man3/daemon.3

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  34 .\"     @(#)daemon.3    8.1 (Berkeley) 6/9/93
  35 .\" Added mentioning of glibc weirdness wrt unistd.h. 5/11/98, Al Viro
  36 .TH DAEMON 3 2016-03-15 "GNU" "Linux Programmer's Manual"
  37 .SH NAME
  38 daemon \- run in the background
  39 .SH SYNOPSIS
  40 .B #include <unistd.h>
  41 .sp
  42 .BI "int daemon(int " nochdir ", int " noclose );
  43 .sp
  44 .in -4n
  45 Feature Test Macro Requirements for glibc (see
  46 .BR feature_test_macros (7)):
  47 .in
  48 .sp
  49 .BR daemon ():
  50 .nf
  51     Since glibc 2.21:
  52 .\"             commit 266865c0e7b79d4196e2cc393693463f03c90bd8
  53         _DEFAULT_SOURCE
  54     In glibc 2.19 and 2.20:
  55         _DEFAULT_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE\ <\ 500)
  56     Up to and including glibc 2.19:
  57         _BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE\ <\ 500)
  58 .fi
  59 .SH DESCRIPTION
  60 The
  61 .BR daemon ()
  62 function is for programs wishing to detach themselves from the
  63 controlling terminal and run in the background as system daemons.
  64 .PP
  65 If
  66 .I nochdir
  67 is zero,
  68 .BR daemon ()
  69 changes the process's current working directory
  70 to the root directory ("/");
  71 otherwise, the current working directory is left unchanged.
  72 .PP
  73 If
  74 .I noclose
  75 is zero,
  76 .BR daemon ()
  77 redirects standard input, standard output and standard error
  78 to
  79 .IR /dev/null ;
  80 otherwise, no changes are made to these file descriptors.
  81 .SH RETURN VALUE
  82 (This function forks, and if the
  83 .BR fork (2)
  84 succeeds, the parent calls
  85 .\" not .IR in order not to underline _
  86 .BR _exit (2),
  87 so that further errors are seen by the child only.)
  88 On success
  89 .BR daemon ()
  90 returns zero.
  91 If an error occurs,
  92 .BR daemon ()
  93 returns \-1 and sets
  94 .I errno
  95 to any of the errors specified for the
  96 .BR fork (2)
  97 and
  98 .BR setsid (2).
  99 .SH ATTRIBUTES
 100 For an explanation of the terms used in this section, see
 101 .BR attributes (7).
 102 .TS
 103 allbox;
 104 lb lb lb
 105 l l l.
 106 Interface       Attribute       Value
 107 T{
 108 .BR daemon ()
 109 T}      Thread safety   MT-Safe
 110 .TE
 111 .SH CONFORMING TO
 112 Not in POSIX.1.
 113 A similar function appears on the BSDs.
 114 The
 115 .BR daemon ()
 116 function first appeared in 4.4BSD.
 117 .SH NOTES
 118 The glibc implementation can also return \-1 when
 119 .I /dev/null
 120 exists but is not a character device with the expected
 121 major and minor numbers.
 122 In this case,
 123 .I errno
 124 need not be set.
 125 .SH BUGS
 126 The GNU C library implementation of this function was taken from BSD,
 127 and does not employ the double-fork technique (i.e.,
 128 .BR fork (2),
 129 .BR setsid (2),
 130 .BR fork (2))
 131 that is necessary to ensure that the resulting daemon process is
 132 not a session leader.
 133 Instead, the resulting daemon
 134 .I is
 135 a session leader.
 136 .\" FIXME https://sourceware.org/bugzilla/show_bug.cgi?id=19144
 137 .\" Tested using a program that uses daemon() and then opens an
 138 .\" otherwise unused console device (/dev/ttyN) that does not
 139 .\" have an associated getty process.
 140 On systems that follow System V semantics (e.g., Linux),
 141 this means that if the daemon opens a terminal that is not
 142 already a controlling terminal for another session,
 143 then that terminal will inadvertently become
 144 the controlling terminal for the daemon.
 145 .SH SEE ALSO
 146 .BR fork (2),
 147 .BR setsid (2)