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.\"

.\"	@(#)daemon.3	8.1 (Berkeley) 6/9/93

.\" Added mentioning of glibc weirdness wrt unistd.h. 5/11/98, Al Viro

.TH DAEMON 3 2017-11-26 "GNU" "Linux Programmer's Manual"

.SH NAME

daemon \- run in the background

.SH SYNOPSIS

.B #include <unistd.h>

.PP

.BI "int daemon(int " nochdir ", int " noclose );

.PP

.in -4n

Feature Test Macro Requirements for glibc (see

.BR feature_test_macros (7)):

.in

.PP

.BR daemon ():

.nf

    Since glibc 2.21:

.\"             commit 266865c0e7b79d4196e2cc393693463f03c90bd8

        _DEFAULT_SOURCE

    In glibc 2.19 and 2.20:

        _DEFAULT_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE\ <\ 500)

    Up to and including glibc 2.19:

        _BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE\ <\ 500)

.fi

.SH DESCRIPTION

The

.BR daemon ()

function is for programs wishing to detach themselves from the

controlling terminal and run in the background as system daemons.

.PP

If

.I nochdir

is zero,

.BR daemon ()

changes the process's current working directory

to the root directory ("/");

otherwise, the current working directory is left unchanged.

.PP

If

.I noclose

is zero,

.BR daemon ()

redirects standard input, standard output and standard error

to

.IR /dev/null ;

otherwise, no changes are made to these file descriptors.

.SH RETURN VALUE

(This function forks, and if the

.BR fork (2)

succeeds, the parent calls

.\" not .IR in order not to underline _

.BR _exit (2),

so that further errors are seen by the child only.)

On success

.BR daemon ()

returns zero.

If an error occurs,

.BR daemon ()

returns \-1 and sets

.I errno

to any of the errors specified for the

.BR fork (2)

and

.BR setsid (2).

.SH ATTRIBUTES

For an explanation of the terms used in this section, see

.BR attributes (7).

.TS

allbox;

lb lb lb

l l l.

Interface	Attribute	Value

T{

.BR daemon ()

T}	Thread safety	MT-Safe

.TE

.SH CONFORMING TO

Not in POSIX.1.

A similar function appears on the BSDs.

The

.BR daemon ()

function first appeared in 4.4BSD.

.SH NOTES

The glibc implementation can also return \-1 when

.I /dev/null

exists but is not a character device with the expected

major and minor numbers.

In this case,

.I errno

need not be set.

.SH BUGS

The GNU C library implementation of this function was taken from BSD,

and does not employ the double-fork technique (i.e.,

.BR fork (2),

.BR setsid (2),

.BR fork (2))

that is necessary to ensure that the resulting daemon process is

not a session leader.

Instead, the resulting daemon

.I is

a session leader.

.\" FIXME . https://sourceware.org/bugzilla/show_bug.cgi?id=19144

.\" Tested using a program that uses daemon() and then opens an

.\" otherwise unused console device (/dev/ttyN) that does not

.\" have an associated getty process.

On systems that follow System V semantics (e.g., Linux),

this means that if the daemon opens a terminal that is not

already a controlling terminal for another session,

then that terminal will inadvertently become

the controlling terminal for the daemon.

.SH SEE ALSO

.BR fork (2),

.BR setsid (2),

.BR daemon (7),

.BR logrotate (8)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.