1.\" Copyright (c) 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.\" @(#)daemon.3 8.1 (Berkeley) 6/9/93 29.\" 30.Dd February 27, 2023 31.Dt DAEMON 3 32.Os 33.Sh NAME 34.Nm daemon 35.Nd run in the background 36.Sh LIBRARY 37.Lb libc 38.Sh SYNOPSIS 39.In stdlib.h 40.Ft int 41.Fn daemon "int nochdir" "int noclose" 42.Ft int 43.Fn daemonfd "int chdirfd" "int nullfd" 44.Sh DESCRIPTION 45The 46.Fn daemon 47function is for programs wishing to detach themselves from the 48controlling terminal and run in the background as system daemons. 49.Pp 50If the argument 51.Fa nochdir 52is zero, 53.Fn daemon 54changes the current working directory to the root 55.Pq Pa / . 56.Pp 57If the argument 58.Fa noclose 59is zero, 60.Fn daemon 61redirects standard input, standard output, and standard error to 62.Pa /dev/null . 63.Pp 64The 65.Fn daemonfd 66function is equivalent to the 67.Fn daemon 68function except that arguments are the descriptors for the current working 69directory and to the descriptor to 70.Pa /dev/null . 71.Pp 72If 73.Fa chdirfd 74is equal to 75.Pq -1 76the current working directory is not changed. 77.Pp 78If 79.Fa nullfd 80is equals to 81.Pq -1 82the redirection of standard input, standard output, and standard error is not 83closed. 84.Sh RETURN VALUES 85.Rv -std daemon daemonfd 86.Sh ERRORS 87The 88.Fn daemon 89and 90.Fn daemonfd 91function may fail and set 92.Va errno 93for any of the errors specified for the library functions 94.Xr fork 2 95.Xr open 2 , 96and 97.Xr setsid 2 . 98.Sh SEE ALSO 99.Xr fork 2 , 100.Xr setsid 2 , 101.Xr sigaction 2 102.Sh HISTORY 103The 104.Fn daemon 105function first appeared in 106.Bx 4.4 . 107The 108.Fn daemonfd 109function first appeared in 110.Fx 12.0 . 111.Sh CAVEATS 112Unless the 113.Fa noclose 114argument is non-zero, 115.Fn daemon 116will close the first three file descriptors and redirect them to 117.Pa /dev/null . 118Normally, these correspond to standard input, standard output, and 119standard error. 120However, if any of those file descriptors refer to something else, they 121will still be closed, resulting in incorrect behavior of the calling program. 122This can happen if any of standard input, standard output, or standard 123error have been closed before the program was run. 124Programs using 125.Fn daemon 126should therefore either call 127.Fn daemon 128before opening any files or sockets, or verify that any file 129descriptors obtained have values greater than 2. 130.Pp 131The 132.Fn daemon 133function temporarily ignores 134.Dv SIGHUP 135while calling 136.Xr setsid 2 137to prevent a parent session group leader's calls to 138.Xr fork 2 139and then 140.Xr _exit 2 141from prematurely terminating the child process. 142