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+.\" Copyright (c) 2013 Jilles Tjoelker
+.\" 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.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR 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 AUTHOR 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.
+.\"
+.\" $FreeBSD$
+.\"
+.Dd August 16, 2013
+.Dt DUP3 3
+.Os
+.Sh NAME
+.Nm dup3
+.Nd duplicate an existing file descriptor
+.Sh LIBRARY
+.Lb libc
+.Sh SYNOPSIS
+.In fcntl.h
+.In unistd.h
+.Ft int
+.Fn dup3 "int oldd" "int newd" "int flags"
+.Sh DESCRIPTION
+The
+.Fn dup3
+function
+duplicates an existing object descriptor
+while allowing the value of the new descriptor to be specified.
+.Pp
+The close-on-exec flag on the new file descriptor is determined by the
+.Dv O_CLOEXEC
+bit in
+.Fa flags .
+.Pp
+If
+.Fa oldd
+\*(Ne
+.Fa newd
+and
+.Fa flags
+== 0,
+the behavior is identical to
+.Li dup2(oldd, newd) .
+.Pp
+If
+.Fa oldd
+==
+.Fa newd ,
+then
+.Fn dup3
+fails, unlike
+.Xr dup2 2 .
+.Sh RETURN VALUES
+The value -1 is returned if an error occurs.
+The external variable
+.Va errno
+indicates the cause of the error.
+.Sh ERRORS
+The
+.Fn dup3
+function fails if:
+.Bl -tag -width Er
+.It Bq Er EBADF
+The
+.Fa oldd
+argument is not a valid active descriptor or the
+.Fa newd
+argument is negative or exceeds the maximum allowable descriptor number
+.It Bq Er EINVAL
+The
+.Fa oldd
+argument is equal to the
+.Fa newd
+argument.
+.It Bq Er EINVAL
+The
+.Fa flags
+argument has bits set other than
+.Dv O_CLOEXEC .
+.El
+.Sh SEE ALSO
+.Xr accept 2 ,
+.Xr close 2 ,
+.Xr dup2 2 ,
+.Xr fcntl 2 ,
+.Xr getdtablesize 2 ,
+.Xr open 2 ,
+.Xr pipe 2 ,
+.Xr socket 2 ,
+.Xr socketpair 2
+.Sh STANDARDS
+The
+.Fn dup3
+function does not conform to any standard.
+.Sh HISTORY
+The
+.Fn dup3
+function appeared in
+.Fx 10.0 .