1.\" $OpenBSD: dup.2,v 1.13 2007/05/31 19:19:32 jmc Exp $ 2.\" $NetBSD: dup.2,v 1.4 1995/02/27 12:32:21 cgd Exp $ 3.\" 4.\" Copyright (c) 1980, 1991, 1993 5.\" The Regents of the University of California. All rights reserved. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that the following conditions 9.\" are met: 10.\" 1. Redistributions of source code must retain the above copyright 11.\" notice, this list of conditions and the following disclaimer. 12.\" 2. Redistributions in binary form must reproduce the above copyright 13.\" notice, this list of conditions and the following disclaimer in the 14.\" documentation and/or other materials provided with the distribution. 15.\" 3. Neither the name of the University nor the names of its contributors 16.\" may be used to endorse or promote products derived from this software 17.\" without specific prior written permission. 18.\" 19.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 20.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 21.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 22.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 23.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 24.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 25.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 26.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 27.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 28.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 29.\" SUCH DAMAGE. 30.\" 31.\" @(#)dup.2 8.1 (Berkeley) 6/4/93 32.\" 33.Dd $Mdocdate: May 31 2007 $ 34.Dt DUP 2 35.Os 36.Sh NAME 37.Nm dup , 38.Nm dup2 39.Nd duplicate an existing file descriptor 40.Sh SYNOPSIS 41.Fd #include <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 the per-process descriptor table. 56The value must be less than the size of the table, which is returned by 57.Xr getdtablesize 3 . 58The new descriptor returned by the call is the lowest numbered descriptor 59currently not in use by the process. 60.Pp 61The object referenced by the descriptor does not distinguish between 62.Fa oldd 63and 64.Fa newd 65in any way. 66Thus if 67.Fa newd 68and 69.Fa oldd 70are duplicate references to an open 71file, 72.Xr read 2 , 73.Xr write 2 74and 75.Xr lseek 2 76calls all move a single pointer into the file, 77and append mode, non-blocking I/O and asynchronous I/O options 78are shared between the references. 79If a separate pointer into the file is desired, a different 80object reference to the file must be obtained by issuing an 81additional 82.Xr open 2 83call. 84The close-on-exec flag on the new file descriptor is unset. 85.Pp 86In 87.Fn dup2 , 88the value of the new descriptor 89.Fa newd 90is specified. 91If this descriptor is already in use, it is first deallocated as if a 92.Xr close 2 93call had been done first. 94When 95.Fa newd 96equals 97.Fa oldd , 98.Fn dup2 99just returns without affecting the close-on-exec flag. 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 close 2 , 122.Xr fcntl 2 , 123.Xr open 2 , 124.Xr pipe 2 , 125.Xr socket 2 , 126.Xr socketpair 2 , 127.Xr getdtablesize 3 128.Sh STANDARDS 129.Fn dup 130and 131.Fn dup2 132are expected to conform to 133.St -p1003.1-88 . 134