1.\" $NetBSD: dup.2,v 1.13 2002/02/08 01:28:17 ross 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 39.Sh NAME 40.Nm dup , 41.Nm dup2 42.Nd duplicate an existing file descriptor 43.Sh LIBRARY 44.Lb libc 45.Sh SYNOPSIS 46.Fd #include \*[Lt]unistd.h\*[Gt] 47.Ft int 48.Fn dup "int oldd" 49.Ft int 50.Fn dup2 "int oldd" "int newd" 51.Sh DESCRIPTION 52.Fn dup 53duplicates an existing object descriptor and returns its value to 54the calling process 55.Fa ( newd 56= 57.Fn dup oldd ) . 58The argument 59.Fa oldd 60is a small non-negative integer index in 61the per-process descriptor table. The value must be less 62than the size of the table, which is returned by 63.Xr getdtablesize 3 . 64The new descriptor returned by the call 65is the lowest numbered descriptor 66currently not in use by the process. 67.Pp 68The object referenced by the descriptor does not distinguish 69between 70.Fa oldd 71and 72.Fa newd 73in any way. 74Thus if 75.Fa newd 76and 77.Fa oldd 78are duplicate references to an open 79file, 80.Xr read 2 , 81.Xr write 2 82and 83.Xr lseek 2 84calls all move a single pointer into the file, 85and append mode, non-blocking I/O and asynchronous I/O options 86are shared between the references. 87If a separate pointer into the file is desired, a different 88object reference to the file must be obtained by issuing an 89additional 90.Xr open 2 91call. 92The close-on-exec flag on the new file descriptor is unset. 93.Pp 94In 95.Fn dup2 , 96the value of the new descriptor 97.Fa newd 98is specified. If this descriptor is already 99in use, the descriptor is first deallocated as if a 100.Xr close 2 101call had been done first. 102.Sh RETURN VALUES 103The value -1 is returned if an error occurs in either call. 104The external variable 105.Va errno 106indicates the cause of the error. 107.Sh ERRORS 108.Fn dup 109and 110.Fn dup2 111fail if: 112.Bl -tag -width Er 113.It Bq Er EBADF 114.Fa oldd 115or 116.Fa newd 117is not a valid active descriptor 118.It Bq Er EMFILE 119Too many descriptors are active. 120.El 121.Sh SEE ALSO 122.Xr accept 2 , 123.Xr close 2 , 124.Xr fcntl 2 , 125.Xr open 2 , 126.Xr pipe 2 , 127.Xr socket 2 , 128.Xr socketpair 2 , 129.Xr getdtablesize 3 130.Sh STANDARDS 131The 132.Fn dup 133and 134.Fn dup2 135functions conform to 136.St -p1003.1-90 . 137