1.\" Copyright (c) 1980, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 3. Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)dup.2 8.1 (Berkeley) 6/4/93 29.\" $FreeBSD: src/lib/libc/sys/dup.2,v 1.9.2.6 2001/12/14 18:34:00 ru Exp $ 30.\" $DragonFly: src/lib/libc/sys/dup.2,v 1.2 2003/06/17 04:26:47 dillon Exp $ 31.\" 32.Dd June 4, 1993 33.Dt DUP 2 34.Os 35.Sh NAME 36.Nm dup , 37.Nm dup2 38.Nd duplicate an existing file descriptor 39.Sh LIBRARY 40.Lb libc 41.Sh SYNOPSIS 42.In unistd.h 43.Ft int 44.Fn dup "int oldd" 45.Ft int 46.Fn dup2 "int oldd" "int newd" 47.Sh DESCRIPTION 48.Fn Dup 49duplicates an existing object descriptor and returns its value to 50the calling process 51.Fa ( newd 52= 53.Fn dup oldd ) . 54The argument 55.Fa oldd 56is a small non-negative integer index in 57the per-process descriptor table. The value must be less 58than the size of the table, which is returned by 59.Xr getdtablesize 2 . 60The new descriptor returned by the call 61is the lowest numbered descriptor 62currently not in use by the process. 63.Pp 64The object referenced by the descriptor does not distinguish 65between 66.Fa oldd 67and 68.Fa newd 69in any way. 70Thus if 71.Fa newd 72and 73.Fa oldd 74are duplicate references to an open 75file, 76.Xr read 2 , 77.Xr write 2 78and 79.Xr lseek 2 80calls all move a single pointer into the file, 81and append mode, non-blocking I/O and asynchronous I/O options 82are shared between the references. 83If a separate pointer into the file is desired, a different 84object reference to the file must be obtained by issuing an 85additional 86.Xr open 2 87call. 88The close-on-exec flag on the new file descriptor is unset. 89.Pp 90In 91.Fn dup2 , 92the value of the new descriptor 93.Fa newd 94is specified. If this descriptor is already in use and 95.Fa oldd 96\*(Ne 97.Fa newd , 98the descriptor is first deallocated as if a 99.Xr close 2 100call had been used. 101If 102.Fa oldd 103is not a valid descriptor, then 104.Fa newd 105is not closed. 106If 107.Fa oldd 108== 109.Fa newd 110and 111.Fa oldd 112is a valid descriptor, then 113.Fn dup2 114is successful, and does nothing. 115.Sh RETURN VALUES 116The value -1 is returned if an error occurs in either call. 117The external variable 118.Va errno 119indicates the cause of the error. 120.Sh ERRORS 121.Fn Dup 122and 123.Fn dup2 124fail if: 125.Bl -tag -width Er 126.It Bq Er EBADF 127.Fa Oldd 128or 129.Fa newd 130is not a valid active descriptor 131.It Bq Er EMFILE 132Too many descriptors are active. 133.El 134.Sh SEE ALSO 135.Xr accept 2 , 136.Xr close 2 , 137.Xr fcntl 2 , 138.Xr getdtablesize 2 , 139.Xr open 2 , 140.Xr pipe 2 , 141.Xr socket 2 , 142.Xr socketpair 2 143.Sh STANDARDS 144The 145.Fn dup 146and 147.Fn dup2 148function calls are expected to conform to 149.St -p1003.1-90 . 150.Sh HISTORY 151A 152.Fn dup 153and a 154.Fn dup2 155function call appeared in 156.At v7 . 157