DAEMON
Section: Linux Programmer's Manual (3)
Updated: 2016-03-15
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NAME
daemon - run in the background
SYNOPSIS
#include <unistd.h>
int daemon(int nochdir, int noclose);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
daemon():
Since glibc 2.21:
_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)
DESCRIPTION
The
daemon()
function is for programs wishing to detach themselves from the
controlling terminal and run in the background as system daemons.
If
nochdir
is zero,
daemon()
changes the process's current working directory
to the root directory ("/");
otherwise, the current working directory is left unchanged.
If
noclose
is zero,
daemon()
redirects standard input, standard output and standard error
to
/dev/null;
otherwise, no changes are made to these file descriptors.
RETURN VALUE
(This function forks, and if the
fork(2)
succeeds, the parent calls
_exit(2),
so that further errors are seen by the child only.)
On success
daemon()
returns zero.
If an error occurs,
daemon()
returns -1 and sets
errno
to any of the errors specified for the
fork(2)
and
setsid(2).
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
| Interface | Attribute | Value
|
|
daemon()
| Thread safety | MT-Safe
|
CONFORMING TO
Not in POSIX.1.
A similar function appears on the BSDs.
The
daemon()
function first appeared in 4.4BSD.
NOTES
The glibc implementation can also return -1 when
/dev/null
exists but is not a character device with the expected
major and minor numbers.
In this case,
errno
need not be set.
BUGS
The GNU C library implementation of this function was taken from BSD,
and does not employ the double-fork technique (i.e.,
fork(2),
setsid(2),
fork(2))
that is necessary to ensure that the resulting daemon process is
not a session leader.
Instead, the resulting daemon
is
a session leader.
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.
SEE ALSO
fork(2),
setsid(2)
COLOPHON
This page is part of release 4.13 of the Linux
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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ATTRIBUTES
-
- CONFORMING TO
-
- NOTES
-
- BUGS
-
- SEE ALSO
-
- COLOPHON
-
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- Running on 
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