated as if a
.Xr close 2
call had been done first.
.Sh RETURN VALUES
Upon successful completion, the new file descriptor is returned.
Otherwise, a value of -1 is returned and the global integer variable
.Va errno
is set to indicate the error.
.Sh ERRORS
The
.Fn dup
and
.Fn dup2
system calls will fail if:
.Bl -tag -width Er
.\" ==========
.It Bq Er EBADF
.Fa fildes
is not an active, valid file descriptor.
.\" ==========
.It Bq Er EINTR
Execution is interrupted by a signal.
.\" ==========
.It Bq Er EMFILE
Too many file descriptors are active.
.El
.Pp
The
.Fn dup2
system call will fail if:
.Bl -tag -width Er
.\" ==========
.It Bq Er EBADF
.Fa fildes2
is negative or greater than the maximum allowable number (see getdtablesize(2)).
.El
.Sh SEE ALSO
.Xr accept 2 ,
.Xr close 2 ,
.Xr fcntl 2 ,
.Xr getdtablesize 2 ,
.Xr open 2 ,
.Xr pipe 2 ,
.Xr socket 2 ,
.Xr socketpair 2
.Sh STANDARDS
.Fn Dup
and
.Fn dup2
are expected to conform to 
.St -p1003.1-88 .