.\" 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 June 4, 1993.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.Fn dup "int oldd".Ft int.Fn dup2 "int oldd" "int newd".Sh DESCRIPTION.Fn Dupduplicates an existing object descriptor and returns its value tothe calling process.Fa ( newd=.Fn dup oldd ) .The argument.Fa olddis a small non-negative integer index inthe per-process descriptor table.  The value must be lessthan the size of the table, which is returned by.Xr getdtablesize 2 .The new descriptor returned by the callis the lowest numbered descriptorcurrently not in use by the process..PpThe object referenced by the descriptor does not distinguishbetween.Fa olddand.Fa newdin any way.Thus if.Fa newdand.Fa olddare duplicate references to an openfile,.Xr read 2 ,.Xr write 2and.Xr lseek 2calls all move a single pointer into the file,and append mode, non-blocking I/O and asynchronous I/O optionsare shared between the references.If a separate pointer into the file is desired, a differentobject reference to the file must be obtained by issuing anadditional.Xr open 2call.The close-on-exec flag on the new file descriptor is unset..PpIn .Fn dup2 ,the value of the new descriptor.Fa newdis specified.  If this descriptor is alreadyin use, the descriptor is first deallocated as if a.Xr close 2call had been done first..Sh RETURN VALUESThe value -1 is returned if an error occurs in either call.The external variable.Va errnoindicates the cause of the error..Sh ERRORS.Fn Dupand.Fn dup2fail if:.Bl -tag -width Er.It Bq Er EBADF.Fa Olddor.Fa newdis not a valid active descriptor.It Bq Er EMFILEToo many descriptors are active..El.Sh SEE ALSO.Xr accept 2 ,.Xr open 2 ,.Xr close 2 ,.Xr fcntl 2 ,.Xr pipe 2 ,.Xr socket 2 ,.Xr socketpair 2 ,.Xr getdtablesize 2.Sh STANDARDS.Fn Dupand.Fn dup2are expected to conformto IEEE Std 1003.1-1988.Pq Dq Tn POSIX .