1.\" $NetBSD: dup.2,v 1.7 1997/07/14 23:19:54 kleink Exp $ 2.\" 3.\" Copyright (c) 1980, 1991, 1993 4.\" The Regents of the University of California. All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 3. All advertising materials mentioning features or use of this software 15.\" must display the following acknowledgement: 16.\" This product includes software developed by the University of 17.\" California, Berkeley and its contributors. 18.\" 4. Neither the name of the University nor the names of its contributors 19.\" may be used to endorse or promote products derived from this software 20.\" without specific prior written permission. 21.\" 22.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 23.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 24.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 25.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 26.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 27.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 28.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 29.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 30.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 31.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 32.\" SUCH DAMAGE. 33.\" 34.\" @(#)dup.2 8.1 (Berkeley) 6/4/93 35.\" 36.Dd June 4, 1993 37.Dt DUP 2 38.Os BSD 4 39.Sh NAME 40.Nm dup , 41.Nm dup2 42.Nd duplicate an existing file descriptor 43.Sh SYNOPSIS 44.Fd #include <unistd.h> 45.Ft int 46.Fn dup "int oldd" 47.Ft int 48.Fn dup2 "int oldd" "int newd" 49.Sh DESCRIPTION 50.Fn Dup 51duplicates an existing object descriptor and returns its value to 52the calling process 53.Fa ( newd 54= 55.Fn dup oldd ) . 56The argument 57.Fa oldd 58is a small non-negative integer index in 59the per-process descriptor table. The value must be less 60than the size of the table, which is returned by 61.Xr getdtablesize 3 . 62The new descriptor returned by the call 63is the lowest numbered descriptor 64currently not in use by the process. 65.Pp 66The object referenced by the descriptor does not distinguish 67between 68.Fa oldd 69and 70.Fa newd 71in any way. 72Thus if 73.Fa newd 74and 75.Fa oldd 76are duplicate references to an open 77file, 78.Xr read 2 , 79.Xr write 2 80and 81.Xr lseek 2 82calls all move a single pointer into the file, 83and append mode, non-blocking I/O and asynchronous I/O options 84are shared between the references. 85If a separate pointer into the file is desired, a different 86object reference to the file must be obtained by issuing an 87additional 88.Xr open 2 89call. 90The close-on-exec flag on the new file descriptor is unset. 91.Pp 92In 93.Fn dup2 , 94the value of the new descriptor 95.Fa newd 96is specified. If this descriptor is already 97in use, the descriptor is first deallocated as if a 98.Xr close 2 99call had been done first. 100.Sh RETURN VALUES 101The value -1 is returned if an error occurs in either call. 102The external variable 103.Va errno 104indicates the cause of the error. 105.Sh ERRORS 106.Fn Dup 107and 108.Fn dup2 109fail if: 110.Bl -tag -width Er 111.It Bq Er EBADF 112.Fa Oldd 113or 114.Fa newd 115is not a valid active descriptor 116.It Bq Er EMFILE 117Too many descriptors are active. 118.El 119.Sh SEE ALSO 120.Xr accept 2 , 121.Xr open 2 , 122.Xr close 2 , 123.Xr fcntl 2 , 124.Xr pipe 2 , 125.Xr socket 2 , 126.Xr socketpair 2 , 127.Xr getdtablesize 3 128.Sh STANDARDS 129The 130.Fn dup 131and 132.Fn dup2 133functions conform to 134.St -p1003.1-90 . 135