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.\" 31.Dd May 6, 2019 32.Dt DUP 2 33.Os 34.Sh NAME 35.Nm dup , 36.Nm dup2 37.Nd duplicate an existing file descriptor 38.Sh LIBRARY 39.Lb libc 40.Sh SYNOPSIS 41.In unistd.h 42.Ft int 43.Fn dup "int oldd" 44.Ft int 45.Fn dup2 "int oldd" "int newd" 46.Sh DESCRIPTION 47.Fn Dup 48duplicates an existing object descriptor and returns its value to 49the calling process 50.Fa ( newd 51= 52.Fn dup oldd ) . 53The argument 54.Fa oldd 55is a small non-negative integer index in 56the per-process descriptor table. The value must be less 57than the size of the table, which is returned by 58.Xr getdtablesize 2 . 59The new descriptor returned by the call 60is the lowest numbered descriptor 61currently not in use by the process. 62.Pp 63The object referenced by the descriptor does not distinguish 64between 65.Fa oldd 66and 67.Fa newd 68in any way. 69Thus if 70.Fa newd 71and 72.Fa oldd 73are duplicate references to an open 74file, 75.Xr read 2 , 76.Xr write 2 77and 78.Xr lseek 2 79calls all move a single pointer into the file, 80and append mode, non-blocking I/O and asynchronous I/O options 81are shared between the references. 82If a separate pointer into the file is desired, a different 83object reference to the file must be obtained by issuing an 84additional 85.Xr open 2 86call. 87The close-on-exec flag on the new file descriptor is unset. 88.Pp 89In 90.Fn dup2 , 91the value of the new descriptor 92.Fa newd 93is specified. If this descriptor is already in use and 94.Fa oldd 95\*(Ne 96.Fa newd , 97the descriptor is first deallocated as if a 98.Xr close 2 99call had been used. 100If 101.Fa oldd 102is not a valid descriptor, then 103.Fa newd 104is not closed. 105If 106.Fa oldd 107== 108.Fa newd 109and 110.Fa oldd 111is a valid descriptor, then 112.Fn dup2 113is successful, and does nothing. 114.Sh RETURN VALUES 115The value -1 is returned if an error occurs in either call. 116The external variable 117.Va errno 118indicates the cause of the error. 119.Sh ERRORS 120.Fn Dup 121and 122.Fn dup2 123fail if: 124.Bl -tag -width Er 125.It Bq Er EBADF 126.Fa Oldd 127or 128.Fa newd 129is not a valid active descriptor 130.It Bq Er EMFILE 131Too many descriptors are active. 132.El 133.Sh SEE ALSO 134.Xr accept 2 , 135.Xr close 2 , 136.Xr fcntl 2 , 137.Xr getdtablesize 2 , 138.Xr open 2 , 139.Xr pipe 2 , 140.Xr socket 2 , 141.Xr socketpair 2 , 142.Xr dup3 3 143.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