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.Dd February 27, 2023
.Dt DAEMON 3
.Os
.Sh NAME
.Nm daemon
.Nd run in the background
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdlib.h
.Ft int
.Fn daemon "int nochdir" "int noclose"
.Ft int
.Fn daemonfd "int chdirfd" "int nullfd"
.Sh DESCRIPTION
The
.Fn daemon
function is for programs wishing to detach themselves from the
controlling terminal and run in the background as system daemons.
.Pp
If the argument
.Fa nochdir
is zero,
.Fn daemon
changes the current working directory to the root
.Pq Pa / .
.Pp
If the argument
.Fa noclose
is zero,
.Fn daemon
redirects standard input, standard output, and standard error to
.Pa /dev/null .
.Pp
The
.Fn daemonfd
function is equivalent to the
.Fn daemon
function except that arguments are the descriptors for the current working
directory and to the descriptor to
.Pa /dev/null .
.Pp
If
.Fa chdirfd
is equal to
.Pq -1
the current working directory is not changed.
.Pp
If
.Fa nullfd
is equals to
.Pq -1
the redirection of standard input, standard output, and standard error is not
closed.
.Sh RETURN VALUES
.Rv -std daemon daemonfd
.Sh ERRORS
The
.Fn daemon
and
.Fn daemonfd
function may fail and set
.Va errno
for any of the errors specified for the library functions
.Xr fork 2
.Xr open 2 ,
and
.Xr setsid 2 .
.Sh SEE ALSO
.Xr fork 2 ,
.Xr setsid 2 ,
.Xr sigaction 2
.Sh HISTORY
The
.Fn daemon
function first appeared in
.Bx 4.4 .
The
.Fn daemonfd
function first appeared in
.Fx 12.0 .
.Sh CAVEATS
Unless the
.Fa noclose
argument is non-zero,
.Fn daemon
will close the first three file descriptors and redirect them to
.Pa /dev/null .
Normally, these correspond to standard input, standard output, and
standard error.
However, if any of those file descriptors refer to something else, they
will still be closed, resulting in incorrect behavior of the calling program.
This can happen if any of standard input, standard output, or standard
error have been closed before the program was run.
Programs using
.Fn daemon
should therefore either call
.Fn daemon
before opening any files or sockets, or verify that any file
descriptors obtained have values greater than 2.
.Pp
The
.Fn daemon
function temporarily ignores
.Dv SIGHUP
while calling
.Xr setsid 2
to prevent a parent session group leader's calls to
.Xr fork 2
and then
.Xr _exit 2
from prematurely terminating the child process.