1.\" Copyright (c) 1980, 1991, 1993 2.\" The Regents of the University of California. 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. Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.Dd December 1, 2017 29.Dt DUP 2 30.Os 31.Sh NAME 32.Nm dup , 33.Nm dup2 34.Nd duplicate an existing file descriptor 35.Sh LIBRARY 36.Lb libc 37.Sh SYNOPSIS 38.In unistd.h 39.Ft int 40.Fn dup "int oldd" 41.Ft int 42.Fn dup2 "int oldd" "int newd" 43.Sh DESCRIPTION 44The 45.Fn dup 46system call 47duplicates an existing object descriptor and returns its value to 48the calling process 49.Fa ( newd 50= 51.Fn dup oldd ) . 52The argument 53.Fa oldd 54is a small non-negative integer index in 55the per-process descriptor table. 56The new descriptor returned by the call 57is the lowest numbered descriptor 58currently not in use by the process. 59.Pp 60The object referenced by the descriptor does not distinguish 61between 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 83system call. 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 and 92.Fa oldd 93\*(Ne 94.Fa newd , 95the descriptor is first deallocated as if the 96.Xr close 2 97system call had been used. 98If 99.Fa oldd 100is not a valid descriptor, then 101.Fa newd 102is not closed. 103If 104.Fa oldd 105== 106.Fa newd 107and 108.Fa oldd 109is a valid descriptor, then 110.Fn dup2 111is successful, and does nothing. 112.Sh RETURN VALUES 113These calls return the new file descriptor if successful; 114otherwise the value -1 is returned and 115the external variable 116.Va errno 117is set to indicate the cause of the error. 118.Sh ERRORS 119The 120.Fn dup 121system call fails if: 122.Bl -tag -width Er 123.It Bq Er EBADF 124The 125.Fa oldd 126argument 127is not a valid active descriptor 128.It Bq Er EMFILE 129Too many descriptors are active. 130.El 131.Pp 132The 133.Fn dup2 134system call fails if: 135.Bl -tag -width Er 136.It Bq Er EBADF 137The 138.Fa oldd 139argument is not a valid active descriptor or the 140.Fa newd 141argument is negative or exceeds the maximum allowable descriptor number 142.El 143.Sh SEE ALSO 144.Xr accept 2 , 145.Xr close 2 , 146.Xr fcntl 2 , 147.Xr getdtablesize 2 , 148.Xr open 2 , 149.Xr pipe 2 , 150.Xr socket 2 , 151.Xr socketpair 2 , 152.Xr dup3 3 153.Sh STANDARDS 154The 155.Fn dup 156and 157.Fn dup2 158system calls are expected to conform to 159.St -p1003.1-90 . 160.Sh HISTORY 161The 162.Fn dup 163function appeared in 164.At v3 . 165The 166.Fn dup2 167function appeared in 168.At v7 . 169