daemon(3)

1DAEMON(3)                  Linux Programmer's Manual                 DAEMON(3)
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NAME

6       daemon - run in the background
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SYNOPSIS

9       #include <unistd.h>
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11       int daemon(int nochdir, int noclose);
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13   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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15       daemon():
16           Since glibc 2.21:
17               _DEFAULT_SOURCE
18           In glibc 2.19 and 2.20:
19               _DEFAULT_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
20           Up to and including glibc 2.19:
21               _BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
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DESCRIPTION

24       The daemon() function is for programs wishing to detach themselves from
25       the controlling terminal and run in the background as system daemons.
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27       If nochdir is zero, daemon()  changes  the  process's  current  working
28       directory  to  the root directory ("/"); otherwise, the current working
29       directory is left unchanged.
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31       If noclose is zero, daemon() redirects standard input, standard  output
32       and  standard  error  to  /dev/null;  otherwise, no changes are made to
33       these file descriptors.
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RETURN VALUE

36       (This function forks, and if the fork(2)  succeeds,  the  parent  calls
37       _exit(2),  so that further errors are seen by the child only.)  On suc‐
38       cess daemon() returns zero.  If an error occurs,  daemon()  returns  -1
39       and  sets errno to any of the errors specified for the fork(2) and set‐
40       sid(2).
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ATTRIBUTES

43       For  an  explanation  of  the  terms  used   in   this   section,   see
44       attributes(7).
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46       ┌──────────┬───────────────┬─────────┐
47       │Interface │ Attribute     │ Value   │
48       ├──────────┼───────────────┼─────────┤
49       │daemon()  │ Thread safety │ MT-Safe │
50       └──────────┴───────────────┴─────────┘

CONFORMING TO

52       Not  in POSIX.1.  A similar function appears on the BSDs.  The daemon()
53       function first appeared in 4.4BSD.
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NOTES

56       The glibc implementation can also return -1 when /dev/null  exists  but
57       is  not  a  character device with the expected major and minor numbers.
58       In this case, errno need not be set.
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BUGS

61       The GNU C library implementation of this function was taken  from  BSD,
62       and  does  not  employ  the  double-fork technique (i.e., fork(2), set‐
63       sid(2), fork(2)) that is necessary to ensure that the resulting  daemon
64       process  is  not  a session leader.  Instead, the resulting daemon is a
65       session leader.  On systems  that  follow  System  V  semantics  (e.g.,
66       Linux),  this  means  that  if  the daemon opens a terminal that is not
67       already a controlling terminal for another session, then that  terminal
68       will inadvertently become the controlling terminal for the daemon.
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COLOPHON

74       This  page  is  part of release 4.16 of the Linux man-pages project.  A
75       description of the project, information about reporting bugs,  and  the
76       latest     version     of     this    page,    can    be    found    at
77       https://www.kernel.org/doc/man-pages/.
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81GNU                               2017-11-26                         DAEMON(3)