daemon(3)

1daemon(3)                  Library Functions Manual                  daemon(3)
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NAME

6       daemon - run in the background
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LIBRARY

9       Standard C library (libc, -lc)
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SYNOPSIS

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

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

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

46       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
47       tributes(7).
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49       ┌────────────────────────────────────────────┬───────────────┬─────────┐
50       │Interface                                   │ Attribute     │ Value   │
51       ├────────────────────────────────────────────┼───────────────┼─────────┤
52       │daemon()                                    │ Thread safety │ MT-Safe │
53       └────────────────────────────────────────────┴───────────────┴─────────┘
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VERSIONS

56       A similar function appears on the BSDs.
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58       The  glibc  implementation can also return -1 when /dev/null exists but
59       is not a character device with the expected major  and  minor  numbers.
60       In this case, errno need not be set.
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STANDARDS

63       None.
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HISTORY

66       4.4BSD.
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BUGS

69       The  GNU  C library implementation of this function was taken from BSD,
70       and  does  not  employ  the  double-fork  technique   (i.e.,   fork(2),
71       setsid(2),  fork(2))  that  is  necessary  to ensure that the resulting
72       daemon process is not a session leader.  Instead, the resulting  daemon
73       is  a session leader.  On systems that follow System V semantics (e.g.,
74       Linux), this means that if the daemon opens  a  terminal  that  is  not
75       already  a controlling terminal for another session, then that terminal
76       will inadvertently become the controlling terminal for the daemon.
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