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.\" @(#)dup.2 6.4 (Berkeley) 3/10/91 33.\" 34.Dd March 10, 1991 35.Dt DUP 2 36.Os BSD 4 37.Sh NAME 38.Nm dup , 39.Nm dup2 40.Nd duplicate an existing file descriptor 41.Sh SYNOPSIS 42.Fd #include <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 95in use, the descriptor is first deallocated as if a 96.Xr close 2 97call had been done first. 98.Sh RETURN VALUES 99The value -1 is returned if an error occurs in either call. 100The external variable 101.Va errno 102indicates the cause of the error. 103.Sh ERRORS 104.Fn Dup 105and 106.Fn dup2 107fail if: 108.Bl -tag -width Er 109.It Bq Er EBADF 110.Fa Oldd 111or 112.Fa newd 113is not a valid active descriptor 114.It Bq Er EMFILE 115Too many descriptors are active. 116.El 117.Sh SEE ALSO 118.Xr accept 2 , 119.Xr open 2 , 120.Xr close 2 , 121.Xr fcntl 2 , 122.Xr pipe 2 , 123.Xr socket 2 , 124.Xr socketpair 2 , 125.Xr getdtablesize 2 126.Sh STANDARDS 127.Fn Dup 128and 129.Fn dup2 130are expected to conform 131to IEEE Std 1003.1-1988 132.Pq Dq Tn POSIX . 133