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 and close-on-fork flags on the new file descriptor 85are unset. 86The resolve-beneath flag on the new file descriptor is set to the same 87state as on the old file descriptor. 88.Pp 89In 90.Fn dup2 , 91the value of the new descriptor 92.Fa newd 93is specified. 94If this descriptor is already in use and 95.Fa oldd 96\*(Ne 97.Fa newd , 98the descriptor is first deallocated as if the 99.Xr close 2 100system call had been used. 101If 102.Fa oldd 103is not a valid descriptor, then 104.Fa newd 105is not closed. 106If 107.Fa oldd 108== 109.Fa newd 110and 111.Fa oldd 112is a valid descriptor, then 113.Fn dup2 114is successful, and does nothing. 115.Sh RETURN VALUES 116These calls return the new file descriptor if successful; 117otherwise the value -1 is returned and 118the external variable 119.Va errno 120is set to indicate the cause of the error. 121.Sh ERRORS 122The 123.Fn dup 124system call fails if: 125.Bl -tag -width Er 126.It Bq Er EBADF 127The 128.Fa oldd 129argument 130is not a valid active descriptor 131.It Bq Er EMFILE 132Too many descriptors are active. 133.El 134.Pp 135The 136.Fn dup2 137system call fails if: 138.Bl -tag -width Er 139.It Bq Er EBADF 140The 141.Fa oldd 142argument is not a valid active descriptor or the 143.Fa newd 144argument is negative or exceeds the maximum allowable descriptor number 145.El 146.Sh SEE ALSO 147.Xr accept 2 , 148.Xr close 2 , 149.Xr fcntl 2 , 150.Xr getdtablesize 2 , 151.Xr open 2 , 152.Xr pipe 2 , 153.Xr socket 2 , 154.Xr socketpair 2 , 155.Xr dup3 3 156.Sh STANDARDS 157The 158.Fn dup 159and 160.Fn dup2 161system calls are expected to conform to 162.St -p1003.1-90 . 163.Sh HISTORY 164The 165.Fn dup 166function appeared in 167.At v3 . 168The 169.Fn dup2 170function appeared in 171.At v7 . 172