gen/FreeBSD/daemon.3

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  28 .\"     @(#)daemon.3    8.1 (Berkeley) 6/9/93
  29 .\" $FreeBSD: src/lib/libc/gen/daemon.3,v 1.15 2007/01/09 00:27:53 imp Exp $
  30 .\"
  31 .Dd June 9, 1993
  32 .Dt DAEMON 3
  33 .Os
  34 .Sh NAME
  35 .Nm daemon
  36 .Nd run in the background
  37 .Sh LIBRARY
  38 .Lb libc
  39 .Sh SYNOPSIS
  40 .In stdlib.h
  41 .Ft int
  42 .Fn daemon "int nochdir" "int noclose"
  43 .Sh DESCRIPTION
  44 The
  45 .Fn daemon
  46 function is for programs wishing to detach themselves from the
  47 controlling terminal and run in the background as system daemons.
  48 The
  49 .Xr fork 2
  50 system call is used; see CAVEATS below about the environment after a
  51 .Fn fork
  52 (without a corresponding call to one of the exec routines).
  53 On Mac OS X, the use of this API is discouraged in favor of using
  54 .Xr launchd 8 .
  55 .Pp
  56 Unless the argument
  57 .Fa nochdir
  58 is non-zero,
  59 .Fn daemon
  60 changes the current working directory to the root
  61 .Pq Pa / .
  62 .Pp
  63 Unless the argument
  64 .Fa noclose
  65 is non-zero,
  66 .Fn daemon
  67 will redirect standard input, standard output, and standard error to
  68 .Pa /dev/null .
  69 .Sh RETURN VALUES
  70 .Rv -std daemon
  71 .Sh ERRORS
  72 The
  73 .Fn daemon
  74 function may fail and set
  75 .Va errno
  76 for any of the errors specified for the library functions
  77 .Xr fork 2
  78 and
  79 .Xr setsid 2 .
  80 .Sh SEE ALSO
  81 .Xr fork 2 ,
  82 .Xr launchd 8 ,
  83 .Xr setsid 2 ,
  84 .Xr sigaction 2
  85 .Sh HISTORY
  86 The
  87 .Fn daemon
  88 function first appeared in
  89 .Bx 4.4 .
  90 .Sh CAVEATS
  91 There are limits to what you can do in the child process.
  92 To be totally safe you should restrict yourself to only
  93 executing async-signal safe operations (see
  94 .Xr sigaction 2 )
  95 until such time
  96 as one of the exec functions is called.
  97 All APIs, including global data symbols, in any framework or library
  98 should be assumed to be unsafe after a
  99 .Fn fork
 100 unless explicitly documented to be safe or async-signal safe.
 101 If you need to use these frameworks in the child
 102 process, you must exec.
 103 In this situation it is reasonable to exec yourself.
 104 .Pp
 105 Unless the
 106 .Fa noclose
 107 argument is non-zero,
 108 .Fn daemon
 109 will close the first three file descriptors and redirect them to
 110 .Pa /dev/null .
 111 Normally, these correspond to standard input, standard output, and
 112 standard error.
 113 However, if any of those file descriptors refer to something else, they
 114 will still be closed, resulting in incorrect behavior of the calling program.
 115 This can happen if any of standard input, standard output, or standard
 116 error have been closed before the program was run.
 117 Programs using
 118 .Fn daemon
 119 should therefore either call
 120 .Fn daemon
 121 before opening any files or sockets, or verify that any file
 122 descriptors obtained have values greater than 2.
 123 .Pp
 124 The
 125 .Fn daemon
 126 function temporarily ignores
 127 .Dv SIGHUP
 128 while calling
 129 .Xr setsid 2
 130 to prevent a parent session group leader's calls to
 131 .Xr fork 2
 132 and then
 133 .Xr _exit 2
 134 from prematurely terminating the child process.