1.\" $OpenBSD: daemon.3,v 1.14 2022/07/30 07:19:30 jsg Exp $ 2.\" 3.\" Copyright (c) 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. Neither the name of the University nor the names of its contributors 15.\" may be used to endorse or promote products derived from this software 16.\" without specific prior written permission. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 19.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 20.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 21.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 22.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 23.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 24.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 25.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 26.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 27.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 28.\" SUCH DAMAGE. 29.\" 30.Dd $Mdocdate: July 30 2022 $ 31.Dt DAEMON 3 32.Os 33.Sh NAME 34.Nm daemon 35.Nd run in the background 36.Sh SYNOPSIS 37.In stdlib.h 38.Ft int 39.Fn daemon "int nochdir" "int noclose" 40.Sh DESCRIPTION 41The 42.Fn daemon 43function is for programs wishing to detach themselves from the 44controlling terminal and run in the background as system daemons. 45.Pp 46If the argument 47.Fa nochdir 48is zero, 49.Fn daemon 50changes the current working directory to the root 51.Pq Pa / . 52.Pp 53If the argument 54.Fa noclose 55is zero, 56.Fn daemon 57redirects standard input, standard output and standard error to 58.Pa /dev/null . 59.Sh RETURN VALUES 60Upon success, 61.Fn daemon 62returns 0; otherwise \-1 is returned. 63.Sh ERRORS 64The function 65.Fn daemon 66may fail and set 67.Va errno 68for any of the errors specified for the library functions 69.Xr fork 2 70and 71.Xr setsid 2 . 72.Sh SEE ALSO 73.Xr fork 2 , 74.Xr setsid 2 75.Sh HISTORY 76The 77.Fn daemon 78function first appeared in 79.Bx 4.3 Reno 80libutil and moved to libc in 81.Bx 4.4 . 82.Sh CAVEATS 83If the 84.Ar noclose 85argument is zero, 86.Fn daemon 87closes the first three file descriptors and redirects them to 88.Pa /dev/null . 89Normally, these correspond to standard input, standard output and 90standard error. 91However, if any of those file descriptors refer to something else they 92will still be closed, resulting in incorrect behavior of the calling program. 93This can happen if any of standard input, standard output or standard error 94have been closed before the program was run. 95Programs using 96.Fn daemon 97should therefore make sure to either call 98.Fn daemon 99before opening any files or sockets or, alternately, verifying that 100any file descriptors obtained have a value greater than 2. 101