man3/daemon.3

   1 '\" t
   2 .\" Copyright (c) 1993
   3 .\"     The Regents of the University of California.  All rights reserved.
   4 .\"
   5 .\" SPDX-License-Identifier: BSD-4-Clause-UC
   6 .\"
   7 .\"     @(#)daemon.3    8.1 (Berkeley) 6/9/93
   8 .\" Added mentioning of glibc weirdness wrt unistd.h. 5/11/98, Al Viro
   9 .TH daemon 3 (date) "Linux man-pages (unreleased)"
  10 .SH NAME
  11 daemon \- run in the background
  12 .SH LIBRARY
  13 Standard C library
  14 .RI ( libc ", " \-lc )
  15 .SH SYNOPSIS
  16 .nf
  17 .B #include <unistd.h>
  18 .PP
  19 .BI "int daemon(int " nochdir ", int " noclose );
  20 .fi
  21 .PP
  22 .RS -4
  23 Feature Test Macro Requirements for glibc (see
  24 .BR feature_test_macros (7)):
  25 .RE
  26 .PP
  27 .BR daemon ():
  28 .nf
  29     Since glibc 2.21:
  30 .\"             commit 266865c0e7b79d4196e2cc393693463f03c90bd8
  31         _DEFAULT_SOURCE
  32     In glibc 2.19 and 2.20:
  33         _DEFAULT_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
  34     Up to and including glibc 2.19:
  35         _BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
  36 .fi
  37 .SH DESCRIPTION
  38 The
  39 .BR daemon ()
  40 function is for programs wishing to detach themselves from the
  41 controlling terminal and run in the background as system daemons.
  42 .PP
  43 If
  44 .I nochdir
  45 is zero,
  46 .BR daemon ()
  47 changes the process's current working directory
  48 to the root directory ("/");
  49 otherwise, the current working directory is left unchanged.
  50 .PP
  51 If
  52 .I noclose
  53 is zero,
  54 .BR daemon ()
  55 redirects standard input, standard output, and standard error
  56 to
  57 .IR /dev/null ;
  58 otherwise, no changes are made to these file descriptors.
  59 .SH RETURN VALUE
  60 (This function forks, and if the
  61 .BR fork (2)
  62 succeeds, the parent calls
  63 .\" not .IR in order not to underline _
  64 .BR _exit (2),
  65 so that further errors are seen by the child only.)
  66 On success
  67 .BR daemon ()
  68 returns zero.
  69 If an error occurs,
  70 .BR daemon ()
  71 returns \-1 and sets
  72 .I errno
  73 to any of the errors specified for the
  74 .BR fork (2)
  75 and
  76 .BR setsid (2).
  77 .SH ATTRIBUTES
  78 For an explanation of the terms used in this section, see
  79 .BR attributes (7).
  80 .TS
  81 allbox;
  82 lbx lb lb
  83 l l l.
  84 Interface       Attribute       Value
  85 T{
  86 .na
  87 .nh
  88 .BR daemon ()
  89 T}      Thread safety   MT-Safe
  90 .TE
  91 .sp 1
  92 .SH VERSIONS
  93 A similar function appears on the BSDs.
  94 .PP
  95 The glibc implementation can also return \-1 when
  96 .I /dev/null
  97 exists but is not a character device with the expected
  98 major and minor numbers.
  99 In this case,
 100 .I errno
 101 need not be set.
 102 .SH STANDARDS
 103 None.
 104 .SH HISTORY
 105 4.4BSD.
 106 .SH BUGS
 107 The GNU C library implementation of this function was taken from BSD,
 108 and does not employ the double-fork technique (i.e.,
 109 .BR fork (2),
 110 .BR setsid (2),
 111 .BR fork (2))
 112 that is necessary to ensure that the resulting daemon process is
 113 not a session leader.
 114 Instead, the resulting daemon
 115 .I is
 116 a session leader.
 117 .\" FIXME . https://sourceware.org/bugzilla/show_bug.cgi?id=19144
 118 .\" Tested using a program that uses daemon() and then opens an
 119 .\" otherwise unused console device (/dev/ttyN) that does not
 120 .\" have an associated getty process.
 121 On systems that follow System V semantics (e.g., Linux),
 122 this means that if the daemon opens a terminal that is not
 123 already a controlling terminal for another session,
 124 then that terminal will inadvertently become
 125 the controlling terminal for the daemon.
 126 .SH SEE ALSO
 127 .BR fork (2),
 128 .BR setsid (2),
 129 .BR daemon (7),
 130 .BR logrotate (8)