1.\" Copyright (c) 1980, 1991 Regents of the University of California. 2.\" 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. All advertising materials mentioning features or use of this software 13.\" must display the following acknowledgement: 14.\" This product includes software developed by the University of 15.\" California, Berkeley and its contributors. 16.\" 4. Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" from: @(#)dup.2 6.4 (Berkeley) 3/10/91 33.\" $Id: dup.2,v 1.3 1993/11/25 00:38:36 jtc Exp $ 34.\" 35.Dd March 10, 1991 36.Dt DUP 2 37.Os BSD 4 38.Sh NAME 39.Nm dup , 40.Nm dup2 41.Nd duplicate an existing file descriptor 42.Sh SYNOPSIS 43.Fd #include <unistd.h> 44.Ft int 45.Fn dup "int oldd" 46.Ft int 47.Fn dup2 "int oldd" "int newd" 48.Sh DESCRIPTION 49.Fn Dup 50duplicates an existing object descriptor and returns its value to 51the calling process 52.Fa ( newd 53= 54.Fn dup oldd ) . 55The argument 56.Fa oldd 57is a small non-negative integer index in 58the per-process descriptor table. The value must be less 59than the size of the table, which is returned by 60.Xr getdtablesize 2 . 61The new descriptor returned by the call 62is the lowest numbered descriptor 63currently not in use by the process. 64.Pp 65The object referenced by the descriptor does not distinguish 66between 67.Fa oldd 68and 69.Fa newd 70in any way. 71Thus if 72.Fa newd 73and 74.Fa oldd 75are duplicate references to an open 76file, 77.Xr read 2 , 78.Xr write 2 79and 80.Xr lseek 2 81calls all move a single pointer into the file, 82and append mode, non-blocking I/O and asynchronous I/O options 83are shared between the references. 84If a separate pointer into the file is desired, a different 85object reference to the file must be obtained by issuing an 86additional 87.Xr open 2 88call. 89The close-on-exec flag on the new file descriptor is unset. 90.Pp 91In 92.Fn dup2 , 93the value of the new descriptor 94.Fa newd 95is specified. If this descriptor is already 96in use, the descriptor is first deallocated as if a 97.Xr close 2 98call had been done first. 99.Sh RETURN VALUES 100The value -1 is returned if an error occurs in either call. 101The external variable 102.Va errno 103indicates the cause of the error. 104.Sh ERRORS 105.Fn Dup 106and 107.Fn dup2 108fail if: 109.Bl -tag -width Er 110.It Bq Er EBADF 111.Fa Oldd 112or 113.Fa newd 114is not a valid active descriptor 115.It Bq Er EMFILE 116Too many descriptors are active. 117.El 118.Sh SEE ALSO 119.Xr accept 2 , 120.Xr open 2 , 121.Xr close 2 , 122.Xr fcntl 2 , 123.Xr pipe 2 , 124.Xr socket 2 , 125.Xr socketpair 2 , 126.Xr getdtablesize 2 127.Sh STANDARDS 128.Fn Dup 129and 130.Fn dup2 131are expected to conform to 132.St -p1003.1-88 . 133