1.\" Copyright (c) 2013 Jilles Tjoelker 2.\" 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.\" 13.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 14.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 15.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 16.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 17.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 18.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 19.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 20.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 21.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 22.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 23.\" SUCH DAMAGE. 24.\" 25.Dd May 17, 2025 26.Dt DUP3 3 27.Os 28.Sh NAME 29.Nm dup3 30.Nd duplicate an existing file descriptor 31.Sh LIBRARY 32.Lb libc 33.Sh SYNOPSIS 34.In fcntl.h 35.In unistd.h 36.Ft int 37.Fn dup3 "int oldd" "int newd" "int flags" 38.Sh DESCRIPTION 39The 40.Fn dup3 41function 42duplicates an existing object descriptor 43while allowing the value of the new descriptor to be specified. 44.Pp 45The close-on-exec flag on the new file descriptor is determined by the 46.Dv O_CLOEXEC 47bit in 48.Fa flags . 49.Pp 50The close-on-fork flag on the new file descriptor is determined by the 51.Dv O_CLOFORK 52bit in 53.Fa flags . 54.Pp 55If 56.Fa oldd 57\*(Ne 58.Fa newd 59and 60.Fa flags 61== 0, 62the behavior is identical to 63.Li dup2(oldd, newd) . 64.Pp 65If 66.Fa oldd 67== 68.Fa newd , 69then 70.Fn dup3 71fails, unlike 72.Xr dup2 2 . 73.Sh RETURN VALUES 74The value -1 is returned if an error occurs. 75The external variable 76.Va errno 77indicates the cause of the error. 78.Sh ERRORS 79The 80.Fn dup3 81function fails if: 82.Bl -tag -width Er 83.It Bq Er EBADF 84The 85.Fa oldd 86argument is not a valid active descriptor or the 87.Fa newd 88argument is negative or exceeds the maximum allowable descriptor number 89.It Bq Er EINVAL 90The 91.Fa oldd 92argument is equal to the 93.Fa newd 94argument. 95.It Bq Er EINVAL 96The 97.Fa flags 98argument has bits set other than 99.Dv O_CLOEXEC 100or 101.Dv O_CLOFORK . 102.El 103.Sh SEE ALSO 104.Xr accept 2 , 105.Xr close 2 , 106.Xr dup2 2 , 107.Xr fcntl 2 , 108.Xr getdtablesize 2 , 109.Xr open 2 , 110.Xr pipe 2 , 111.Xr socket 2 , 112.Xr socketpair 2 113.Sh STANDARDS 114The 115.Fn dup3 116function does not conform to any standard. 117.Sh HISTORY 118The 119.Fn dup3 120function appeared in 121.Fx 10.0 . 122The 123.Dv O_CLOFORK 124flag appeared in 125.Fx 15.0 . 126