16545cf5d5
The daemonfd function is equivalent to the daemon(3) function expect that arguments are descriptors. For example dhclient(8) which is sandboxed is unable to open /dev/null to close stdio instead it's allows to fail daemon(3) function to close the descriptors and then do it explicit in code. Instead of such hacks we can use now daemonfd. This API can be also helpful to migrate system to platforms like CheriBSD. Reviewed by: brooks@, bcr@, jilles@ (earlier version) Differential Revision: https://reviews.freebsd.org/D13433
143 lines
3.9 KiB
Groff
143 lines
3.9 KiB
Groff
.\" Copyright (c) 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)daemon.3 8.1 (Berkeley) 6/9/93
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.\" $FreeBSD$
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.\"
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.Dd December 23, 2017
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.Dt DAEMON 3
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.Os
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.Sh NAME
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.Nm daemon
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.Nd run in the background
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In stdlib.h
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.Ft int
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.Fn daemon "int nochdir" "int noclose"
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.Ft int
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.Fn daemonfd "int chdirfd" "int nullfd"
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.Sh DESCRIPTION
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The
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.Fn daemon
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function is for programs wishing to detach themselves from the
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controlling terminal and run in the background as system daemons.
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.Pp
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Unless the argument
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.Fa nochdir
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is non-zero,
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.Fn daemon
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changes the current working directory to the root
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.Pq Pa / .
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.Pp
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Unless the argument
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.Fa noclose
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is non-zero,
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.Fn daemon
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will redirect standard input, standard output, and standard error to
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.Pa /dev/null .
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.Pp
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The
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.Fn daemonfd
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function is equivalent to the
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.Fn daemon
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function except that arguments are the descriptors for the current working
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directory and to the descriptor to
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.Pa /dev/null .
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.Pp
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If
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.Fa chdirfd
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is equal to
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.Pq -1
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the current working directory is not changed.
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.Pp
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If
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.Fa nullfd
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is equals to
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.Pq -1
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the redirection of standard input, standard output, and standard error is not
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closed.
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.Sh RETURN VALUES
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.Rv -std daemon daemonfd
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.Sh ERRORS
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The
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.Fn daemon
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and
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.Fn daemonfd
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function may fail and set
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.Va errno
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for any of the errors specified for the library functions
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.Xr fork 2
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.Xr open 2,
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and
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.Xr setsid 2 .
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.Sh SEE ALSO
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.Xr fork 2 ,
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.Xr setsid 2 ,
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.Xr sigaction 2
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.Sh HISTORY
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The
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.Fn daemon
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function first appeared in
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.Bx 4.4 .
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The
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.Fn daemonfd
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function first appeared in
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.Fx 12.0 .
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.Sh CAVEATS
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Unless the
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.Fa noclose
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argument is non-zero,
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.Fn daemon
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will close the first three file descriptors and redirect them to
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.Pa /dev/null .
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Normally, these correspond to standard input, standard output, and
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standard error.
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However, if any of those file descriptors refer to something else, they
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will still be closed, resulting in incorrect behavior of the calling program.
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This can happen if any of standard input, standard output, or standard
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error have been closed before the program was run.
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Programs using
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.Fn daemon
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should therefore either call
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.Fn daemon
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before opening any files or sockets, or verify that any file
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descriptors obtained have values greater than 2.
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.Pp
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The
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.Fn daemon
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function temporarily ignores
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.Dv SIGHUP
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while calling
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.Xr setsid 2
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to prevent a parent session group leader's calls to
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.Xr fork 2
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and then
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.Xr _exit 2
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from prematurely terminating the child process.
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