gen/daemon.3

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  32 .\"     @(#)daemon.3    8.1 (Berkeley) 6/9/93
  33 .\" $FreeBSD: src/lib/libc/gen/daemon.3,v 1.14 2003/11/10 22:04:51 ghelmer Exp $
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  35 .Dd June 9, 1993
  36 .Dt DAEMON 3
  37 .Os
  38 .Sh NAME
  39 .Nm daemon
  40 .Nd run in the background
  41 .Sh LIBRARY
  42 .Lb libc
  43 .Sh SYNOPSIS
  44 .In stdlib.h
  45 .Ft int
  46 .Fn daemon "int nochdir" "int noclose"
  47 .Sh DESCRIPTION
  48 The
  49 .Fn daemon
  50 function is for programs wishing to detach themselves from the
  51 controlling terminal and run in the background as system daemons.
  52 On Mac OS X, the use of this API is discouraged in favor of using
  53 .Xr launchd 8 .
  54 .Pp
  55 Unless the argument
  56 .Fa nochdir
  57 is non-zero,
  58 .Fn daemon
  59 changes the current working directory to the root
  60 .Pq Pa / .
  61 .Pp
  62 Unless the argument
  63 .Fa noclose
  64 is non-zero,
  65 .Fn daemon
  66 will redirect standard input, standard output, and standard error to
  67 .Pa /dev/null .
  68 .Sh RETURN VALUES
  69 .Rv -std daemon
  70 .Sh ERRORS
  71 The
  72 .Fn daemon
  73 function may fail and set
  74 .Va errno
  75 for any of the errors specified for the library functions
  76 .Xr fork 2
  77 and
  78 .Xr setsid 2 .
  79 .Sh SEE ALSO
  80 .Xr fork 2 ,
  81 .Xr setsid 2 ,
  82 .Xr sigaction 2
  83 .Sh HISTORY
  84 The
  85 .Fn daemon
  86 function first appeared in
  87 .Bx 4.4 .
  88 .Sh CAVEATS
  89 Unless the
  90 .Fa noclose
  91 argument is non-zero,
  92 .Fn daemon
  93 will close the first three file descriptors and redirect them to
  94 .Pa /dev/null .
  95 Normally, these correspond to standard input, standard output, and
  96 standard error.
  97 However, if any of those file descriptors refer to something else, they
  98 will still be closed, resulting in incorrect behavior of the calling program.
  99 This can happen if any of standard input, standard output, or standard
 100 error have been closed before the program was run.
 101 Programs using
 102 .Fn daemon
 103 should therefore either call
 104 .Fn daemon
 105 before opening any files or sockets, or verify that any file
 106 descriptors obtained have values greater than 2.
 107 .Pp
 108 The
 109 .Fn daemon
 110 function temporarily ignores
 111 .Dv SIGHUP
 112 while calling
 113 .Xr setsid 2
 114 to prevent a parent session group leader's calls to
 115 .Xr fork 2
 116 and then
 117 .Xr _exit 2
 118 from prematurely terminating the child process.