man3/daemon.3

   1 .\" Copyright (c) 1993
   2 .\"     The Regents of the University of California.  All rights reserved.
   3 .\"
   4 .\" SPDX-License-Identifier: BSD-4-Clause-UC
   5 .\"
   6 .\"     @(#)daemon.3    8.1 (Berkeley) 6/9/93
   7 .\" Added mentioning of glibc weirdness wrt unistd.h. 5/11/98, Al Viro
   8 .TH DAEMON 3 2021-03-22 "Linux man-pages (unreleased)" "Linux Programmer's Manual"
   9 .SH NAME
  10 daemon \- run in the background
  11 .SH LIBRARY
  12 Standard C library
  13 .RI ( libc ", " \-lc )
  14 .SH SYNOPSIS
  15 .nf
  16 .B #include <unistd.h>
  17 .PP
  18 .BI "int daemon(int " nochdir ", int " noclose );
  19 .fi
  20 .PP
  21 .RS -4
  22 Feature Test Macro Requirements for glibc (see
  23 .BR feature_test_macros (7)):
  24 .RE
  25 .PP
  26 .BR daemon ():
  27 .nf
  28     Since glibc 2.21:
  29 .\"             commit 266865c0e7b79d4196e2cc393693463f03c90bd8
  30         _DEFAULT_SOURCE
  31     In glibc 2.19 and 2.20:
  32         _DEFAULT_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
  33     Up to and including glibc 2.19:
  34         _BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
  35 .fi
  36 .SH DESCRIPTION
  37 The
  38 .BR daemon ()
  39 function is for programs wishing to detach themselves from the
  40 controlling terminal and run in the background as system daemons.
  41 .PP
  42 If
  43 .I nochdir
  44 is zero,
  45 .BR daemon ()
  46 changes the process's current working directory
  47 to the root directory ("/");
  48 otherwise, the current working directory is left unchanged.
  49 .PP
  50 If
  51 .I noclose
  52 is zero,
  53 .BR daemon ()
  54 redirects standard input, standard output, and standard error
  55 to
  56 .IR /dev/null ;
  57 otherwise, no changes are made to these file descriptors.
  58 .SH RETURN VALUE
  59 (This function forks, and if the
  60 .BR fork (2)
  61 succeeds, the parent calls
  62 .\" not .IR in order not to underline _
  63 .BR _exit (2),
  64 so that further errors are seen by the child only.)
  65 On success
  66 .BR daemon ()
  67 returns zero.
  68 If an error occurs,
  69 .BR daemon ()
  70 returns \-1 and sets
  71 .I errno
  72 to any of the errors specified for the
  73 .BR fork (2)
  74 and
  75 .BR setsid (2).
  76 .SH ATTRIBUTES
  77 For an explanation of the terms used in this section, see
  78 .BR attributes (7).
  79 .ad l
  80 .nh
  81 .TS
  82 allbox;
  83 lbx lb lb
  84 l l l.
  85 Interface       Attribute       Value
  86 T{
  87 .BR daemon ()
  88 T}      Thread safety   MT-Safe
  89 .TE
  90 .hy
  91 .ad
  92 .sp 1
  93 .SH STANDARDS
  94 Not in POSIX.1.
  95 A similar function appears on the BSDs.
  96 The
  97 .BR daemon ()
  98 function first appeared in 4.4BSD.
  99 .SH NOTES
 100 The glibc implementation can also return \-1 when
 101 .I /dev/null
 102 exists but is not a character device with the expected
 103 major and minor numbers.
 104 In this case,
 105 .I errno
 106 need not be set.
 107 .SH BUGS
 108 The GNU C library implementation of this function was taken from BSD,
 109 and does not employ the double-fork technique (i.e.,
 110 .BR fork (2),
 111 .BR setsid (2),
 112 .BR fork (2))
 113 that is necessary to ensure that the resulting daemon process is
 114 not a session leader.
 115 Instead, the resulting daemon
 116 .I is
 117 a session leader.
 118 .\" FIXME . https://sourceware.org/bugzilla/show_bug.cgi?id=19144
 119 .\" Tested using a program that uses daemon() and then opens an
 120 .\" otherwise unused console device (/dev/ttyN) that does not
 121 .\" have an associated getty process.
 122 On systems that follow System V semantics (e.g., Linux),
 123 this means that if the daemon opens a terminal that is not
 124 already a controlling terminal for another session,
 125 then that terminal will inadvertently become
 126 the controlling terminal for the daemon.
 127 .SH SEE ALSO
 128 .BR fork (2),
 129 .BR setsid (2),
 130 .BR daemon (7),
 131 .BR logrotate (8)